Overview

There is not a 1:1 relationship between a gene and a trait. Genes are pleiotropic: they affect many traits. Conversely, traits are multigenic – influenced by many genes. The environment, too, plays a role in determining phenotype. Further complexity arises from the quantitative nature of many traits – for example, size, fertility, or the probability of developing a tumor. Genetic effects considered in this chapter include quantitative dominance, epistasis (interactions between genes), penetrance and expressivity (variation in phenotype exhibited by one genotype), and genotype-environment interactions.

Multigenic Determination of Phenotype

Only the nucleic acid sequence of the primary transcript is surely and invariably determined by a single gene. All other classes of phenotype require the participation of two or more genes. However, from a practical standpoint, a single gene often can be considered to code for the amino acid sequence of a polypeptide, as other determinants of amino acid sequence usually do not act in a gene product-specific way. At high levels of biological organization, all traits are multigenic. Nearly all individuals of a species may be phenotypically alike for some trait, no matter how many genes influence it, because there may be no genetic variation for the trait, or else such variation may be hidden.

Example of Multigenic Influence. When peas dry they shrink. Excessive shrinkage, due to a shortage of starch, causes the seed coat to wrinkle.