rav65819_ch12_219-236
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rav65819_ch12_219-236

Course Number: BIO BIO1, Fall 2009

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;;;;;;;; 12 chapter Patterns of Inheritance i ntroduction EVERY LIV I NG CREATURE IS A PRODUCT of the long evolutionary history of life on Earth. All organisms share this h istory, but as far as we know, only humans wonder about the p rocesses that led to their origin and investigate the possibilities. We are far from understanding everything about our origins, but we have learned a great deal. Like a partially...

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of ;;;;;;;; 12 chapter Patterns Inheritance i ntroduction EVERY LIV I NG CREATURE IS A PRODUCT of the long evolutionary history of life on Earth. All organisms share this h istory, but as far as we know, only humans wonder about the p rocesses that led to their origin and investigate the possibilities. We are far from understanding everything about our origins, but we have learned a great deal. Like a partially completed jigsaw puzzle, the boundaries of this elaborate question have fallen into p lace, and much of the internal structure is becoming apparent. I n this chapter, we discuss one piece of the puzzlethe enigma of heredity. Why do individuals, like the children in this picture, d iffer so much in appearance despite the fact that we are all members of the same species? And, why do members of a single family tend to resemble one another more than they resemble members of other families? 12.4 Probability: Predicting the Results of Crosses s Mendels Principle of Independent Assortment explains dihybrid results concept outline 12.1 The Mystery of Heredity s Early plant biologists produced hybrids and saw puzzling results s Mendel used mathematics to analyze his crosses 12.2 Monohybrid Crosses: The Principle of Segregation s The F 1 generation exhibits only one of two traits, without blending s The F 2 generation exhibits both traits in a 3:1 ratio s The 3:1 ratio is actually 1:2:1 s Mendels Principle of Segregation explains monohybrid observations s The Punnett square allows symbolic analysis s Some human traits exhibit dominant / recessive inheritance 12.3 D ihybrid Crosses: The Principle of Independent Assortment s The F 1 generation displays two of four traits, without blending s The F 2 generation exhibits four types of progeny in a 9:3:3:1 ratio s Two probability rules help predict monohybrid cross results s D ihybrid cross probabilities are based on monohybrid cross probabilities 12.5 The Testcross: Revealing Unknown Genotypes 12.6 Extensions to Mendel s I n polygenic inheritance, more than one gene can affect a single trait s I n pleiotropy, a single gene can affect more than one trait s Genes may have more than two alleles s Dominance is not always complete s Genes may be affected by the environment s I n epistasis, interactions of genes alter genetic ratios 219 rav65819_ch12_219-236.indd 219 rav65819_ch12_219-236.indd 219 1/2/07 6:04:21 PM 1/2/07 6:04:21 PM 12.1 The Mystery of Heredity As far back as written records go, patterns of resemblance among the members of particular families have been noted and commented on (figure 12.1), but there was no coherent model to explain these patterns. Before the 20th century, two concepts provided the basis for most t hinking about heredity. The first was that heredity occurs within species. The second was t hat traits are transmitted directly from parents to offspring. Taken together, these ideas led to a view of inheritance as resulting from a blending of traits within fixed, unchanging species. Inheritance itself was viewed as traits being borne through fluid, usually identified as blood, that led to their blending in offspring. This older idea persists today in the use of the term bloodlines when referring to the breeding of domestic animals such as horses. Taken together, however, these two classical assumptions led to a paradox. If no variation enters a species from outside, and if the variation within each species blends in every generation, then all members of a species should soon have the same appearance. It is clear that this does not happenindividuals within most species differ from one another, and t hey differ in characteristics that are transmitted from generation to generation. E arly plant biologists produced hybrids and saw puzzling r esults The first investigator to achieve and document successful experimental hybridizations was Josef Klreuter, who in 1760 cross-fertilized (or crossed, for short) different strains of tobacco and obtained fertile offspring. The hybrids differed in appearance from both parent strains. When individuals within the hybrid generation were crossed, their offspring were highly variable. Some of these offspring resembled plants of the hybrid generation (their parents), but a few resembled the original strains (their grandparents). Klreuters work represents the beginning of modern genetics. The patterns of i nheritance observed in his hybrids contradicted the theory of direct transmission because of t he variation observed in second-generation offspring. Over the next hundred years, other investigators elaborated on Klreuters work. In one such series of experiments, carried out in 1823, T. A. Knight, an English landholder, crossed two varieties of the garden pea, Pisum sativum (figure 12.2). One of these varieties had green seeds, and the other had yellow seeds. Both varieties were t rue- breeding, meaning that the offspring produced from self-fertilization would remain uniform from one generation to the next. All of the progeny (offspring) of the cross between the two varieties had yellow seeds. Among the offspring of these hybrids, however, some plants produced yellow seeds and others, less common, produced green seeds. Other investigators made observations similar to Knights, namely that alternative forms of observed characters were being distributed among the offspring. Referring to a heritable feature as a character, a modern geneticist would say the alternative forms of each character were segregating among the progeny of a mating, meaning that some offspring exhibited one form of a character (yellow seeds), and other offspring from the same mating exhibited a different form (green seeds). This segregation of alternative forms of a character, or t rait, provided the clue that led Gregor Mendel to his understanding of the nature of heredity. Within these deceptively simple results were the makings of a scientific revolution. Nevertheless, another century passed before the process of segregation was fully appreciated. Mendel used mathematics to analyze his crosses Born in 1822 to peasant parents, Gregor Mendel (figure 12.3) was educated in a monastery and went on to study science and mathematics at the University of Vienna, where he failed h is examina- gure 12.1 gure 12.2 HERED IT Y AND FAMI LY RESEMBLANCE. Family resemblances are often stronga visual manifestation of the mechanism of heredity. THE GARDEN PEA, P isum sativum. Easy to cultivate and able to produce many distinctive varieties, the garden pea was a popular experimental subject in investigations of heredity as long as a century before Gregor Mendels experiments. rav65819_ch12_219-236.indd 220 rav65819_ch12_219-236.indd 220 1/2/07 6:04:26 PM 1/2/07 6:04:26 PM tions for a teaching certificate. He returned to the monastery and spent the rest of his life t here, eventually becoming abbot. In the garden of the monastery, Mendel initiated his own series of experiments on plant hybridization. The results of these experiments would u ltimately change our views of heredity irrevocably. Practical considerations for use of the garden pea For his experiments, Mendel chose the garden pea, the same plant Knight and others had studied. The choice was a good one for several reasons. First, many earlier investigators had p roduced hybrid peas by crossing different varieties, so Mendel knew that he could expect to observe segregation of traits among the offspring. Second, a large number of pure varieties of peas were available. Mendel initially examined 34 varieties. Then, for further study, he selected lines that differed with respect to seven easily distinguishable traits, such as round versus wrinkled seeds and yellow versus green seeds, the latter a trait that Knight had studied. Third, pea plants are small and easy to grow, and they have a relatively short generation time. A researcher can therefore conduct experiments involving numerous plants, grow several generations in a single year, and obtain results relatively quickly. A fourth advantage of studying peas is that both the male and female sexual organs are enclosed within each pea flower (figure 12.3), and gametes produced by the male and female parts of the same flower can fuse to form viable offspring, a process termed selffertilization. This self-fertilization takes place automatically within an individual flower if i t is not disturbed, resulting in offspring that are the progeny from a single individual. It is also possible to prevent self-fertilization by removing a flowers male parts before fertilization occurs, then introduce pollen from a different strain, thus performing crosspollination t hat results in c ross-fertilization (see figure 12.3). Mendels experimental design Mendel was careful to focus on only a few specific differences between the plants he was using and to ignore the countless other differences he must have seen. He also had the i nsight to realize that the differences he selected must be comparable. For example, he appreciated that trying to study the inheritance of round seeds versus tall height would be useless. Mendel usually conducted his experiments in three stages: 1. By allowing plants of a given variety to self-cross for multiple generations, Mendel was able to assure himself that the traits he was studying were indeed true-breeding, that is, transmitted unchanged from generation to generation. 2. Pollen is obtained from the white flower. 1. The anthers are cut away on the purple flower. Petals Stigma Style Anthers (male) Carpel (female) 3. Pollen is transferred to the purple flower. 4. All progeny result in purple flowers. gure 12.3 HOW MENDEL CONDUCTED HIS EXPERIME NTS. I n a pea plant flower, petals enclose the male anther (containing pollen grains, which give rise to haploid sperm) and the female carpel (containing ovules, which give rise to haploid eggs). This ensures self-fertilization will take place unless the flower is disturbed. Mendel collected pollen from the anthers of a white flower, then placed that pollen onto t he stigma of a purple flower with anthers removed. This cross fertilization yields all hybrid seeds that give rise to purple flowers. Using pollen from a white flower to fertilize a purple flower gives the same result. inquiry ? What confounding problems could have been seen if Mendel had chosen another plant w ith exposed male and female structures? www.ravenbiology.com chapter 12 patterns of inheritance 221 rav65819_ch12_219-236.indd 221 rav65819_ch12_219-236.indd 221 1/2/07 6:04:28 PM 1/2/07 6:04:28 PM 1. Flower Color Dominant Recessive F2 Generation Purple White 705 Purple: 224 White 5474 Round: 1850 Wrinkled 2.96:1 Dominant Recessive F2 Generation 787 Tall: 277 Short 2.84:1 5. Pod Shape 882 Inflated: 299 Constricted XXXX X 3.15:1 2.95:1 Inflated Constricted 2. Seed Color 6. Flower Position 7. Plant Height 4. Pod Color 3. Seed Texture 651 Axial: 207 Terminal 6022 Yellow: 2001 Green 3.01:1 X Yellow Green Round Wrinkled 3.14:1 Axial Terminal Tall Short 2.82:1 428 Green: 152 Yellow X Green Yellow gure 12.4 MENDELS SEVEN TRAITS. Mendel studied how differences among varieties of peas were i nherited when the varieties were crossed. Similar experiments had been done before, but Mendel was the first to quantify the results and appreciate their significance. Results are shown for seven different monohybrid crosses. The F 1 generation is not shown in the table. 