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Unformatted text preview: Lecture 3: Transmission Genetics continued Monohybrid and Dihybrid crosses Start sex chromosomes and sex linkage Chapter 2, pp 31 50; 57  65 Do problems # 20, 22, 26, 28, 32, 33, 34, 35, 45, 46, 53 Chapter 3, pp 89 97; 102 109 Parental generation First filial generation Second filial generation yellow yellow green Monohybrid cross (a cross between two true breeding lines for a trait) Green seed color disappears in the F 1 But reappears in the F 2 How does this translate to large numbers of progeny? Large numbers of progeny represent many independent fertilization events. The probabilities of dominant and recessive phenotypes translate into fractions of progeny. 3/4 of the F2 progeny should show the dominant phenotype and 1/4 the recessive for the single gene under study. This is Mendels famous 3:1 ratio . How did Mendel know that the dominant F2s consisted of two different genotypes? Self the F2 and examine the F3 The recessive phenotype should breed true But now there are two classes of dominant phenotypes. One class breeds true. The other class recapitulates the 3:1 ratio. Fraction of F2 Phenotype of F2 Phenotype of F3 Genotype of F3 1/4 1/2 1/4 dominant dominant recessive ALL dominant dominant recessive ALL recessive Y/Y Y/y y/y Mendels principle of segregation The genotype of the F1 generation from a monohybrid cross between homozygous dominant (yellow seed) and homozygous recessive (green seed) parents will all be heterozygous , and display the dominant (yellow seed) phenotype When an F1 individual forms gametes (* remember gametes are formed by meiosis ) the the paired Y/y alleles will separate ( segregate ) into different gamete cells (there will be LOTS of these) About 1/2 of the gametes will carry the dominant Y allele and 1/2 will carry the recessive y allele. This is true for both the male and female gametes During the formation of the F2 zygote , gametes combine randomly Mendel and probability Two elements of chance in Mendels model: The chance a given F1 gamete will be Y or y. The chance a given gamete from one F1 parent will combine with a given gamete from the other F1 parent. Mendels principle of segregation asserts that dominant and recessive alleles will segregate with equal probability into gametes Mendel also assumed that these gametes have an equal likelihood of combining in the next generation Two rules of probability The product ( both/and) rule: the probability that two (independent) events occur at the same time is the product of their respective probabilities. What is the probability of rolling two six sided dice and obtaining a six both times?...
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 Spring '08
 Schultze
 Genetics

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