Unformatted text preview: In a Moravian monastery in the mid1800's, a humble monk tended his garden... ... and provided the world with the mechanisms underlying inheritance and evolution by natural selection. ~~~~~~~~~~~~~~~~~~~~~~~~
"It requires indeed some courage to undertake a labor of such far-reaching extent; this appears, farhowever, to be the only right way by which we can finally reach the solution of a question the importance of which cannot be overestimated in connection with the history of the evolution of organic forms." - Mendel, 1866 forms." Mendel examined several traits of peas, including pea color (yellow or green), pea shape (round or wrinkled), and pea flower color (red or white). Mendel began with "pure-breeding" lines pure- breeding" of peas (Pisum sativum). (Pisum sativum). Pure-breeding lines are those that are Puregenetically homogeneous. They "breed true", e.g., green peas that yield only true" green pea offspring. Pure-breeding lines are homozygous; Purehomozygous; both alleles at the gene locus are the same. The "locus" is the physical place on the locus" chromosome where the information (the gene) for a particular trait is. The "allele" is allele" the form of the gene locus (e.g., the pea color gene can have a yellow allele or a green allele). In a "homozygote", both homozygote" alleles are the same. Having chromosomes in pairs makes you "diploid". diploid" 1 Crosses between different pure lines yield offspring with both alleles. The "phenotype" is simply the outward phenotype" appearance of the organism, e.g., the pea is yellow or green, not both. "Heterozygous" individuals have two Heterozygous" different alleles at a given locus. The "dominant" allele is the one that is dominant" apparent in the phenotype of the heterozygote. For pea color, yellow is dominant. And green is "recessive". recessive" A Punnett square predicts the progeny for a cross between two hybrids (= heterozygotes): heterozygotes): A mating between 2 heterozygous pea plants yields offspring in characteristic "Mendelian" ratios... Mendelian" ratios... The ratio of phenotypes is: Yellow : green = 3 : 1. It disguises the ratios of homozygous to heterozygous genotypes: YY : Yy : yy = 1 : 2 : 1. Mendel confirmed that his "factors" came factors" in pairs with the recessive "testcross": testcross" Mendel's 1st Law: Mendel' "Segregation" means the two different Segregation" alleles do not blend in any way. Mendel's 2nd Law: Mendel' "Independent Assortment" means that Assortment" loci on different chromosomes behave independently of one another. 2 Two loci on different chromosomes (i.e., where rows & columns = gametes) yields... This is the simplest application of the two basic rules of probability used in genetics: 1. The "Multiplication Rule": the Rule" probability of the co-occurrence of two coindependent events is the product of their probabilities. 2. The "Addition Rule": the probability Rule" of the occurrence of either of two independent events is the sum of their probabilities. The coin-flipping example coinFlip a coin once: the probability of getting heads is 1/2; the probability of getting tails is 1/2. Flip a coin twice: the probability of getting 2 heads is 1/2 x 1/2 = 1/4. The probability of getting 2 tails is 1/2 x 1/2 = 1/4. This is the multiplication rule. The coin-flipping example coinWhat is the probability of getting 1 head & 1 tail when you flip a coin twice? You must consider all possible ways to get 1 head & 1 tail. You can get a head then a tail or you can get a tail than a head. The 1st outcome has a 1/2 x 1/2 = 1/4 probability, as does the 2nd outcome. So, you add up the probabilities of all relevant outcomes: 1/4 + 1/4 = 1/2. This is the addition rule. Anyone ever hear the claim that "Mendel forged his data"? data" For one set of experiments, the fit of Mendel's data to expectations are too good! Mendel' Everything is copacetic for the 3:1 and 1:1 experiments... but not the "2:1" experiments... 2:1" experiments... experiments... Based on the Punnett square, one expects a 2:1 ratio of hetero-:homozygotes. hetero- homozygotes. In two sets of crosses, Mendel self-fertilized selfdominant individuals and raised 10 offspring. If they yielded no recessive phenotypes, they were called homozygotes. homozygotes. What are the odds a selfed heterozygote produces 0 recessive offspring out of 10? 3 The odds of any particular offspring not being the recessive phenotype is 3/4. If there are 10 offspring, the odds are: (0.75)10 = 0.0563. Since 2/3 of the dominant phenotypes are heteroheterozygous, you predict that zygous, (2/3)(0.0563) = 0.0375 of the genotypes are misclassified. Punnett Heterozyg's Heterozyg' 2 0.6667 Homozyg's Homozyg' 1 0.3333 Fisher (0.6667 - 0.0375) (0.3333 + 0.0375) 0.629 0.371 0.665 0.680 0.335 0.320 Mendel 1 Mendel 2 4 ...
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- Spring '08