2. Mendel then performed crosses between true-breeding varieties exhibiting alternative forms of traits. He also performed r eciprocal crosses: using pollen from a white-flowered plant to fertilize a purple-flowered plant, then using pollen from a purple-flowered plant to fertilize a white-flowered plant. 3. Finally, Mendel permitted the hybrid offspring produced by these crosses to self-fertilize for several generations, allowing him to observe the inheritance of alternative forms of a t rait. Most important, he counted the numbers of offspring exhibiting each trait in each succeeding generation. This quantification of results is what distinguished Mendels research from that of earlier i nvestigators, who only noted differences i n a qualitative way. Mendels mathematical analysis of experimental results led to the i nheritance model that we still use today. Ideas about inheritance before Mendel did not form a consistent model. The dominant view was of blending inheritance, but plant hybridizers before Mendel had already cast doubt on this model. M endel followed up on the work of early plant hybridizers by systematizing and quantifying his observations. Monohybrid Crosses: The Principle of Segregation 12.2 A monohybrid cross is a cross that follows only two variations on a single trait, such as white- and purple-colored flowers. This deceptively simple kind of cross can lead to important conclusions about the nature of inheritance. The seven characters Mendel studied in his experiments possessed two variants that differed from one another in ways that were easy to recognize and score (see figure 12.4). We examine in detail Mendels crosses with flower color. His experiments with other characters were similar, and they produced similar results. The F 1 generation exhibits only one of two traits, without blending When Mendel crossed white-flowered and purple-flowered plants, the hybrid offspring he obtained did not have flowers of intermediate color, as the hypothesis of blending inheritance would predict. Instead, in every case the flower color of the offspring resembled that of one of t heir parents. These off-spring are customarily referred to as the first filial generation, 222 part I II genetic and molecular biology rav65819_ch12_219-236.indd 222 rav65819_ch12_219-236.indd 222 1/2/07 6:04:31 PM 1/2/07 6:04:31 PM gure 12.5 True-breeding Purple Parent True-breeding White Parent SEED SHAPE: A MENDEL IA N CHARACTER. One of the differences Mendel studied involved the shape of pea plant seeds. In some varieties, the seeds were round, but in others, they were wrinkled. or F 1. I n a cross of white-flowered and purple-flowered plants, the F 1 offspring all had purple flowers, as others had reported earlier. Mendel referred to the form of each trait expressed in the F 1 plants as dominant, and to the alternative form that was not expressed in the F 1 plants as r ecessive. For each of the seven pairs of contrasting traits that Mendel examined, one of the pair proved to be dominant and the other recessive. Parent generation Cross-fertilize Self-cross Purple Offspring The F 2 generation exhibits both traits in a 3:1 ratio After allowing individual F 1 plants to mature and self-fertilize, Mendel collected and planted t he seeds from each plant to see what the offspring in the second filial generation, or F 2, would look like. He found that although most F 2 plants had purple flowers, some exhibited white flowers, the recessive trait. Although hidden in the F 1 generation, the recessive trait had reappeared among some F 2 i ndividuals. Believing the proportions of the F 2 t ypes would provide some clue about the mechanism of heredity, Mendel counted the numbers of each type among the F 2 p rogeny. In the cross between the purple-flowered F 1 plants, he obtained a total of 929 F 2 i ndividuals. Of these, 705 (75.9%) had purple flowers, and 224 (24.1%) had white flowers (see figure 12.4). Approximately 1/4 of the F 2 i ndividuals, therefore, exhibited the recessive form of the character. Mendel obtained the same numerical result with the other six characters he examined: Of the F 2 i ndividuals, 3/4 exhibited the dominant trait, and 1/4 displayed the recessive trait. I n other words, the dominant- to-recessive ratio among the F 2 plants was always close to 3:1. Mendel carried out similar experiments with other traits, such as wrinkled versus round seeds (figure 12.5), and obtained the same result. The 3:1 ratio is actually 1:2:1 Mendel went on to examine how the F 2 plants passed traits to subsequent generations. He found that plants exhibiting the recessive trait were always true-breeding. For example, the white-flowered F 2 i ndividuals reliably produced white-flowered offspring when they were allowed to self-fertilize. By contrast, only 1/3 of the dominant, purple-flowered F 2 i ndividuals (1/4 of all F 2 offspring) proved pure-breeding, but 2/3 were not. This last class of plants produced dominant and recessive individuals in the third filial generation (F 3) in a 3:1 ratio. This result suggested that, for the entire sample, the 3:1 ratio that Mendel observed in t he F 2 generation was really a disguised 1:2:1 ratio: 1/4 true-breeding dominant individuals, 1/2 not-true-breeding dominant individuals, and 1/4 true-breeding recessive individuals (figure 12.6). F1 generation F2 generation F3 generation Purple Dominant True-breeding Purple Dominant Non-true-breeding Purple Dominant Non-true-breeding White Recessive True-breeding Self-cross Self-cross Self-cross Self-cross gure 12.6 THE F 2 G ENERAT ION IS A DISGUISED 1:2:1 RATIO. By allowing the F 2 generation to self-fertilize, Mendel found from the offspring (F 3) that the ratio of F 2 p lants was 1 pure-breeding dominant: 2 not-pure-breeding dominant: and 1 pure-breeding recessive. www.ravenbiology.com chapter 12 patterns of inheritance 223 rav65819_ch12_219-236.indd 223 rav65819_ch12_219-236.indd 223 1/2/07 6:04:32 PM 1/2/07 6:04:32 PM Mendels Principle of Segregation explains monohybrid observations From his experiments, Mendel was able to understand four things about the nature of heredity: The plants he crossed did not produce progeny of intermediate appearance, as a hypothesis of blending inheritance would have predicted. Instead, different plants i nherited each trait intact, as a discrete characteristic. For each pair of alternative forms of a trait, one alternative was not expressed in the F 1 hybrids, although it reappeared in some F 2 i ndividuals. The trait that disappeared must therefore be latent (present but not expressed) in the F 1 i ndividuals. The pairs of alternative traits examined were segregated among the progeny of a particular cross, some individuals exhibiting one trait and some the other. These alternative traits were expressed in the F 2 generation in the ratio of 3/4 dominant to 1/4 recessive. This characteristic 3:1 segregation is referred to as the M endelian ratio for a monohybrid cross. Mendels five-element model To explain these results, Mendel proposed a simple model that has become one of the most famous in the history of science, containing simple assumptions and making clear p redictions. The model has five elements: 1. Parents do not transmit physiological traits directly to their offspring. Rather, they t ransmit discrete information for the traits, what Mendel called factors. We now call t hese factors genes. 2. Each individual receives two genes that encode each trait. We now know that the two factors are carried on chromosomes, and each adult individual is diploid. Gametes, p roduced by meiosis, are haploid. 3. Not all copies of a gene are identical. The alternative forms of a gene are called a lleles. When two haploid gametes containing the same allele fuse during fertilization, t he resulting offspring is said to be homozygous. When the two haploid gametes contain different alleles, the resulting offspring is said to be heterozygous. 4. The two alleles remain discretethey neither blend with nor alter each other. Therefore, when the individual matures and produces its own gametes, the alleles segregate randomly into these gametes. 5. The presence of a particular allele does not ensure that the trait it encodes will be expressed. In heterozygous individuals, only one allele is expressed (the dominant one), and the other allele is present but unexpressed (the recessive one). Geneticists now refer to the total set of alleles that an individual contains as the individuals genotype. The physical appearance or other observable characteristics of that individual, which result from an alleles expression, is termed the individuals phenotype. I n other words, the genotype is the blueprint, and the phenotype is the visible outcome. This also allows us to present Mendels ratios in more modern terms. The 3:1 ratio of dominant to recessive is the monohybrid phenotypic ratio. The 1:2:1 ratio of homozygous dominant to heterozygous to homozygous recessive is the monohybrid genotypic ratio. The genotypic ratio collapses into the phenotypic ratio due to the action of the dominant allele making the heterozygote appear the same as homozygous dominant. The principle of segregation Mendels model accounts for the ratios he observed in a neat and satisfying way. His main conclusionthat alternative alleles for a character segregate from each other during gamete formation and remain distincthas since been verified in many other organisms. It is commonly referred to as Mendels first law of heredity, or the P rinciple of Segregation. I t can be simply stated as: The two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization. The physical basis for allele segregation is the behavior of chromosomes during meiosis. As you saw in chapter 11, homologues for each chromosome disjoin during anaphase I of meiosis. The second meiotic division then produces ga metes that contain only one homologue for each chromosome. It is a tribute to Mendels intellect that his analysis arrived at the correct scheme, even t hough he had no knowledge of the cellular mechanisms of inheritance; neither chromosomes nor meiosis had yet been described. The Punnett square allows symbolic analysis To test his model, Mendel first expressed it in terms of a simple set of symbols. He then used t he symbols to interpret his results. Consider again Mendels cross of purple-flowered with white-flowered plants. By convention, we assign the symbol P (uppercase) to the dominant allele, associated with the p roduction of purple flowers, and the symbol p (lowercase) to the recessive allele, associated w ith the production of white flowers. In this system, the genotype of an individual that is true-breeding for the recessive white-flowered trait would be designated pp. Similarly, the genotype of a truebreeding purple-flowered individual would be designated PP. I n contrast, a heterozygote would be designated Pp (dominant allele first). Using these conventions and denoting a cross between two strains with !, we can symbolize Mendels original purple !white cross as PP! pp. Because a white-flowered parent (pp) can produce only p gametes, and a truebreeding purple-flowered parent (PP, homozygous dominant ) can produce only P gametes, t he union of these gametes can produce only heterozygous Pp offspring in the F 1 generation. Because the P allele is dominant, all of these F 1 i ndividuals are expected to have purple f lowers. When F 1 i ndividuals are allowed to self-fertilize, the P and p alleles segregate during gamete formation to produce both P gametes and p gametes. Their subsequent union at fertilization to form F 2 i ndividuals is random. 