Unformatted text preview: What we'll cover in the next few lectures we' Lectures 9: Mendelian Genetics I Jan 28, 2009
We won't make it through all of today's slides NOTES Midterm 2 covers chapters 10, 21, 22 and part of 55. Those chapter practice quiz questions are fair game. Two of the questions that stumped the most people on midterm 1 were verbatim from the book chapters. HEREDITY is the study of how the genetic composition of an organism (its GENOTYPE) and the environment GENOTYPE) influence its the physical appearance (its PHENOTYPE) PHENOTYPE) What are the Mendelian laws of inheritance? What is the relationship between genes and chromosomes? chromosomes? How do alleles interact? How do genes interact? What about traits that are controlled by multiple genes? genes? We have gotten a little ahead of schedule. The syllabus has us finishing the Darwin / Wallace story today, but we will get to Gregor Mendel! III. The Theory of Evolution by Natural Selection C. Observations leading to the theory
INFERENCE 1: There is a "struggle" for existence due to struggle" excess fecundity. INFERENCE 2: The best competitors win in the struggle for existence: differential survival. INFERENCE 3: Heritable traits that enhance survival and reproduction in nature will increase in frequency in the population through time POPULATION Transformational (top) vs. variational evolution
Transformational (Lamarckian) Natural selection
Variational (Darwinian) This is Natural Selection: Selection: As defined in the textbook: "The differential contribution of offspring to the next generation by various genetic types belonging to a population" population" 1 CD ARW III. The Theory of Evolution by Natural Selection D. Things that were hard for Darwin to explain...
In Darwin's day, they believed in Blending Inheritance Darwin'
Problem: Eventually, the population should become Problem: phenotypically uniform, leaving no variation upon which Requisite Superbowl week metaphor selection can act Father Is credit of Darwin's for this innovation warranted? x Mother P1 Offspring Offspring Offspring F1 Darwin's great fumble? II. Mendelian Genetics A. Gregor Mendel Mendel was a contemporary of Darwin He was unusually well-trained in the physical wellsciences (& it showed in his entire statistical/probabilistic approach to a biological problem) In 1866, he sent Darwin his pea paper, but Darwin either didn't understand the implications, or never didn' read it Darwin never mentions the paper in his notebooks, though there is a copy of the paper in Darwin's collection Darwin' Darwin performed similar crosses on snapdragons, but did not grasp the significance! Pre-Mendelian thinking about inheritance PreLong before about 1700 AD, most females knew that there was an association between sex & reproduction... But it was not until ~1700, that Camerarius showed that plant sex required the union of pollen & ovules And it wasn't until 1830 that Hertwig showed that sexual reproduction in wasn' animals also required the union of sperm & egg So, whatever it was that caused the resemblance between parents & offspring had to fit inside a sperm & egg But, no one had the slightest idea what the stuff was inside sperm & eggs, much less what happened during fertilization Pre-Mendelian thinking about inheritance PreIn 1694, Nicolas Hartsoeker `discovered' "animalcules" in the discovered' animalcules" semen of humans and other animals This was the beginning of spermists' theory, who held the spermists' belief that the sperm was in fact a "little man" (homunculus) that (homunculus) was placed inside a woman for growth into a child (Wikipedia: "Homunculus") Homunculus"
homunculi 2 Pre-Mendelian thinking about inheritance PreBy the mid-19th century, plant & animal breeders worked under a midreasonable (but incorrect) assumption about inheritance: When you breed 2 different parental lines that differ for some trait(s), each of which is true-breeding, to produce hybrid true-breeding, offspring, these offspring all look pretty much alike, and (often) look (often) like a mix of the 2 parental types. Thus there is Blending Inheritance Pre-Mendelian thinking about inheritance PreThis was a problem for Darwin's theory of natural selection: Darwin' With Blending, populations should eventually become phenotypically uniform, with no variation upon which selection can act Gregor Mendel (1822-1884) (1822- II. Mendelian Genetics A. Gregor Mendel
