TABLE 20-5 Synonymous and Nonsynonymous Polymorphisms and Species Differences for Alcohol Dehydrogenase in Three Species of Drosophila Organism Species differences Polymorphisms Nonsynonymous Synonymous 7 2 17 42 Ratio 0.29:0.71 0.05:0.95 Source: J. McDonald and M. Kreitman, “Adaptive Protein Evolution at the Adh locus in Drosophila," Nature 351, 1991, 652–654.
Genetic Variation
Genetic variation refers to the variation in the genome sequences between individual organisms of a species. Individual differences or population differences can both be referred to as genetic variations. It is primarily caused by mutation, but other factors such as genetic drift and sexual reproduction also play a major role.
Quantitative Genetics
Quantitative genetics is the part of genetics that deals with the continuous trait, where the expression of various genes influences the phenotypes. Thus genes are expressed together to produce a trait with continuous variability. This is unlike the classical traits or qualitative traits, where each trait is controlled by the expression of a single or very few genes to produce a discontinuous variation.
Examining Table 20-5, how would the interpretation of
the McDonald–Kreitman test results differ if the number
of nonsynonymous observed species differences was
1 instead of 7?
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