224 part I II genetic and molecular biology rav65819_ch12_219-236.indd 224 rav65819_ch12_219-236.indd 224 1/2/07 6:04:36 PM 1/2/07 6:04:36 PM 2. P+p=Pp. 1. p+p=pp. White parent pp F2 generation 3 Purple:1 White (1PP:2Pp:1pp) P p P p P P Pp p p p pp p pp Purple parent PP P Pp Pp P p P p P Pp Pp P Pp P Pp PP F1 generation p pp pP p pp pP 3. p+P=pP. 4. P+P=PP. Purple heterozygote Pp a. P p gure 12.7 Purple heterozygote Pp USING A PUNNETT SQUARE TO ANALYZE MENDELS C ROSS. a . To make a Punnett square, place the different possible types of female gametes along the side of a square and the different possible types of male gametes along the top. Each potential zygote is represented as the intersection of a vertical line and a horizontal line. b. I n Mendels cross of purple by white flowers, the original parents each only make one type of gamete. The resulting F 1 generation are all Pp heterozygotes with purple flowers. These F 1 t hen each make two types of gametes t hat can be combined to produce three kinds of F 2 offspring: PP homozygotes (purple flowers); Pp heterozygotes (also purple flowers); and pp homozygotes (white flowers). The ratio of dominant to recessive phenotypes is 3:1. The ratio of genotypes is 1:2:1 (1 PP: 2 Pp: 1 pp ). P PP Pp p pP pp b. The F 2 possibilities may be visualized in a simple diagram called a P unnett square, named after its originator, the English geneticist R. C. Punnett (figure 12.7a). Mendels model, analyzed in terms of a Punnett square, clearly predicts that the F 2 generation should consist of 3/4 purple-flowered plants and 1/4 white-flowered plants, a phenotypic ratio of 3:1 (figure 12.7b). Some human traits exhibit dominant/recessive inheritance A number of human traits have been shown to display both dominant and recessive i nheritance (table 12.1 provides a sample of these). Researchers cannot perform controlled crosses in humans the way Mendel did with pea plants, so to analyze human inheritance, geneticists study crosses that have been T A B L E 12 .1 Some Dominant and Recessive Traits in H umans Recessive Traits Phenotypes Dominant Traits Phenotypes Albinism Lack of melanin pigmentation Middigital hair Presence of hair on middle segment of ngers Alkaptonuria Inability to metabolize homogentisic acid Brachydactyly Short ngers Red-green color blindness Inability to distinguish red or green wavelengths of light Huntington disease Degeneration of nervous system, starting in middle age Cystic brosis Abnormal gland secretion, leading to liver degeneration and lung failure Phenylthiocarbamide (PTC) sensitivity Ability to taste PTC as bitter Duchenne muscular dystrophy Wasting away of muscles during childhood Camptodactyly Inability to straighten the l ittle nger Hemophilia Inability of blood to clot properly, some clots form but the process is delayed H ypercholesterolemia (the most common human Mendelian disorder) Elevated levels of blood cholesterol and risk of heart attack Sickle cell anemia Defective hemoglobin that causes red blood cells to curve and stick together Polydactyly Extra ngers and toes www.ravenbiology.com chapter 12 patterns of inheritance 225 rav65819_ch12_219-236.indd 225 rav65819_ch12_219-236.indd 225 1/2/07 6:04:36 PM 1/2/07 6:04:36 PM Mating between first cousins Generation IV One of these persons is heterozygous Heterozygous performed alreadyin other words, family histories. The organized methodology we use is a pedigree, a consistent graphical representation of matings and offspring over multiple generations for a particular trait. The information in the pedigree may allow geneticists to deduce a model for the mode of inheritance of the trait. A dominant pedigree: Juvenile glaucoma One of the most extensive pedigrees yet produced traced the inheritance of a form of blindness caused by a dominant allele. The disease allele causes a form of hereditary juvenile glaucoma. The disease causes degeneration of nerve fibers in the optic nerve, leading to blindness. This pedigree followed inheritance over three centuries following the origin back to a couple in a small town in northwestern France who died in 1495. A small portion of this pedigree is shown in figure 12.8. The dominant nature of the trait is obvious from the fact t hat every generation shows the trait. This is extremely unlikely for a recessive trait as it would require large numbers of unrelated individuals to be carrying the disease allele. A recessive pedigree: Albinism An example of inheritance of a recessive human trait is albinism, a condition in which the pigment melanin is not produced. Long thought to be due to a single gene, there are actually multiple genes that can all lead to albinism, the common feature is the loss of pigment from hair, skin, and eyes. The loss of pigment makes albinistic individuals sensitive to the sun. The tanning effect we are all familiar with from exposure to the sun is due to increased numbers of pigment- producing cells, and Recessive Pedigree Key Generation I 12 Generation II Generation III 12 123 4 5 3 4 567 123 Homozygous recessive unaffected male unaffected female affected male affected female male carrier female carrier gure 12.9 RECESSIVE PEDIGREE FOR ALBINISM. One of the two individuals in the first generation must be heterozygous and individuals II- 2 and II- 4 must be heterozygous. Notice that for each affected i ndividual, neither parent is affected, but both must be heterozygous (carriers). The double line i ndicates a consanguineous mating (between relatives) that, in this case. produced affected offspring. inquiry unaffected male unaffected female 23451 Dominant Pedigree ? From a genetic disease standpoint, why is it never advisable for close relatives to mate and h ave children? Generation I Generation II 1 2 increased production of pigment. This is lacking in albinistic individuals due to the lack of any pigment to begin with. The pedigree in figure 12.9 is for a form of albinism due to a nonfunctional allele of the enzyme tyrosinase, which is required for the formation of melanin pigment. The genetic characteristics of this form of albinism are: females and males are affected equally, most affected individuals have unaffected parents, a single affected parent usually does not have affected offspring, and affected offspring are more frequent when parents are related. Each of t hese features can be see in figure 12.9, and all of this fits a recessive mode of inheritance quite well. Generation III 1 23 Key affected male affected female gure 12.8 DOMI NANT PEDIGREE FOR HEREDI TARY JUVEN IL E G LAUCOMA. Males are shown as squares and females are shown as circles. Affected individuals are shown shaded. The dominant nature of this trait can be seen in the trait appearing in every generation, a feature of dominant traits. Monohybrid crosses show that traits are due to factors inherited intact with no blending. Traits that appear in the F 1 generation are called dominant; traits that are not observed are called recessive. In the F 2 generation, both traits are observed in a predictable ratio of 3 dominant to 1 recessive. The Principle of Segregation states that during gamete formation, alleles segregate into different gametes and are then randomly combined during fertilization. Dominant/recessive inheritance is analyzed in humans using pedigrees. inquiry ? If one of the affected females in the third generation married an unaffected male, could she produce unaffected offspring? If so, what are the chances of having unaffected offspring? 226 part I II genetic and molecular biology rav65819_ch12_219-236.indd 226 rav65819_ch12_219-236.indd 226 1/2/07 6:04:37 PM 1/2/07 6:04:37 PM D ihybrid Crosses: The Principle of Independent Assortment 12.3 The Principle of Segregation explains the behavior of alternative forms of a single trait in a monohybrid cross. The next step is to extend this to follow the behavior of two different traits i n a single cross: a dihybrid cross. With an understanding of the behavior of single traits, Mendel went on to ask if different traits behaved independently in hybrids. He first established a series of truebreeding lines of peas that differed in two of the seven characters he had studied. He then crossed contrasting pairs of the true-breeding lines to create heterozygotes. These heterozygotes are now doubly heterozygous, or dihybrid. Finally, he self-crossed the dihybrid F 1 plants to produce an F 2 generation, and counted all progeny types. The F 1 generation displays two of four traits, without blending Consider a cross involving different seed shape alleles (round, R, and wrinkled, r ) and different seed color alleles (yellow, Y, and green, y). Crossing round yellow (RR YY ) with w rinkled green (rr yy), p roduces heterozygous F 1 i ndividuals having the same phenotype (namely round and yellow) and the same genotype (Rr Yy). A llowing these dihybrid F 1 i ndividuals to self-fertilize produces an F 2 generation. The F 2 generation exhibits four types of progeny in a 9:3:3:1 r atio In analyzing these results, we first consider the number of possible phenotypes. We expect to see the two parental phenotypes: round yellow and wrinkled green. If the traits behave i ndependently, then we can also expect one trait from each parent to produce plants with round green seeds and others with wrinkled yellow seeds. Next consider what types of gametes the F 1 i ndividuals can produce. Again, we expect t he two types of gametes found in the parents: R Y and r y. I f the traits behave i ndependently, then we can also expect the gametes R y and r Y. Using modern language, t wo genes each with two alleles can be combined four ways to produce these gametes: R Y, r y, R y, and r Y. A dihybrid Punnett square We can then construct a Punnett square with these gametes to generate all possible progeny. This is a 4!4 square with 16 possible outcomes. Filling in the Punnett square produces all possible offspring (figure 12.10). From this we can see that there are 9 round yellow, 3 w rinkled yellow, 3 round green, and 1 wrinkled green. This predicts a phenotypic ratio of 9:3:3:1 for traits that behave independently. Mendels data What did Mendel actually observe? From a total of 556 seeds from self-fertilized dihybrid plants, he observed the following results: 315 round yellow (signified R__ Y__, where the underscore indicates the presence of either allele), 108 round green (R__ yy), 101 wrinkled yellow (rr Y__), and 32 wrinkled green (rr yy). RR YY rr yy Parent generation Meiosis Meiosis Cross-Fertilization Meiosis (chromosomes assort independently into four types of gametes) Rr Yy F1 generation RY Ry rY ry F1 X F1 (RrYy X RrYy) RY Ry rY ry RR YY RR Yy Rr YY RR Yy RR yy Rr Yy Rr yy Rr YY Rr Yy rr YY rr Yy Rr Yy Rr yy rr Yy rr yy RY Ry rY ry Rr Yy F2 generation 9/16 3/16 3/16 1/16 round, yellow round, green wrinkled, yellow wrinkled, green gure 12.10 ANALYZING A DIHYBR I D CROSS. This Punnett square shows the results of Mendels dihybrid cross between plants with round yellow seeds and plants with wrinkled green seeds. The ratio of the four possible combinations of phenotypes is predicted to be 9:3:3:1, the ratio that Mendel found. www.ravenbiology.com chapter 12 patterns of inheritance 227 rav65819_ch12_219-236.indd 227 rav65819_ch12_219-236.indd 227 1/2/07 6:04:37 PM 1/2/07 6:04:37 PM These results are very close to a 9:3:3:1 ratio. (The expected 9:3:3:1 ratio from this many offspring would be 313:104:104:35.) The alleles of two genes appeared to behave independently of each other. Mendel referred to this phenomenon as the traits assorting independently. Note that this i ndependent assortment of different genes in no way alters the segregation of individual pairs of alleles for each gene. Round versus wrinkled seeds occur in a ratio of approximately 3:1 (423:133); so do yellow versus green seeds (416:140). Mendel obtained similar results for other pairs of traits. Mendels Principle of Independent Assortment explains d ihybrid results Mendels discovery is often referred to as Mendels second law of heredity, or the P rinciple of Independent Assortment. This can also be stated simply: I n a dihybrid cross, the alleles of each gene assort i ndependently. L ike segregation, independent assortment arises from the behavior of chromosomes during meiosis to produce haploid gametes (chapter 11)in this case, the i ndependent alignment of different homologous pairs during metaphase I. Mendels analysis of dihybrid crosses revealed that the segregation of allele pairs for different genes is i ndependent, known as Mendels Principle of Independent Assortment. When individuals that differ in t wo traits are crossed, and their progeny are intercrossed, the result is four different types that occur i n a ratio of 9:3:3:1, or Mendels dihybrid ratio. 