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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 Mendel’s famous 3:1 ratio . How did Mendel know that the dominant F2’s 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 ¾ dominant ¼ recessive ALL recessive Y/Y Y/y y/y Mendel’s 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 Mendel’s 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. • Mendel’s 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, Sex linkage, r/r y/y

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