He headed a monastery in Moravia, Austria (now the Czech Republic), bred sweet peas, and solved Darwin's problem Darwin' Father x Mother P1 Looks like blending inheritance Offspring Offspring Offspring F1 Mendel to the rescue! II. Mendelian Genetics A. Why Mendel was so clever... He asked the right question: everyone else was trying to figure out why mice gave birth to mice, cows to cows, etc. etc. Mendel knew that to study heredity, he had to study organism that could that interbreed (not usually possible with different species) He knew he had to study traits that VARIED within species He knew he had to study traits that could be classified into two or more DISCRETE classes, with no intermediates He extended Dalton's atomic theory of chemistry to inheritance: Dalton' Dalton never saw an atom, but reasoned that atoms must exist because because of the fixed and simple proportions in which elements combine to form compounds (e.g., H2O) He knew he had to study organisms that he could easily breed and that produced lots of progeny because he understood that he needed to count frequencies (his mathematical approach was unusual at the time) Remember, Mendel knew nothing about genes, chromosomes, DNA... DNA... THE CHROMOSOMAL THEORY OF INHERITANCE 1822-1884 Name that hero of Mendelian genetics 1856-1863 1809-1882 Erich von Tschermak 1871-1962 Carl Correns 1864-1933 Hugo de Vries 1848-1935 3 II. Mendelian Genetics B. What Mendel did He repeatedly self-fertilized plants so that they were TRUE BREEDING selffor each of 7 BINARY, discrete characters in sweet peas He crossed these plants in various combinations in careful experiments experiments He analyzed the results from these experiments using simple mathematics and statistics, and deduced the nature of genes and the laws governing heredity ... Without any knowledge of chromosomes or ... DNA! (smart guy...) This mathematics of crossing single gene characters remains essentially unchanged to today (really smart guy). The field of statistics wasn't even invented yet (how smart a guy was wasn' he?) II. Mendelian Genetics B. What Mendel did, 1. General Methods
Mendel's controlled crosses with true-breeding lines Mendel' true- II. Mendelian Genetics B. What Mendel did, 2. Monohybrid Crosses
Methods (STEP 1): Make true-breeding lines for each trait true Cross parental plants (P) differing in just one character F1 generation are thus monohybrids Results (STEP 1): One trait of each pair disappeared in the F1 generation these traits are recessive The trait that appears in the F1 is the dominant trait stimga anthers ovary Plant the seeds Parental plants F1 generation
100% 100% F1 plant
Transfer pollen from one parent to the stigma of the other parent Parental generation = P Resulting offspring = first filial generation or F1 Self-pollinate or cross F1 plants to Selfproduce second filial generation or F2 100% 100% Hypothesis: Traits from the parents are irreversibly blended in the offspring offspring Prediction: When smooth and wrinkled peas are crossed, their offspring will have Prediction: will partially-wrinkled peas (intermediate) and the parental types will not be recoverable partially- II. Mendelian Genetics B. What Mendel did, 2. Monohybrid Crosses
Methods (STEP 2): Cross 2 of these resulting F1 plants Count the numbers of each trait type in the F2 generation II. Mendelian Genetics B. What Mendel did, 2. Monohybrid Crosses
Results (STEP 2): The recessive trait always re-appeared in the F2's reThe ratio of dominant to recessive phenotypes in the F2 was about 3:1 Reciprocal crosses yielded the same results (didn't matter which parent gave pollen or ovule) (didn' II. Mendelian Genetics B. What Mendel did, 3. Test of Blending
Hypothesis: Traits from the parents are irreversibly blended in the offspring offspring Prediction: When smooth and wrinkled peas are crossed, their offspring will have Prediction: will partially-wrinkled peas (intermediate) and the parental types will not be recoverable partiallyMethods:
Fig 10.3 in text Results (STEP 2): See next slide... Cross two F1 plants
Self fertilization 3 smooth and 1 wrinkled pea ~3:1