12.4 Probability: Predicting the Results of Crosses Probability allows us to predict the likelihood of the outcome of random events. Because the behavior of different chromosomes during meiosis is independent, we can use probability to p redict the outcome of crosses. The probability of an event that is certain to happen is equal to 1. In contrast, an event that can never happen has a probability of 0. Therefore, p robabilities for all other events have fractional values, between 0 and 1. For instance, when you flip a coin, two outcomes are possible; there is only one way to get the event heads so t he probability of heads is one divided by two, or 1/2. In the case of genetics, consider a pea plant heterozygous for the flower color alleles P and p. This individual can produce two types of gametes in equal numbers, again due to the behavior of chromosomes during meiosis. There is one way to get a P gamete, so the p robability of any particular gamete carrying a P allele is 1 divided by 2 or 1/2, just like the coin toss. Two probability rules help predict monohybrid cross results We can use probability to make predictions about the outcome of genetic crosses using only t wo simple rules. Before we describe these rules and their uses, we need another definition. We say that two events are mutually exclusive i f both cannot happen at the same time. The heads and tails of a coin flip are examples of mutually exclusive events. Notice that this is different from two consecutive coin flips where you can get two heads or two tails. In this case, each coin flip represents an i ndependent event and it is the distinction between i ndependent and mutually exclusive events that forms the basis for our two rules. The rule of addition If we consider a six-sided die instead of a coin, for any roll of the die, only one outcome is possible; each of the possible outcomes are mutually exclusive. The probability of any particular number coming up is 1/6. The probability of either of two different numbers is the sum of the individual probabilities, or to restate as the r ule of addition: For two mutually exclusive events, the probability of either event occurring is the sum of t he individual probabilities. Probability of rolling either a 2 or a 6 is =1/6+ =2/6=1/3 1/6 To apply this to our cross of heterozygous purple F 1, four mutually exclusive outcomes are possible: PP, Pp, pP, and pp. The probability of being heterozygous is the same as the p robability of being either Pp or pP, or 1/4 plus 1/4, or 1/2. Probability of F 2 heterozygote=1/4Pp+ pP=1/2 1/4 In the previous example, of 379 total offspring, we would expect about 190 to be heterozygotes. (The actual number is 189.5.) The rule of multiplication The second rule, and by far the most useful for genetics, deals with the outcome of i ndependent events. This is called the p roduct rule, or r ule of multiplication, and it states that the probability of two independent events both occurring is the p roduct of their i ndividual probabilities. We can apply this to a monohybrid cross where offspring are formed by gametes from each of two parents. For any particular outcome then, this is due to two independent events: t he formation of two different gametes. Consider the purple F 1 parents from earlier. They are all Pp (heterozygotes), so the probability that a particular F 2 i ndividual will be pp (homozygous recessive) is the probability of receiving a p gamete from the male (1/2) times t probability he of receiving a p gamete from the female (1/2), or 1/4: Probability of pp homozygote=1/2p (male parent) !1/2p (female parent) =1/4pp This is actually the basis for the Punnett square that we used before. Each cell in the square was the product of the probabilities of the gametes that contribute to the cell. We t hen use the addition rule to sum the probabilities of the mutually exclusive events that make up each cell. 228 part I II genetic and molecular biology rav65819_ch12_219-236.indd 228 rav65819_ch12_219-236.indd 228 1/2/07 6:04:38 PM 1/2/07 6:04:38 PM We can use the result of a probability calculation to predict the number of homozygous r ecessive offspring in a cross between heterozygotes. For example, out of 379 total offspring, w e would expect about 95 to exhibit t he homozygous recessive phenotype. (The actual calculated number is 94.75.) Dihybrid cross probabilities are based on monohybrid cross p robabilities Probabilit y analysis can be extended to t he dihybrid case. For our purple F 1 by F 1 cross, there are four possible outcomes, three of which show the dominant phenotype. Thus the p robability of any offspring showing the dominant phenotype is 3/4, and the probability of any offspring showing the recessive phenotype is 1/4. Now we can use this and the product r ule to predict the outcome of a dihybrid cross. We will use our example of seed shape and color from earlier, but now examine it using probability. If the alleles affecting seed shape and seed color segregate independently, then the p robability that a particular pair of seed shape alleles would occur together with a particular pair of seed color alleles is the product of the individual probabilities for each pair. For example, the probability that an individual with wrinkled green seeds (rryy ) would appear in t he F 2 generation would be equal to the probability of obtaining wrinkled seeds (1/4) times t he probability of obtaining green seeds (1/4), or 1/16. Probability of r ryy =1/4 r r !1/4 yy=1/16 r ryy Because of independent assortment, we can think of the dihybrid cross of consisting of t wo independent monohybrid crosses; since these are independent events, the product rule applies. So, we can calculate the probabilities for each dihybrid phenotype: Probability of round yellow (R__ Y__)= 3/4 R__!3/4 Y__=9/16 Probability of round green (R__ yy)= 3/4 R__!1/4 yy=3/16 Probability of wrinkled yellow (rr Y__)= 1/4 r r !3/4 Y_=3/16 Probability of wrinkled green (rryy )= 1/4 r r !1/4 yy=1/16 The hypothesis that color and shape genes are independently assorted thus predicts t hat the F 2 generation will display a 9:3:3:1 phenotypic ratio. These ratios can be applied to an observed total offspring to predict the expected number in each phenotypic group. The underlying logic and the results are the same as obtained using the Punnett square. The probability of either of two events occurring is the sum of the individual probabilities. The p robability of two independent events both occurring is the product of the individual probabilities. T hese can be applied to genetic crosses to determine the probability of particular genotypes and phenotypes. 12.5 The Testcross: Revealing Unknown Genotypes To test his model further, Mendel devised a simple and powerful procedure called the testcross. I n a testcross, an individual with unknown genotype is crossed with the homozygous recessive genotypethat is, the recessive parental variety. The contribution of t he homozygous recessive parent can be ignored, because this parent can contribute only recessive alleles. Consider a purple-flowered pea plant. It is impossible to tell whether such a plant is homozygous or heterozygous simply by looking at it. To learn its genotype, you can per- f orm a testcross to a white-flowered plant. In t his cross, t he t wo possible test plant genotypes w ill give different results (figure 12.11): Alternative 1: Unknown individual is homozygous dom inant (PP ) PP!pp: All offspring have purple flowers (Pp). Alternative 2: Unknown individual is heterozygous (Pp) Pp!pp: 1 /2 of offspring have white flowers (pp ), and 1/2 have purple flowers (Pp). gure 12.11 PP or Pp? Dominant Phenotype (unknown genotype) A TESTCROSS. To determine whether an individual exhibiting a dominant phenotype, such as purple f lowers, is homozygous or heterozygous for the dominant allele, Mendel crossed the individual in question with a plant that he knew to be homozygous recessive in this case, a plant with white f lowers. Homozygous recessive Alternative 1: All offspring are purple and the unknown flower is homozygous dominant Homozygous recessive Alternative 2: Half of the offspring are white and the unknown flower is heterozygous dominant p p p p Homozygous dominant Heterozygous dominant P P Pp Pp Pp Pp P If Pp If PP then then p Pp Pp pp pp www.ravenbiology.com chapter 12 patterns of inheritance 229 rav65819_ch12_219-236.indd 229 rav65819_ch12_219-236.indd 229 1/2/07 6:04:38 PM 1/2/07 6:04:38 PM TA B L E 12 . 2 D ihybrid Testcross Put simply, the appearance of the recessive phenotype in the offspring of a testcross i ndicates that the test individuals genotype is heterozygous. For each pair of alleles Mendel investigated, he observed phenotypic F 2 ratios of 3:1 (see f igure 12.4) and testcross ratios of 1:1, just as his model had predicted. Testcrosses can also be used to determine the genotype of an individual when two genes are involved. Mendel often performed testcrosses to verify the genotypes of dominant- appearing F 2 i ndividuals. An F 2 i ndividual exhibiting both dominant traits (A__ B__ ) might have any of the following genotypes: AABB, AaBB, AABb, or AaBb. By crossing dominant- appearing F 2 i ndividuals with homozygous recessive individuals (that is, A__ B__!aabb), Mendel was able to determine whether either or both of the traits bred true among the progeny, and so to determine the genotype of the F 2 parent (table 12.2). Testcrossing is a powerful tool that simplifies genetic analysis. We will use this method of analysis in the next chapter, when we explore genetic mapping. Individuals showing the dominant phenotype can be either homozygous dominant or heterozygous. The genotype can be determined using a testcross, which involves crossing the individual of unknown genotype to a homozygous recessive individual. Heterozygous individuals produce both dominant and r ecessive phenotypes in equal numbers as a result of the testcross. Actual Genotype Results of Testcross Trait A Trait B AABB Trait A breeds true Trait B breeds true AaBB Trait B breeds true AABb T rait A breeds true AaBb 12.6 Extensions to Mendel Although Mendels results did not receive much notice during his lifetime, three different i nvestigators independently redis-covered his pioneering paper in 1900, 16 years after his death. They came across it while searching the literature in preparation for publishing their own findings, which closely resembled those Mendel had presented more than 30 years earlier. In the decades following the rediscovery of Mendels ideas, many investigators set out to test them. However, scientists attempting to confirm Mendels theory often had trouble obtaining the same simple ratios he had reported. The reason that Mendels simple ratios were not obtained had to do with the traits that others examined. A number of assumptions are built into Mendels model that are oversimplifications. These assumptions include that each trait is specified by a single gene w ith two alternative alleles; that there are no environmental effects; and that gene products act independently. The idea of dominance also hides a wealth of biochemical complexity. In t he following sections, youll see how Mendels simple ideas can be extended to provide a more complete view of genetics (table 12.3). In polygenic inheritance, more than one gene can affect a single trait Often, the relationship between genotype and phenotype is more complicated than a single allele producing a single trait. Most phenotypes also do not reflect simple two-state cases like purple or white flowers. Consider Mendels crosses between tall and short pea plants. In reality, the tall plants actually have normal height, and the short plants are dwarfed by an allele at a single gene. But in most species, including humans, height varies over a continuous range, rather than having discrete values. This continuous distribution of a phenotype has a simple genetic explanation: the action of more than one gene. The mode of inheritance that takes place in this case is often called polygenic inheritance. TA B L E 12 . 