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Unformatted text preview: SIMPLE PATTERNS OF INHERITANCE CHAPTER 16 Mendel Chose Pea Plants as His Experimental Organism Mendel chose the garden pea (Pisum sativum) to study the natural laws governing plants hybridization - many hybrids were previously produced The garden pea was advantageous because 1. It existed in several varieties with distinct characteristics 2. Its structure allowed for easy crosses 3. Large number of true-breeding varieties - small and easy to grow short generation time (20 - 30 days from bloom to harvest) 4. Sexual organs enclosed in flower could be manipulated self-fertilization cross fertilization Mendel carried out two types of crosses 1. Self-fertilization Pollen and egg are derived from the same plant 2. Cross-fertilization Pollen and egg are derived from different plants Mendel Studied Seven Traits That Bred True The morphological characteristics of an organism are termed characters or traits A variety that produces the same trait over and over again is termed a true-breeder The seven traits that Mendel studied are Mendel's experimental design allowed pea plants to self-fertilize for several generations assured pure-breeding traits or true breeding performed crosses between varieties exhibiting alternative character forms permitted hybrid offspring to self-fertilize for several generations Hybridization The mating or crossing between two individuals that have different characteristics Purple-flowered plant X whiteflowered plant Hybrids The offspring that result from such a mating Mendel's Experiments Mendel did not have a hypothesis to explain the formation of hybrids Rather, he believed that a quantitative analysis of crosses may provide a mathematical relationship He experimented on single factor crosses resulting in the formation of monohybrids and double factor crosses resulting in the formation of dihybrids. Single-factor cross Experimenter follows the variants of only 1 trait P generation True-breeding parents F1 generation Offspring of P cross Monohybrids if parents differ in 1 trait F2 generation F1 self-fertilizes Mendel crossed two variants that differ in only one trait. This is termed a monohybrid cross F1 generation All tall All round F2 generation 787 tall, dwarf 5,474 round, 1,850 wrinkled 277 Ratio 2.84:1 2.96:1 P Cross Tall X dwarf stem Round X wrinkled seeds Yellow X Green seeds Purple X white flowers All yellow All purple 6,022 yellow, 2,001 green 705 purple, white 224 3.01:1 3.15:1 Axial X terminal flowers Smooth X constricted pods All axial All smooth 651 axial, terminal 882 smooth, constricted 207 3.14:1 229 2.95:1 Green X yellow pods All green 428 green, yellow 152 2.82:1 3 important ideas 1. Dominant and recessive traits Dominant is displayed trait Recessive trait masked by dominant trait 2. Genes and alleles Particulate mechanism of inheritance His "unit factors" are genes Every individual has 2 genes for a trait Gene has two variant forms or alleles 3. Segregation of alleles Approximately 3:1 ratio Two copies of a gene carried by an F1 plant segregate (separate) from each other, so that each sperm or egg carries only one allele Mendel's Law of Segregation 2 copies of a gene segregate from each other during the transmission from parent to offspring Genotype and phenotype Genotype Genetic composition of individual TT or tt homozygous Tt heterozygous Phenotype Characteristics that are the result of gene expression TT and Tt are tall Tt is dwarf Punnett Squares A Punnett square is a grid that enables one to predict the outcome of simple genetic crosses It was proposed by the English geneticist, Reginald Punnett Punnett square Step 1. Write down the genotypes of both parents Male parent: Tt Female parent: Tt Step 2. Write down the possible gametes that each parent can make Male gametes: T or t Female gametes: T or t Step 3. Create an empty Punnett square. Male gametes T t T Female gametes t Step 4. Fill in the possible gentoypes. Male gametes T t T Female gametes TT Tt t T t tt Step 5. Determine relative proportions of genotypes and phenotypes. Male gametes T t T Female gametes TT Tt Genotype ratio TT:Tt:tt 1:2:1 Phenotype ratio Tall: dwarf 3:1 t T t tt Testcross A dwarf pea plant must be tt A tall pea plant could be either TT or Tt Cross unknown individual to a homozygous recessive individual If some offspring are dwarf, unknown individual must have been Tt If all offspring are tall, the unknown individual was TT Two-factor cross Follow inheritance of 2 different traits Possible patterns 2 genes linked so that variants found together in parents are always inherited as a unit 2 genes are independent and randomly distributed Dihybrid offspring- offspring are hybrids with respect to both traits Data for F2 hybrids consistent only with independent assortment Law of Independent Assortment Alleles of different genes assort independently of each other during gamete formation Probability Chance that an event will have a particular outcome For a single coin toss Self-fertilization of a pea plant that was heterozygous for the height gene (Tt) Punnett square predicted that one-fourth of the offspring would be dwarf Sample size Accuracy of prediction depends on the number of events observed or sample size Random sampling error deviation between observed and expected outcome Larger samples have smaller sampling errors Humans have small families and observed data may be very different from expected outcome Pedigree Analysis When studying human traits, it is not ethical to control parental crosses (as Mendel did with peas) Rather, we must rely on information from family trees or pedigrees Pedigree analysis is used to determine the pattern of inheritance of traits in humans Pedigree analysis Inherited trait is analyzed over the course of a few generations in one family Cystic fibrosis (CF) example Approximately 3% of Americans of European descent are heterozygous carriers of the recessive CF allele and phenotypically normal Individuals who are homozygous exhibit disease symptoms Many of the alleles causing human genetic disease are recessive like CF Some are dominant like Huntington disease Huntington disease has an autosomal inheritance pattern Gene on one of 22 pairs of autosomes Genes also found on sex chromosomes Sex chromosomes Found in many but not all species with 2 sexes Several mechanisms for sex determination X-Y system males are XY and females XX X-O system females are XX and males X or XO Z-W system male is ZZ and female ZW Not all chromosomal mechanisms involve sex chromosomes Bees are haplo - diploid male is haploid and female is diploid Other mechanisms also exist Sex is controlled by environment (temperature) in some reptiles and fish Plants Some have a single type of plant making male and female gametophytes Others have sexually distinct plants making male or female gametophytes only X-linked In humans, X chromosome is larger and carries more genes than the Y chromosome Genes found on the X but not the Y are Xlinked genes Sex linked genes are found on one sex chromosome but not the other Males are hemizygous for X-linked genes Hemophilia A example Hemophilia A caused by recessive X-linked gene Encodes defective clotting protein Wild-type allele Prevalent allele in a population Encodes a protein made in the proper amount and functioning normally Mutant alleles Altered by mutation Tend to be rare in natural populations Defective in its ability to express a functional protein In simple dominance, the recessive allele does not affect the phenotype of the heterozygote A single copy of the dominant allele is sufficient to mask the recessive allele Purple pigment, P One P allele makes enough functional protein to provide a normal phenotype In other cases, the heterozygote may make more than 50% of the normal amount of protein up-regulated In incomplete dominance the heterozygote exhibits a phenotype that is intermediate between the corresponding homozygotes Example: Flower color in the four o'clock plant Two alleles CR = wild-type allele for red flower color CW = allele for white flower color Phenylketonuria (PKU) Heterozygotes appear phenotypically normal but heterozygotes have double the normal phenylalanine levels 1:2:1 phenotypic ratio NOT the 3:1 ratio observed in simple Mendelian inheritance In this case, 50% of the CR protein is not sufficient to produce the red phenotype Codominance - No single allele is dominant, and each allele has its own effect. ABO blood groups human gene that encodes enzyme that adds sugar molecules to lipids on the surface of red blood cells IB adds galactose IA adds N-acetylgalactosamine i adds no sugar The ABO blood group provides another example of multiple alleles It is determined by the type of antigen present on the surface of red blood cells Antigens are substances that are recognized by antibodies produced by the immune system Antigen A, which is controlled by allele IA Antigen B, which is controlled by allele IB Antigen O, which is controlled by allele i Multiple alleles 3 or more variants in a population Phenotype depends on which 2 alleles are inherited ABO blood types in humans Type AB is codominance- expressing both alleles equally Allele i is recessive to both IA and IB Alleles IA and IB are codominant They are both expressed in a heterozygous individual Sex-influenced inheritance Allele is dominant in one sex but recessive in the other Pattern baldness Baldness allele dominant in men but not women Only a woman homozygous for baldness allele would be bald Not X-linked Role of environment Norm of reaction effects of environmental variation on a phenotype Genetically identical plants grow to different heights in different temperatures People with PKU can develop normally if given a diet free of phenylalanine If their diet contains phenylalanine, they develop mental retardation, underdeveloped teeth and foulsmelling urine COMPLEX PATTERNS OF INHERITANCE CHAPTER 17 All or nearly all traits are influenced by many genes Mendel studied true-breeding strains that differed with regard to only one gene Gene interaction a single trait is controlled by 2 or more genes, each of which has 2 or more alleles Epistasis Alleles of one gene mask the expression of the alleles of another gene Often arise because 2 or more different proteins involved in a single cellular function - For example, an enzymatic pathway In sweet peas, a cross between a true-breeding purple-flowered plant and a true-breeding white-flowered produced F1 with all purple-flowered plants F2 3:1 purple- to white-flowered 2 varieties of white-flowered peas crossed F1 with all purple-flowered plants Unexpected! F2 9:7 purple- to white-flowered 2 genes involved in flower color C (purple) dominant to c (white) P (different purple) dominant to p (white) Cc masks P or pp masks C producing white flowers Enzyme A Enzyme B Colorless precursor The recessive a allele encodes an inactive enzyme (aa) Colorless intermediate Purple pigment The recessive b allele encodes an inactive enzyme (bb) If an individual is homozygous for either recessive allele It will not make any functional enzyme A or enzyme B Therefore, they remain white Types of traits Discrete or discontinuous Clearly defined phenotypic variants Purple or white flowers, red or white eyes Continuous or quantitative Majority of traits Show continuous variation over a range of phenotypes Height, skin color, number of apples on a tree Polygenic- several or many genes contribute to the outcome Environment also plays a role ...
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This note was uploaded on 04/29/2008 for the course CELL 101 taught by Professor Burdsal during the Spring '08 term at Tulane.
- Spring '08