He repeated this with 7 traits and got the same results... Result: F1 seeds are not intermediate and both parental traits reappear in F2's Result: Conclusions: Conclusions: 1. There is no blending of factors 2. Each plant has two discrete particles for each character, one from each parent Particulate Theory Of Inheritance 4 II. Mendelian Genetics B. What Mendel did, 4. Law of Segregation
Mendel's inference from the results of the monohybrid crosses: Mendel' Each gamete (pollen/sperm or egg) contains only one "factor", but the factor" zygote contains two--because it is produced from the fusion of two two-- gametes We now call these "factors" genes factors" This lead to Mendel's First Law of Heredity: The Law of Segregation Mendel' When any individual produces gametes, the two copies of a gene separate/segregate, so that each gamete receives only one copy Thus from every parent of the P generation, every individual of the F1 generation receives one gene copy chosen randomly II. Mendelian Genetics B. What Mendel did, 4. Law of Segregation
Terms you should make yourself very comfortable with: II. Mendelian Genetics B. What Mendel did, 4. Law of Segregation
Segregation HENRY The location of a gene on a chromosome is called a locus (plural = loci) loci) Variants of a gene are called alleles (e.g. purple vs. white flowers) When an individual has two of the same alleles, they are homozygous (homozygotes); when different alleles, heterozygous (heterozygotes) homozygotes) heterozygotes) The combination of each pair of alleles for a given gene is an organism's organism' genotype, it's outward characteristics are the phenotype genotype, it' The totality of all genes in an organism is the genome
Allele for purple flowers What is this process? Where does it occur? Is the parent homozygous or heterozygous? A rare New Zealand reptile has become a father, possibly for the first time, at the age of 111. The keepers of Henry, a tuatara, had thought he was past his prime especially after showing no interest in females during 40 years in captivity. But he mated with 80-year-old Mildred last July and 11 of the eggs she produced have now hatched. They are the only surviving members of a family of species which walked the Earth with the dinosaurs more than 200 million years ago. genotype
Locus for flower color gene Homologous pair of chromosomes phenotype SS Ss ss Allele for white flowers 5 II. Mendelian Genetics B. What Mendel did, 4. Law of Segregation
Back to Mendel's original experiment, Mendel' now with a Punnett Square
1. Cross a plant homozygous for spherical seeds (SS) with a plant homozygous for (SS) wrinkled seeds (ss). (ss). 2. Parental gametes combine to produce offspring with Ss genotype and sphericalsphericalseeded phenotype. 3. Heterozygous F1 plant produces haploid gametes & self-pollinates. self4. Male & female gametes produced by F1 are arrayed outside the Punnett Square. 5. Different combinations of alleles from each parent produce 2 different seed phenotypes in the F2 generation. 6. Seed PHENOTYPES appear in 3:1 ratio II. Mendelian Genetics B. What Mendel did, 4. Law of Segregation
The R allele is dominant and codes for red flowers, r is recessive and codes for orange. If an Rr individual is crossed with an RR individual, what will be the resulting ratio of phenotypes? phenotypes? A. All red flowers B. Half red, half orange flowers C. Half homozygotes and half heterozygotes D. All orange flowers E. Both A & C II. Mendelian Genetics B. What Mendel did, 4. Law of Segregation
The R allele is dominant and codes for red flowers, r is recessive and codes for orange. If an Rr individual is crossed with an RR individual, what will be the resulting ratio of phenotypes? phenotypes? R R RR
red R RR
red Phenotype = All red Genotype = Half homozygotes and half heterozygotes r rR
red II. Mendelian Genetics B. What Mendel did, 4. Law of Segregation
The R allele is dominant and codes for red flowers, r is recessive and codes for orange. If an Rr individual is crossed with an rr individual, what will be the resulting ratio of phenotypes? phenotypes? A. All red flowers B. Half red, half orange flowers C. Half homozygotes and half heterozygotes D. All orange flowers E. Both B & C II. Mendelian Genetics B. What Mendel did, 4. Law of Segregation
The R allele is dominant and codes for red flowers, r is recessive and codes for orange. If an Rr individual is crossed with an rr individual, what will be the resulting ratio of phenotypes? phenotypes? Mendel's 2nd Law of Heredity: Independent Assortment Mendel'