3 When Mendels Laws/Results May Not Be Observed Genetic Occurrence Denition Examples Polygenic inheritance More than one gene can affect a single trait. Four genes are involved in determining eye color. Human height Pleiotropy A single gene can affect more than one trait. A pleiotropic allele dominant for yellow fur in mice is recessive for a lethal developmental defect. Cystic brosis Sickle cell anemia Multiple alleles for one gene Genes may have more than two alleles. ABO blood types in humans Dominance is not always complete In incomplete dominance the heterozygote is intermediate. In codominance no single allele is dominant, and the heterozygote shows some aspect of both homozygotes. Japanese four oclocks Human blood groups Environmental factors Genes may be affected by the environment. Siamese cats Gene interaction Products of genes can interact to alter genetic ratios. The production of a purple pigment in corn Coat color in mammals 230 part I II genetic and molecular biology rav65819_ch12_219-236.indd 230 rav65819_ch12_219-236.indd 230 1/2/07 6:04:39 PM 1/2/07 6:04:39 PM In reality, few phenotypes result from the action of only one gene. Instead, most characters reflect multiple additive contributions to the phenotype by several genes. When multiple genes act jointly to influence a character, such as height or weight, the character often shows a range of small differences. When these genes segregate independently, a gradation in the degree of difference can be observed when a group consisting of many i ndividuals is examined (figure 12.12). We call this gradation continuous variation, and we call such traits quantitative traits. The greater the number of genes that influence a character, the more continuous the expected distribution of the versions of that character. This continuous variation in traits is similar to blending different colors of paint: Combining one part red with seven parts white, for example, produces a much lighter pink shade than does combining five parts red with three parts white. Different ratios of red to white result in a continuum of shades, ranging from pure red to pure white. Often, variations can be grouped into categories, such as different height ranges. Plotting the numbers in each height category produces a curve called a h istogram, such as t hat shown in figure 12.12. The bell-shaped histogram approximates an idealized normal d istribution, i n which the central tendency is characterized by the mean, and the spread of t he curve indicates the amount of variation. Even simple-appearing traits can have this kind of polygenic basis. For example, human eye colors are often described in simple terms with brown dominant to blue, but t his is actually incorrect. Extensive analysis indicates that at least four genes are involved in determining eye color. This leads to more complex inheritance patterns than initially reported. For example, blue-eyed parents can have brown-eyed offspring, although it is rare. In pleiotropy, a single gene can affect more than one trait Not only can more than one gene affect a single trait, but a single gene can affect more than one trait. Considering the complexity of biochemical pathways and the interdependent nature of organ systems in multicellular organisms, this should be no surprise. An allele that has more than one effect on phenotype is said to be pleiotropic. The pioneering French geneticist Lucien Cuenot studied yellow fur in mice, a dominant trait, and found he was unable to obtain a pure-breeding yellow strain by crossing individual yellow m ice with each other. Individuals homozygous for the yellow allele died, because the yellow allele was pleiotropic: One effect was yellow coat color, but another was a lethal developmental defect. A pleiotropic allele may be dominant with respect to one phenotypic consequence (yellow fur) and recessive with respect to another (lethal developmental defect). Pleiotropic effects are difficult to predict, because a gene that affects one trait often performs other, unknown functions. Pleiotropic effects are characteristic of many inherited disorders in humans, including cystic fibrosis and sickle cell anemia (discussed in the following chapter). In these disorders, multiple symptoms (phenotypes) can be traced back to a single gene defect. Cystic fibrosis patients exhibit clogged blood vessels, overly sticky mucus, salty sweat, liver and pancreas failure, and a battery of other symptoms. It is often difficult to deduce the nature of the p rimary defect from the range of a genes pleiotropic effects. As it turns out, all these symptoms of cystic fibrosis are pleiotropic effects of a single defect, a mutation in a gene that encodes a chloride ion transmembrane channel. Number of Individuals 0 5'0'' 5'6'' Height 30 20 10 0 6'0'' Genes may have more than two alleles Mendel always looked at genes with two alternative alleles. Although any diploid individual can carry only two alleles for a gene, there may be more than two alleles in a population. The example of ABO blood types in humans, described later on, involves an allelic series with t hree alleles. If you think of a gene as a sequence of nucleotides in a DNA molecule, then the number of possible alleles is huge because even a single nucleotide change could produce a new allele. I n reality, the number of alleles possible for any gene is constrained, but usually more than t wo alleles exist for any gene in an outbreeding population. The dominance relationships of t hese alleles cannot be predicted, but can be determined by observing the phenotypes for the various heterozygous combinations. Dominance is not always complete Mendels idea of dominant and recessive traits can seem hard to explain in terms of modern biochemistry. For example, if a recessive trait is caused by the loss of function of an enzyme gure 12.12 HEIGH T IS A CONTI N UOUSLY VARYING TRAIT. The photo and accompanying graph show variation in height among students of the 1914 class at the Connecticut Agricultural College. Because many genes contribute to height and tend to segregate independently of one another, the cumulative contribution of different combinations of alleles to height forms a continuous distribution of possible heights, in which the extremes are much rarer than the intermediate values. Variation can also arise due to environmental factors such as nutrition. www.ravenbiology.com chapter 12 patterns of inheritance 231 rav65819_ch12_219-236.indd 231 rav65819_ch12_219-236.indd 231 1/2/07 6:04:39 PM 1/2/07 6:04:39 PM encoded by the recessive allele, then why should a heterozygote, with only half the activity of t his enzyme, have the same appearance as a homozygous dominant individual? The answer is that enzymes usually act in pathways and not alone. These pathways, as you have seen in earlier chapters, can be highly complex in terms of inputs and outputs, and t hey can sometimes tolerate large reductions in activity of single enzymes in the pathway w ithout reductions in the level of the end-product. When this is the case, complete dominance will be observed; however, not all genes act in this way. Incomplete dominance In i ncomplete dominance, t he heterozygote is intermediate in appearance between the t wo homozygotes. For example, in a cross between red- and white-flowering Japanese four oclocks, described in figure 12.13, all the F 1 offspring have pink flowers indicating that neither red nor white flower color was dominant. Looking only at the F 1, we might conclude t hat this is a case of blending inheritance. But when two of the F 1 pink flowers are crossed, t hey produce red-, pink- , and white-flowered plants in a 1:2:1 ratio. In this case the phenotypic ratio is the same as the genotypic ratio because all three genotypes can be distinguished. Codominance Most genes in a population possess several different alleles, and often no single allele is dominant; instead, each allele has its own effect, and the heterozygote shows some aspect of t he phenotype of both homozygotes. The alleles are said to be codominant. Codominance can be distinguished from incomplete dominance by the appearance of t he heterozygote. In incomplete dominance, the heterozygote is intermediate between the t wo homozygotes, whereas in codominance, some aspect of both alleles is seen in the heterozygote. One of the clearest human examples is found in the human blood groups. The different phenotypes of human blood groups are based on the response of the i mmune system to proteins on the surface of red blood cells. In homozygotes a single type of p rotein is found on the surface of cells, and in heterozygotes, two kinds of protein are found, leading to codominance. The human ABO blood group system The gene that determines ABO blood types encodes an enzyme that adds sugar molecules to p roteins on the surface of red blood cells. These sugars act as recognition markers for the i mmune system (chapter 51). The gene that encodes the enzyme, designated I , has three common alleles: I A , whose product adds galactosamine; I B , whose product adds galactose; and i, which codes for a protein that does not add a sugar. The three alleles of the I gene can be combined to produce six different genotypes. An i ndividual heterozygous for the I A and I B alleles produces both forms of the enzyme and exhibits both galactose and galactosamine on red blood cells. Because both alleles are expressed simultaneously in heterozygotes, the I A and I B alleles are codominant. Both I A and I B are dominant over the i allele, because both I A and I B alleles lead to sugar addition, whereas the i allele does not. The different combinations of the three alleles produce four different phenotypes (figure 12.14): 1. Type A individuals add only galactosamine. They are either I A I A homozygotes or I A i heterozygotes (two genotypes). 2. Type B individuals add only galactose. They are either I B I B homozygotes or I B i heterozygotes (two genotypes). 3. Type AB individuals add both sugars and are I A I B heterozygotes (one genotype). 