Overview

To learn anything meaningful about a gene – its identity, its effects on an organism's traits, its interaction with other genes – one must study its behavior and inheritance in vivo. It is alarming how common are the patently false beliefs that a gene is a piece of DNA divorced from the organism and that a gene's existence, characteristics, functioning, and genetic interactions can be known from molecular analysis alone.

The inheritance of genes in complex organisms has two hallmarks: (1) it is inextricably tied to sex, in which chromosomes segregate, assort, and recombine, and (2) it works according to quantitative rules. This chapter introduces the rules of inheritance and ties them to the behavior of chromosomes in sexual reproduction.

Genetic Terminology

A diploid organism's genetic makeup is its genotype; depending on the context, this can refer to a single gene or to many genes (Figure 31.1). An organism's traits are its phenotype(s). Genotype and environment both contribute to the phenotype. Variant forms of a gene (variant DNA sequences) are alleles. Functional alleles that occur in nature and make for a normal phenotype are wild type (this term is not applied to humans); abnormal alleles, especially those induced experimentally, are mutant. A gene's name usually relates to its phenotypic effects, and that name is abbreviated with one or a few letters; alleles are ideally designated by meaningful superscripts.