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Chapter03_SSM - 65781_CH03_040_063.qxd 12:54 PM Page 40...

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Chapter 3: Transmission Genetics: The Principle of Segregation Chapter Summary Mendelian genetics deals with the hereditary transmission of genes from one generation to the next. One key principle is segregation, in which the two alleles in an individual separate during the formation of gametes so that each gamete is equally likely to contain either member of the pair. In a cross such as AA × aa , in which only one gene is considered, the genotype of the offspring (constituting the F 1 generation) is heterozygous Aa . The phenotype of the F 1 progeny depends on the dominance relationships among the alleles. For many morphological traits, the wildtype allele, here denoted A , is dominant, and the phenotype of heterozygous Aa is indistinguishable from that of homozygous AA . In contrast, the alleles of molecular genetic markers are often codominant, and the phenotypes of AA , Aa , and aa are all distinct. For an RFLP marker, for example, the homozygous AA and aa genotypes have a phenotype consisting of a single band differing in electrophoretic mobility, whereas the heterozygous Aa genotype has a phenotype consisting of both bands. In the formation of gametes, an Aa genotype produces A -bearing and a -bearing gametes in equal propor- tions. Hence, in the F 1 × F 1 cross Aa × Aa , assuming random union of gametes in fertilization, the progeny (the F 2 generation) are expected to consist of genotypes AA : Aa : aa in the proportions 1 : 2 : 1. The distri- bution of phenotypes in the F 2 generation again depends on the dominance relationships. If A is dominant to a , then the F 2 ratio of dominant : recessive phenotypes is expected to be 3 : 1. With codominance all three genotypes are distinguishable, and the ratio of F 2 phenotypes is 1 : 2 : 1. For crosses involving two genes—for example, AA BB × aa bb —the F 1 genotype is Aa Bb . Segregation of each gene implies that the ratios of A : a and of B : b gametes are both 1 : 1. If the genes are unlinked, they undergo independent segregation (independent assortment), which means that the gametic types A B : A b : a B : a b are formed in the ratio 1 : 1 : 1 : 1. Hence, the F 2 generation formed from the cross F 1 × F 1 is expected to have genotypes given by the product of the expression (1/4 AA + 1/2 Aa + 1/4 aa ) × (1/4 BB + 1/2 Bb + 1/4 bb ). Using a dash to represent an allele of unspecified type, we can write the F 2 genotypes as 9 A– B– : 3 A– bb : 3 aa B– : 1 aa bb , and if both A and B are dominant, the phenotypic 40
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41 ratio in the F 2 generation is 9 : 3 : 3 : 1. This ratio can be modified in various ways by interaction between the genes (epistasis). Different types of epistasis may result in dihybrid ratios such as 9 : 7 or 12 : 3 : 1. The rules of probability provide the basis for predicting the outcomes of genetic crosses based on the principles of segregation and independent assortment. Two basic rules for combining probabilities are the addition rule and the multiplication rule. The addition rule applies to mutually exclusive events; it states
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