4. Type O individuals add neither sugar and are i i homozygotes (one genotype). These four different cell surface phenotypes are called the ABO blood groups. A persons immune system can distinguish among these four phenotypes. If a type A i ndividual receives a transfusion of type B blood, the recipients immune system recognizes t he foreign antigen (galactose) and attacks the donated blood cells, causing them to clump, or agglutinate. The same thing would happen if the donated blood is type AB. However, if the donated blood is type O, no immune attack occurs, because there are no galactose antigens. In general, any individuals immune system will tolerate a transfusion of type O blood, and so type O is termed the universal donor. Because neither galactose nor galactosamine CRCR CWCW Parent generation Cross-fertilization CRCW F1 generation F2 generation 1:2:1 CRCR:CRCW:CWCW CR CW CR CW CRCR CRCW CRCW CWCW gure 12.13 INCOMP LE T E DOMI NANCE. I n a cross between a red-flowered (genotype CRCR ) Japanese four oclock and a white-flowered one (CWCW), neither allele is dominant. The heterozygous progeny have p ink flowers and the genotype CRCW. I f two of these heterozygotes are crossed, the phenotypes of their p rogeny occur in a ratio of 1:2:1 (red:pink:white). 232 part I II genetic and molecular biology rav65819_ch12_219-236.indd 232 rav65819_ch12_219-236.indd 232 1/2/07 6:04:40 PM 1/2/07 6:04:40 PM Sugars Exhibited Blood AllelesType Donates and Receives IAIA , IAi (IA dominant to i) Galactose B Receives B and O Donates to B and AB IBIB , IBi (IB dominant to i) ii (i is recessive) Both galactose and galactosamine AB Universal receiver Donates to AB None O Receives O Universal donor II AB (codominant) Galactosamine A Receives A and O Donates to A and AB gure 12.14 ABO BLOOD GROUPS ILLUSTRATE BOTH CODOMI NANCE A ND MULT I P L E ALLELES. There are three alleles of the I gene: I A, I B and i . I A and I B are both dominant to i (see types A and B), but codominant to each other (see type AB). The genotypes that give r ise to each blood type are shown with the associated phenotypes in terms of sugars added to surface p roteins and the behavior in blood transfusions. is foreign to type AB individuals (whose red blood cells have both sugars), those individuals may receive any type of blood, and type AB is termed the universal recipient. Nevertheless, matching blood is preferable for any transfusion. Genes may be affected by the environment Another assumption, implicit in Mendels work, is that the environment does not affect the relationship between genotype and phenotype. For example, the soil in the abbey yard where Mendel performed his experiments was probably not uniform, and yet its possible effect on t he expression of traits was ignored. But in reality, although the expression of genotype p roduces pheno-type, the environment can affect this relationship. Environmental effects are not limited to the external envi ronment. For example, t he alleles of some genes encode heat-sensitive products, that are affected by differences in i nternal body temperature. The ch allele in Himalayan rabbits and Siamese cats encodes a heat-sensitive version of the enzyme tyrosinase, which as you may recall is involved in albinism (figure 12.15). The Ch version of the enzyme is inactivated at temperatures above about 33C. At the surface of the torso and head of these animals, the temperature is above 33C and tyrosinase is inactive, producing a whitish coat. At the extremities, such as the tips of the ears and tail, the temperature is usually below 33C and the enzyme is active allowing p roduction of melanin that turns the coat in these areas a dark color. Temperature below 33C, tyrosinase active, dark pigment Temperature above 33C, tyrosinase inactive, no pigment inquiry ? Many studies of identical twins separated at birth have revealed phenotypic differences in t heir development (height, weight, etc.). If these are identical twins, can you propose an explanation for these differences? In epistasis, interactions of genes alter genetic ratios The last simplifying assumption in Mendels model is that the products of genes do not i nteract. But the products of genes may not act independently of one another, and the i nterconnected behavior of gene products can change the ratio expected by independent assortment, even if the genes are on different chromosomes that do exhibit independent assortment. Given the interconnected nature of metabolism, it should not come as a surprise that many gene products are not independent. Genes that act in the same metabolic pathway, for example, should show some form of dependence at the level of function. In such cases, the ratio Mendel would predict is not readily observed, but it is still there in an altered form. Epistasis in corn In the tests of Mendels ideas that followed the rediscovery of his work, scientists had trouble obtaining Mendels simple ratios, particularly with dihybrid crosses. Sometimes, it was not possible to identify successfully each of the four phenotypic classes expected, because two or more of the classes looked alike. An example of this comes from the analysis of particular varieties of corn, Zea mays. Some commercial varieties exhibit a purple pigment called anthocyanin in their seed coats, whereas others do not. In 1918, geneticist R. A. Emerson crossed two true-breeding corn varieties, each lacking anthocyanin pigment. Surprisingly, all of the F 1 plants produced purple seeds. When two of these pigment-producing F 1 plants were crossed to produce an F 2 generation, 56% were pigment producers and 44% were not. This is clearly not the Mendelian expectation. Emerson correctly deduced that two genes were involved in p roducing pigment, and that the second cross had thus been a dihybrid cross. According to Mendels theory, gametes in a di-hybrid cross could combine in 16 equally possible waysso t he puzzle was to figure out how these 16 combinations could occur in the two phenotypic groups of progeny. Emerson multiplied the fraction that were pigment producers (0.56) by 16 to obtain 9, and multiplied the fraction that lacked pigment (0.44) by 16 to obtain 7. Emerson t herefore had a modified ratio of 9:7 instead of the usual 9:3:3:1 ratio (figure 12.16). gure 12.15 SIAMESE CAT. The pattern of coat color is due to an allele that encodes a temperature-sensitive form of the enzyme tyrosinase. www.ravenbiology.com chapter 12 patterns of inheritance 233 rav65819_ch12_219-236.indd 233 rav65819_ch12_219-236.indd 233 1/2/07 6:04:40 PM 1/2/07 6:04:40 PM This modified ratio is easily rationalized by considering the function of the products encoded by these genes. When gene products act sequentially, as in a biochemical pathway, an allele expressed as a defective enzyme early in the pathway blocks the flow of material t hrough the rest of the pathway. In this case, it is impossible to judge whether the later steps of the pathway are functioning properly. This type of gene interaction, in which one gene can interfere with the expression of another, is the basis of the phenomenon called epistasis. The pigment anthocyanin is the product of a two-step biochemical pathway: enzyme 1 enzyme 2 starting molecule D i ntermediate D anthocyanin (colorless) (colorless) (purple) To produce pigment, a plant must possess at least one functional copy of each enzymes gene. The dominant alleles encode functional enzymes, and the recessive alleles encode nonfunctional enzymes. Of the 16 genotypes predicted by random assortment, 9 contain at least one dominant allele of both genes; they therefore produce purple progeny. The remaining 7 geno-types lack dominant alleles at either or both loci (3+3+1=7) and so produce colorless progeny, giving the phenotypic ratio of 9:7 that Emerson observed (see figure 12.16). You can see that although this ratio is not the expected dihybrid ratio, it is a modification of the expected ratio. Epistasis in Labrador retrievers In many animals, coat color is the result of epistatic interactions among genes. Coat color in Labrador retrievers, a breed of dog, is due primarily to the interaction of two genes. The E gene determines whether a dark pigment, eumelanin, will be deposited in the fur. A dog having the genotype ee has no dark pigment deposited, and its fur is yellow. A dog having the genotype EE or Ee (E__) does have dark pigment deposited in the fur. A second gene, the B gene, determines how dark the pigment will be. This gene controls the distribution of melanosomes in a hair. Dogs with the genotype E__bb have brown fur and are called chocolate labs. Dogs with the genotype E__B__ have black fur. Even in yellow dogs, however, the B gene does have some effect. Yellow dogs with the genotype eebb exhibit brown pigment on their nose, lips, and eye rims, but yellow dogs with the genotype eeB__ have black pigment in these areas. White (AAbb) Cross-fertilization All Purple (AaBb) White (aaBB) Parental generation F1 generation AB Ab aB ab AABB AABb AaBB AaBb AABb AAbb AaBb Aabb AaBb Aabb aaBb aabb AaBB AaBb aaBB aaBb F2 generation AB Ab aB ab 9/16 Purple: 7/16 White a. Enzyme Precursor (colorless) A Intermediate (colorless) Enzyme B Pigment (purple) b. gure 12.16 HOW EPISTASIS AFFECTS GRAIN COLOR. a. Crossing some white varieties of corn yields an all purple F 1. If the white kernels were due to a recessive allele for a single gene we would expect white offspring. Self-crossing the F 1 y ields 9 purple:7 white. This can be explained by the presence of two genes, each encoding an enzyme necessary for pigment production. Unless both enzymes are active (the p lant has a dominant allele for each of the two genes, A_B_), no pigment is expressed. b. The biochemical pathway for pigment production with enzymes encoded by A and B genes. Mendels model is correct, but incomplete. Some traits are produced by the action of multiple genes (polygenic inheritance), and one gene can affect more than one trait (pleiotropy). Genes may have more t han two alleles that may not show simple dominance. In incomplete dominance, the heterozygote is i ntermediate between the two homozygotes, and in codominance the heterozygote shows aspects of both homozygotes. The action of genes is also not always independent. This can lead to modified d ihybrid ratios although the alleles of each gene are segregating independently. In epistasis, the action of one gene obscures the action of other genes. 234 part I II genetic and molecular biology rav65819_ch12_219-236.indd 234 rav65819_ch12_219-236.indd 234 1/2/07 6:04:42 PM 1/2/07 6:04:42 PM concept review 12.1 The Mystery of Heredity Our understanding of inheritance is the result of the scientific observations and Mendels pea hybridization research. s T raits, or characters, are transmitted directly to offspring, but they do not necessarily blend. s I nherited characters can disappear in one generation only to reappear later, that is, the traits segregate among the offspring of a cross. s Some traits are observed more often in the offspring of crosses. s Mendels experiments involved reciprocal crosses between pure-breeding pea varieties followed by one or more generations of self-fertilization. s Mendels mathematic analysis of experimental results lead to the present model of inheritance. 