Introduction to Genetic Analysis 650

Introduction to Genetic Analysis 650 - 44200_20_p643-678...

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Two features of the total distribution are notewor- thy. First, there is only a single mode, the most frequent observation represented by the location on the height axis of the peak of the curve. Despite the existence of three separate genotypic distributions underlying it, the population distribution as a whole does not reveal the separate modes. Second, any individual plant whose height lies between the two arrows could have any one of the three genotypes, because the phenotypes of those three genotypes overlap so much. The result is that we cannot carry out a simple Mendelian analysis to deter- mine the genotype of an individual plant. For example, suppose that the three genotypes are the two homozy- gotes and the heterozygote for a pair of alleles at a locus. Let a / a be the short homozygote and A / A be the tall one, with the heterozygote being of intermediate height. Because the phenotypic distributions overlap so much, we cannot know to which genotype a given individual plant belongs. Conversely, if we cross a homozygote a / a and a heterozygote A / a , the offspring will not fall into two discrete classes, A/a and a/a, in a 1:1 ratio but will
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