Mendel's next experiment: Mendel' Crossing peas that differed in two characters--seed shape and seed characters-- color. This is a DIHYBRID cross True-breeding parents: True- r R Rr
red r Rr
red SSYY: ssyy: ssyy: spherical yellow seeds wrinkled green seeds Phenotype = Half Red, Half Orange Genotype = Half homozygotes and half heterozygotes r rr
orange 6 Mendel's 2nd Law of Heredity: Independent Assortment Mendel' F1 generation is SsYy: all spherical yellow. SsYy: Crossing (self-pollinating) the F1 generation is a dihybrid cross (self(two traits vs. one). Mendel asked whether, in the gametes produced by SsYy, SsYy, the traits would be linked, or segregate independently. independently. The results of the dihybrid cross: Independent assortment of the 2 loci Mendel's 2nd Law: The Law of Independent Assortment Mendel' 1. Alleles of different genes assort independently during gamete formation if they are on different chromosomes. SY sy That is, chromosomes assort independently. Predictions: Predictions:
sy sy LINKED: If the S and Y loci are linked, then gametes would be SY or sy LINKED: F2 would have 3 times more spherical yellow than wrinkled green. UNLINKED: If the S and Y loci are independent, gametes could be SY, sy, UNLINKED: sy, Sy, or sY. Sy, sY. F2 would have nine different genotypes; phenotypes would be in 9:3:3:1 9:3:3:1 ratio. Results in recombinant (non-parental) phenotypes. (non- SsYy ssyy SsYy 2. Genes on the same chromosome are physically linked and generally do no assort independently. As you'll see, when genes on the same are widely separated they may recombine so frequently that it appears as if they assort independently. With Independent Assortment Without Independent Assortment From Punnett Squares to Probabilities If an event is certain to happen, probability = 1 If an event cannot possibly happen, probability = 0 All other events have a probability between 0 and 1 So, for SS homozygotes, the probability that a gamete will carry S = 1 And for ss homozygotes, the probability that a gamete will carry the s allele = 1 For an Ss heterozygote, the probability that a gamete will carry an... S allele = 1/2 (0.5) s allele = 1/2 (0.5) Some Simple Rules of Probability Theory
The Addition Rule: The probability of an event that can occur in two different ways is the sum of the individual probabilities. Tossing two coins: there are 2 ways to get a heads/tail combination: toss heads 1st, then tails OR toss tails 1st, then heads ( x ) + ( x ) = + = In F2, there are two ways gametes can combine to get a heterozygote [Ss () or sS ()]; thus + = The Multiplication Rule: Probability of two independent events happening together: multiply the probabilities of the individual events. Tossing two coins: probability that both will come up heads: x= In the F2's, there is 1 way to get an SS offspring ( x = ) & 1 way to get an ss offspring ( x = ).
That is, you have two "draw" 2 S-alleles in a row, or 2 s-alleles in a row. Result: 1(SS):2(Ss):1(ss) ratio of genotypes; Result: 3:1 ratio of phenotypes But, what about heterozygotes? heterozygotes? 7 Using probabilities instead of Punnett Squares
Gamete from parent 1 Gamete from parent 2 The results of the dihybrid cross: Independent assortment of the 2 loci Probabilities & Dihybrid Crosses Let's work an example: Probability that F2 seed will be spherical is ____ = probability of heterozygote + probability of homozygote or ___ + ____ = ____ Probability that F2 seed will be yellow: ____ + ____ = ______ Probability that a seed will be both spherical and yellow: ___ x ___= _____ (joint probability( Joint probability that a seed will be spherical and green? Joint probability that a seed will be wrinkled and yellow? Joint probability that a seed will be wrinkled and green? The results of the dihybrid cross: Independent assortment of the 2 loci Making sure you understand Now imagine that you have three loci, each has two alleles that sort independently: Yellow / green (Y/y); Smooth / wrinkled (Ss); purple flowers / white flowers (Pp). Take a true breeding yellow, wrinkled, white plant and cross it to a true breeding green smooth purple one to make the F1. Now cross the F1's to get an F2 What fraction will be green____, smooth___, purple flowered____? How many will be green, smooth and purple flowered____? 8 ...
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