12.2 Monohybrid Crosses: The Principle of Segregation (gure 12.6) A monohybrid cross follows only two forms of a single trait. s T raits are determined by discrete factors we now call genes. s A lleles are alternative forms of a gene that produce alternative forms of a trait. s A genotype refers to the total set of alleles possessed by an individual. s A phenotype refers to the physical appearance or other observable characteristic of the individual t hat is the result of the genotypes expression. s The offspring of a parental cross (P) are the rst lial generation (F 1). s I n crosses between pure-breeding parents the dominant trait is expressed and the alternative or recessive trait is not expressed until the F 2 generation. s I n the F 2 generation, the Mendelian ratio is expressed as 75% dominant to 25% recessive; also expressed as a 3:1 ratio. s The F 2 generation disguises a 1:2:1 ratio in which 1/4 are true-breeding dominants, 2/4 (1/2) are not true-breeding and 1/4 are true-breeding recessives. s The Principle of Segregation states that alternative alleles for a gene segregate during gamete formation and are randomly rejoined during fertilization. s A homozygous individual carries two alleles of a gene that are the same. s A heterozygous individual carries two alleles of a gene that are different. s A t rait determined by a dominant allele will be seen in both the homozygous dominant and the heterozygote. s A t rait determined by a recessive allele will only be seen in the homozygous recessive. s The results of Mendelian crosses can be predicted with a Punnett square or by probability theory (gure 12.7). s H uman inheritance is studied using family pedigrees. 12.3 D ihybrid Crosses: The Principle of Independent Assortment (gure 12.10) During meiosis, the segregation of different pairs of alleles is independent of each other. s A dihybrid cross follows the behavior of two different traits during a single cross. s During a dihybrid cross the F 1 generation displays only two of four possible combination of traits with no blending. s The F 2 generation of a dihybrid cross exhibits all four possible combinations of traits in a 9:3:3:1 ratio. s The Principle of Independent Assortment states that the alleles of each gene assort independently. 12.4 Probability: Predicting the Behavior of Crosses Because the behavior of different chromosomes during meiosis is independent we can use probability to p redict the outcome of crosses. s Two rules of probability help predict genotypes and phenotypes from monohybrid cross results. s The rule of addition states that the probability of two mutually exclusive events occuring is the v xr of the individual probabilities. s The rule of multiplication states that the probability of two independent events both occurring is t he tcxbw of individual probabilities. s D ihybrid cross probabilities are based on monohybrid cross probabilities using the product rule. 12.5 The Testcross: Revealing Unknown Genotypes (gure 12.11) In a testcross, an unknown genotype is crossed with a homozygous recessive genotype. s I f the unknown genotype is homozygous dominant, the F 1 offspring will be the same. s I f the unknown genotype is heterozygous, the F 1 offspring will exhibit a 1:1 ratio. s The results of a testcross support the Principle of Segregation. 12.6 Extensions to Mendel In subsequent research scient ists concluded t hat Mendels basic model is correct, but it is incomplete a nd m akes assum pt ions t hat are not valid. s I n polygenic inheritance m ore t han one gene contributes to a phenotype. s M any complex t rait s are due to m ult iple addit ive contributions by several genes, result ing in a cont inuous variat ion of quant itat ive t raits. s A p leiotropic effect occurs when an allele affects more t han one t rait and t heir effects are dif cult t o predict. s Genes m ay have m ore t han t wo (m ult iple), alleles. s I ncomplete dom inance occurs when t he heterozygous condition exhibits an intermediate p henotype result ing in a 1:2:1 rat io ( gure 12.13). s Codom inant alleles each exhibit t heir own effect on the phenotype because one allele is not d om inant over t he other. s The environment m ay affect t he expression of a genotype, result ing in different phenotypes. s I n epistasis genes interact, and one gene interferes wit h t he expression of a second. www.ravenbiology.com chapter 1 2 patterns of inheritance 235 rav65819_ch12_219-236.indd 235 rav65819_ch12_219-236.indd 235 1/2/07 6:04:43 PM 1/2/07 6:04:43 PM review questions SELF TEST 1. A t rue-breeding plant is one t hat a. produces offspring t hat are different from t he parent b . form s hybrid offspring t hrough cross pollinat ion c. produces offspring t hat are always t he same as the parent d. can only reproduce wit h it self 2. What property dist inguished Mendels investigation from previous studies? a . Mendel used t rue-breeding pea plants. b. Mendel quant i ed his results. c. Mendel exam ined m any different t raits. d. Mendel exam ined t he segregation of t raits. 3. A m onohybrid cross a. is t he same as self-fert ilizat ion b. exam ines a single variant of a t rait c. produces a single offspring d. exam ines t wo variants of a single t rait 4. What was t he appearance of t he F 1 generation of a monohybrid cross of purple (PP) and white (pp ) ower pea plants? a. All the F 1 plants had white owers. b. The F 1 p lants had a light purple or blended appearance. c. All the F 1 plants had purple owers. d. The most of the F 1 (3/4) had purple owers, but 1/4 of the plants had white. 5. The F 1 p lants from t he previous question are allowed to self-fertilize. What will the phenotypic ratio be for this F 2? a. All purple b. 1 purple:1 white c. 3 purple:1 white d. 3 white:1 purple 6. Which of the following is not a part of Mendels ve-element model? a. Traits have alternative forms (what we now call alleles). b. Parents transmit discrete traits to their offspring. c. If an allele is present it will be expressed. d. Traits do not blend. 7. A heterozygous i ndividual is one that carries a. two completely different sets of genes b. two identical alleles for a particular gene c. only one functional allele d. two different alleles for a given gene 8. An organisms ___________ is determined by its _____________ a. genotype; phenotype b. phenotype; genotype c. alleles; phenotype d. F 1 generation; alleles 9. Which of the following represent the phenotype for the recessive human trait, albinism ? a. Absence of the pigment melanin b. Presence of a nonfunctional allele for the enzyme tyrosinase c. Absence of the enzyme tyrosinase from the individuals cells d . Bot h a and c 10. A dihybrid cross between a plant with long smooth leaves and a plant wit h short h airy leaves produces a long smooth F 1. If this F 1 is allowed to self-cross to produce an F 2, what would you predict for the ratio of F 2 phenotypes? a. 9 long smooth:3 long hairy:3 short hairy:1 short smooth b. 9 long smooth:3 long hairy:3 short smooth:1 short hairy c. 9 short hairy:3 long hairy:3 short smooth:1 long smooth d. 1 long smooth:1 long hairy:1 short smooth:1 short hairy 11. A testcross is used to determine if an individual is a. homozygous dominant or heterozygous b. homozygous recessive or homozygous dominant c. heterozygous or homozygous recessive d. true-breeding 12. What is a polygenic trait? a. A set of multiple phenotypes determined by a single gene b. A single phenotypic trait determined by two alleles c. A single phenotypic trait determined by more than one gene d. The collection of traits possessed by an individual 13. When a single gene in uences multiple phenotypic traits the effect is called a. Codominance b. Epistasis c. Incomplete dominance d. Pleiotropy 14. What is the probability of obtaining an individual with the genotype bb f rom a cross between two individuals with the genotype Bb ? a. 1/2 b. 1/4 c. 1/8 d. 0 15. What is the probability of obtaining an individual with the genotype CC f rom a cross between t wo individuals with the genotypes CC and Cc? a. 1/2 b. 1/4 c. 1/8 d. 1/16 C HALLENGE QUESTIONS 1. Create a Punnett square for the following crosses and use this to predict phenotypic ratio for dominant and recessive traits. Dominant alleles are indicated by uppercase letters and recessive a re indicated by lowercase letters. a. A monohybrid cross between individuals with the genotype Aa and Aa b. A dihybrid cross between two individuals with the genotype AaBb c. A dihybrid cross between individuals with the genotype AaBb and aabb 2. Use probability to predict the following. a. What is the probability of obtaining an individual with the genotype r r f rom the self-fertilization of a plant with the genotype Rr ? b. What is the probability that a testcross with a heterozygous individual will produce homozygous recessive offspring? c. A plant with the genotype Gg is self-fertilized. Use probability to determine the proportion of the offspring that will have the dominant phenotype. d. Use probability to determine the proportion of offspring from a dihybrid cross (GgRr !GgRr ) that w ill have the phenotype ggR_. Do you need additional review? V isit www.ravenbiology.com for practice quizzes, animations, videos, and activities designed to help you master the material in this chapter. rav65819_ch12_219-236.indd 236 rav65819_ch12_219-236.indd 236 1/2/07 6:04:43 PM 1/2/07 6:04:43 PM

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;134 mmchapterChromosomes, Mapping, and the MeiosisInheritance ConnectionintroductionMENDELS EXPERIMENTS OPENED the door to understanding inheritance, but many questionsremained. In the early part of the 20th century, we did not know the nature of
CUNY Brooklyn - BIO - BIO1
;14chapterD NA: The Genetic MaterialintroductionTHE REALIZATION THAT PATTERNS OFheredity can be explained by the segregation of chromosomes in meiosis raised a question that occupied biologists for over 50 years: What is t he exact nature of the con
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;151.2 mchapterGenes and How They Workconcept outlineintroduction15.1 The Nature of Geness Garrod concluded that inherited disorders can involve specific enzymes s Beadle and Tatum showed that genes specify enzymes s The central dogma describes in
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;partIII16genetic and molecular biology40 mm 40 mchapterIN MUSIC, DIFFERENT INSTRUMENTS PLAY their own parts atdifferent times during a piece; a musical score determines which instruments play when. Similarly, in an organism different genes are ex
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;170.3 mmchapterBiotechnologyintroductionOVER THE PAST DECADES, the development of new andpowerful techniques for studying and manipulating DNA has revolutionized biology. The knowledge gained in the last 25 years is greater than the rest of the hi
CUNY Brooklyn - BIO - BIO1
;18chapterGenomicsTHE PACE OF DISCOVERY IN BIOLOGY in the last 30 years hasbeen like the exponential growth of a population. Starting with the isolation of the first genes in the mid-1970s, researchers had accomplished the first complete genome seque
CUNY Brooklyn - BIO - BIO1
;194000 mchapterCellular Mechanisms of Developmentintroduction19.1 Overview of Development 19.2 Cell Divisions Development begins with cell division s Every cell division is known in the development of C. elegans s Stem cells continue to divide and
CUNY Brooklyn - BIO - BIO1
;partIVevolution20Genes Within Populationschapter introductionNO OTHER HUMAN BEING is exactly like you (unless you have an identical twin). Often the particular characteristics of an individual have an important bearing on its survival, on its chan
CUNY Brooklyn - BIO - BIO1
;21chapterThe Evidence for EvolutionintroductionAS WE DISCUSSED IN CHAPTER 1, when Darwin proposed his revolutionary theory of evolution bynatural selection, little actual evidence existed to bolster his case. Instead, Darwin relied on observations
CUNY Brooklyn - BIO - BIO1
;22The Origin of Specieschapteri ntroductionALTHOUGH DARWIN TITLED HIS BOOKSpecies, he never actually discussed what he referred to as that mystery of mysterieshow one species gives rise to another. Rather, his argument concerned evolution by natura
CUNY Brooklyn - BIO - BIO1
;chapter23Systematics and the Phylogenetic RevolutionintroductionALL ORGANISMS SHARE MANY biological characteristics.Theyare composed of one or more cells, carry out metabolism and transfer energy with ATP, and encode hereditary information in DNA.
