Multihybrid crosses can involve genes that are inherited independently but that exert their phenotypic effect on the same character. In this light, explain the conditions of "complimentary genes," "epistasis," "collaboration," modifier genes," and "multiple-gene inheritance."
Complimentary genes are mutually dependent on one another being unable to exert their phenotypic effects unless both genes are present. In epistasis, the genes coding for one phenotypic effect mask the phenotypic effect of another completely different set of genes. Unlike dominance, in which alleles interact with each other, epistasis is an interaction of non-allelic genes. Collaboration is the situation when two non-allelic genes influence the same character to produce a single-character phenotype neither gene could produce alone. Modifier genes influence the expression of characters nominally determined by one principle gene; for example, modifier genes for human eye color affect the principle recessive expression of blue eyes by causing the color to be expressed as gray and affect the principle dominant expression of brown eyes by causing the color to be expressed as black. Multiple-gene inheritance describes the situation when two or more genes affect the same character in an additive way creating a gradient of possible phenotypic expressions; the infinite gradation of skin color that exists in humans is an example.
