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Modern Genetic Analysis Chapter 6. Gene Interaction
Summary

Although it is possible by genetic analysis to identify a single gene whose alleles dictate two alternative phenotypes for one character, this gene does not control that character by itself; the gene must interact with many other genes in the genome. There are two types of interaction, between alleles of a single gene and between alleles of different genes. The complementation test decides if two related mutant phenotypes determined by recessive alleles are due to mutations of one gene or of two different genes. The mutant genotypes are brought together in an F1 individual, and if the phenotype is mutant, then no complementation has occurred and the two alleles must be of the same gene. If complementation is observed, the alleles must be of different genes.

Alleles of a single gene interact in several ways. First, their specific cellular actions result in variations of dominance: incomplete dominance results from interme-diate amounts of the protein in the heterozygote, and codominance results from the production of two distinct protein products. Alleles also interact through a kind of dosage effect: one mutant allele in heterozygous condition might produce a mildly deleterious effect, whereas two doses in the homozygous mutant might cause severe effects or even lethality.

The interaction of alleles of different genes can be detected through modified 9:3:3:1 dihybrid ratios. Mutations of genes in a single pathway give a 9:7 ratio if the mutant phenotypes are the same, or a 9:3:4 ratio if the mutant phenotypes are different. The 9:3:4 ratio is said to result from recessive epistasis, a situation in which the mutation in the first-acting protein in the sequence obliterates expression of a later-acting gene.

Another two-gene interaction is suppression: suppressor mutations convert a mutant phenotype caused by a mutant allele of another locus into a wild-type phenotype.

Penetrance is the proportion of individuals in which a mutation is expressed, and expressivity is the severity of the phenotype attributed to one allele. Variable penetrance and expressivity are caused by interactions with other genes and with the environment.

© 1999 by W. H. Freeman and Company. All rights reserved.