CUNY Brooklyn - BIO - BIO1
;24Genome EvolutionintroductionGENOMES CONTAIN THE RAW MATERIALhidden in the ever-changing nature of genomes. As more genomes have been sequenced, the new and exciting field of comparative genomics has emerged and has yielded some surprising results
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;25chapterEvolution of Developmenti ntroductionHOW IS IT THAT CLOSELYrelated species of frogs can have completely different patterns of development? One frog goes f rom fertilized egg to adult frog with no intermediate tadpole stage. The sister spec
CUNY Brooklyn - BIO - BIO1
;partVdiversity of life on earth26IN PRECEDING CHAPTERS, youve seen that many commonfeatures are found in living things. To name a few, they are composed of one or more cells, they carry out metabolism and transfer energy with ATP, and they encode h
CUNY Brooklyn - BIO - BIO1
;27.036 mm .036 mchapterV irusesintroductionWE BEGIN OUR EXPLORATIO N of the diversity of life with v iruses. Viruses are genetic elements enclosed in protein; they a re not considered organisms since they lack many of the features associated with l
CUNY Brooklyn - BIO - BIO1
;28chapterProkaryotesintroductionONE OF THE HALLMARKS OF LIVING organisms is their cellular organization. You learned earlier that living things come in two basic cell types: tp`twdv and d xp`twdvTo review, prokaryotes lack the membrane-bounded nucl
CUNY Brooklyn - BIO - BIO1
;29FOR MORE THAN HALF OF the long history of life on Earth, all life was microscopic. The biggestorganisms that existed for over 2 billion years were single-celled bacteria fewer than 6 mm thick. These prokaryotes lacked internal membranes, except for
CUNY Brooklyn - BIO - BIO1
;30Overview of Green PlantsintroductionchapterPLANT EVOLUTION IS THE STORY of adaptation to terrestriallife by green algal ancestors. All green algae and land plants share a common ancestor, composing a monophyletic group called the green plants. Fo
CUNY Brooklyn - BIO - BIO1
;31chapterFungiintroductionTHE FUNGI, AN OFTEN-OVERLOOKED group of unicellular and multicellular organisms, have aprofound influence on ecology and human health. Along with bacteria, they are important decomposers and disease-causing organisms. Fung
CUNY Brooklyn - BIO - BIO1
;32chapter introductionOverview of Animal DiversityWE NOW EXPLORE THE GREAT diversity of modern animals,the result of a long evolutionary history. Animals are among the most abundant living organisms. Found in almost every conceivable habitat, they b
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;33chapterNoncoelomate InvertebratesintroductionWE START OUR EXPLORATION of the great diversity of animals with the simplest members of theanimal kingdomsponges, jellyfish, and simple worms. These animals lack a body cavity(coelom), and they are th
CUNY Brooklyn - BIO - BIO1
;34chapterCoelomate InvertebratesALTHOUGH ACOELOMATES AND PSEUDOCOELOMATEShave proven very successful, a third way of organizing the animal body has also evolved, one that occurs in many protostomes and in all deuterostomes. We begin our discussion o
CUNY Brooklyn - BIO - BIO1
;35chapter introductionMEMBERS OF THE PHYLUM CHORDATA exhibit great changes in the endoskeleton compared with what is seen in echinoderms. As you saw in chapter 34, the endoskeleton of echinoderms is functionally similar to the exoskeleton of arthropod
CUNY Brooklyn - BIO - BIO1
;partVIplant form and function36chapterPlant FormintroductionALTHOUGH THE SIMILARITIES AMONG a cactus, an orchid, anda hardwood tree might not be obvious at first sight, most plants have a basic unity of structure. This unity is reflected in how
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;37chapterVegetative Plant DevelopmentintroductionHOW DOES A FERTILIZED EGG DEVELOP into a complex adultplant body? Because plant cells cannot move, the timing and directionality of each cell division must be carefully orchestrated. Cells need infor
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;chapter38T ransport in PlantsintroductionTERRESTRIAL PLANTS FACE TWOmajor challenges: maintaining water and n utrient balance, and providing sufficient structural support for upright growth. The vascular system t ransports water, minerals, and orga
CUNY Brooklyn - BIO - BIO1
;39chapterPlant Nutrition and SoilsVAST ENERGY INPUTS ARE REQUIRED for the ongoingconstruction of a plant. In this chapter, youll learn what inputs, besides energy from the Sun, a plant needs to survive. Plants, like animals, need various nutrients t
CUNY Brooklyn - BIO - BIO1
;40chapterPlant Defense ResponsesPLANTS ARE CONSTANTLY UNDER ATTACK by viruses,bacteria, fungi, animals, and even other plants. An amazing array of defense mechanisms has evolved to block or temper an invasion. Many plantpest relationships undergo co
CUNY Brooklyn - BIO - BIO1
;41chapterALL ORGANISMS SENSE AND INTERACT with their environments. This is particularly true ofplants. Plant survival and growth are critically influenced by abiotic factors, including water, wind, and light. The effect of the local environment on pl
CUNY Brooklyn - BIO - BIO1
;42chapterPlant ReproductionintroductionTHE REMARKABLE EVOLUT IONARY SUCCESS of floweringp lants can be linked to their novel reproductive strategies. In t his chapter, we explore the reproductive strategies of the angiosperms and how their unique f
CUNY Brooklyn - BIO - BIO1
;partVIIanimal form and function431 mchapterThe Animal Body and Principles of RegulationWHEN PEOPLE THINK OF ANIMALS, they may think of petdogs and cats, the animals in a zoo, on a farm, in an aquarium, or wild animals living outdoors. When think
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;20 mmchapter44The Nervous SystemintroductionALL ANIMALS EXCEPT SPONGES use a network of nerve cells togather information about the bodys condition and the external environment, to process and integrate that information, and to issue commands to th
CUNY Brooklyn - BIO - BIO1
;455 mmchapterALL INPUT FROM SENSORY NEURONS to the central nervoussystem arrives in the same form, as action potentials. Sensory neurons receive input from a variety of different kinds of sense receptor cells, such as the rod and cone cells found in
CUNY Brooklyn - BIO - BIO1
;46chapterThe Endocrine SystemintroductionDIABETES IS A DISEASE IN WHICH well-fed patients appearto starve to death. The disease was known to Roman and Greek physicians, who described a melting away of flesh coupled with excessive urine production l
CUNY Brooklyn - BIO - BIO1
;47chapterThe Musculoskeletal SystemTHE ABILI TY TO MOVEis so much a part of our daily lives that we tend to take it for granted. It is made possible by the combination of a semirigid skeletal system, joints that act as h inges, and a muscular system
CUNY Brooklyn - BIO - BIO1
;48chapter963The Digestive SystemPLANTS AND OTHER PHOTOSYNTHETIC ORGANISMScan produce the organic molecules they need from inorganic components. Therefore, they are autotrophs, or self-sustaining. Animals, such as the chipmunk shown, are heterotroph
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;concept outline4949.1 Invertebrate Circulatory Systems s Open circulatory systems move uids in a one-way path s Closed circulatory systems move uids in a loop 49.2 Vertebrate Circulatory Systems s In shes, more efcient circulation developed concurrent
CUNY Brooklyn - BIO - BIO1
;50chapterTemperature, Osmotic Regulation, and the Urinary Systemconcept outline50.1 Regulating Body Temperature s Q10 is a measure of temperature sensitivity s Temperature is determined by internal and external factors s Organisms are classied based
CUNY Brooklyn - BIO - BIO1
;51chapterThe Immune Systemconcept outlineWHEN YOU CONSIDER HOW ANIMALS defend themselves, it is natural to think of turtles and armadillos with their obvious external armor. However, armor offers little protection against the greatest dangers verteb
CUNY Brooklyn - BIO - BIO1
;chapter52The Reproductive SystemBIRD SONG IN THE SPRING, insects chirping outside thewindow, frogs croaking in swamps, and wolves howling in a frozen northern forest are all sounds of evolutions essential act, reproduction. These distinctive noises,
CUNY Brooklyn - BIO - BIO1
;chapter53i ntroductionAnimal DevelopmentSEXUAL REPRODUCT IO Ni n all but a few animals unites twohaploid gametes to form a single diploid cell called a zygote. T he zygote develops by a process of cell division and differentiation i nto a complex
CUNY Brooklyn - BIO - BIO1
;partVIIIecology and behavior54Behavioral BiologyintroductionORGANISMS INTERACT WITH their environment in many ways. To understand these interactions, we need to appreciate the internal factors that shape the way an animal behaves, as well as aspec
CUNY Brooklyn - BIO - BIO1
;55chapterPopulation EcologyintroductionECOLOGY, THE STUDY OF HOW organisms relate to oneanother and to their environments, is a complex and fascinating area of biology that has important implications for each of us. In our exploration of ecological
CUNY Brooklyn - BIO - BIO1
;56chapterintroductionALL THE ORGANISMS THAT LIVE together in a place are members of a community. The myriad ofspecies that inhabit a tropical rain forest are a community. Indeed, every inhabited place on Earth supports its own particular array of or
CUNY Brooklyn - PSYCH - PSYCH1
Adolescence&AdulthoodLectureOverview Biopsychosocialdevelopmentin adolescence AdolescentRiskTaking AdolescentIdentityFormation MaslowsHierarchyofNeeds Love&Workasdefiningfeaturesof AdulthoodAdolescence Adolescenceismarkedby biologicaldevelopments p
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Week10:MechanismsofMotivation&Emotion,CH6 Mon.(Nov.2nd):ASSIGNMENT:Readpp.179195(GeneralPrinciples,Hunger,) Wed.(Nov.4th):ASSIGNMENT:Readpp.195212(Sex&Sleep) Week11:MemoryandConsciousness,CH9 Mon.(Nov.9th):ASSIGNMENT:Readpp.303318(Overview,Attention& Work
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APA FormatMichael Brown Brooklyn College - CUNYWhat is APA Format? APA format was developed by the American Psychological Association. It focuses on the needs of presenting psychological information, but is used for essentially all college writing. It
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TheCerebralCortexBrainDamage,BrainGrowth& theEffectsofDrugsCerebralCortex Accountsfor80%ofourbrainsvolume Dividedintotwohemispheresconnectedbythecorpus callosum Dividedinto4lobes: FrontalLobeassociatedwithreasoning,planning, producingspeech,planningmo
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Introduction to the Brain and Nervous SystemThe Nervous SystemSympathetic Division (for Fight or Flight)Parasympathetic Division (for energy conservation) Sensory NeuronsThree Basic Varieties of Neurons Carry info from sensory organs (eyes, ears, no
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Sleep,Attention&MemoryLectureOverview WhyWeSleep&EffectsofSleep Deprivation Attention&PreAttention SensoryMemory SubliminalPrimingSleepReviewfromLastTime 5StagesofSleep:Stage14NonREM&REM Asleepcyclelasts90110minutes,andpeople usuallyhave45sleepcyclesp
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IntroductiontoPsychology,1.1,Fall2009Syllabus NeedtoKnowInfo Instructor:AdamJohnson Hall AdamJ@brooklyn.cuny.edu Office:5111JamesHall Class:Mon/Wed3:404:55,5301James Section:MW3Code:1003 OfficeHours:ByAppointmentRequiredTextBook:Gray,P.(2007).Psychology
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FinalExamReviewNewMaterialforFinal(roughly60%offinal) CH.10:Intelligence&IntelligenceTesting EarlyIntelligenceTesting SimonBinetScale&IQ StanfordBinetscale ThehistoricalexplosionofintelligencetestingintheUS PartsoftheWechslerAdultIntelligenceScale(WAISIV
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PersonalityQuiz Question Name one of the Big Five personality dimensions OR Name the statistical method that is used in Personality research in order to determine which traits correlate with each other to form broader FactorsThe $64,000 Question Why d