Many phenotypes are characterized by a variation of multiple discrete characters. This is usually due to polygenic inheritance: two or more genes affecting a single phenotype. Such is the case of the genes in this problem. In this cross, there are eight possible gamete combinations for this corss: ABC, ABc, AbC, Abc, aBC, aBc, abC, and abc. A Punnet square using this number of gametes would produce 64 possible outcomes.
Fruit color in a particular plant is controlled by a set of three QTLs (quantitative trait loci, or "polygenes") that work in an equal and additive manner. Each QTL has two alleles (i.e. A and a); each allele represented by a capital letter produces "one dose" of yellow pigment, while alleles represented with lower case letters do not produce any pigment at all. A trihybrid plant (A/a;B/b;C/c) is selfed. Assuming no effects of the environment, what proportion of the offspring will have thesame fruit color phenotype as the trihybrid parent?
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