As described in Chapter 5, the information encoded in the DNA sequence of genes specifies the sequence—and therefore the structure and function—of every protein molecule in a cell. The power of genetics as a tool for studying cells and organisms lies in the ability of researchers to selectively alter every copy of one specific type of protein in a cell by making a change in the gene for that protein. Genetic analyses of mutants defective in a particular process can reveal (1) genes required for the process to occur, (2) the order in which gene products act in the process, and (3) whether and how the proteins encoded by different genes interact with one another. Before we see how genetic studies of this type can provide insights into the mechanism of a complicated cellular or developmental process, let’s first review some basic genetic terms used throughout our discussion.

The different forms or variants of a gene are referred to as alleles. Geneticists commonly refer to the numerous naturally occurring genetic variants that exist in populations, particularly human populations, as alleles. The term mutation is usually reserved for instances in which an allele is known to have been newly formed, such as after treatment of an experimental organism with a mutagen, an agent that causes a heritable change in the DNA sequence.

Strictly speaking, the particular set of alleles for all the genes carried by an individual constitutes its genotype. However, this term is most often used in a more restricted sense to denote the alleles of a particular gene or genes under examination. For experimental organisms, the term wild type is often used to designate a standard genotype for use as a reference in breeding experiments. Thus the normal, nonmutant allele is usually designated as the wild type. Because of the enormous allelic variation that naturally exists in human populations, the term wild type usually denotes an allele that is present at a much higher frequency than any of the other possible alternatives.

Geneticists draw an important distinction between the genotype and the phenotype of an organism. The term phenotype refers to all the physical attributes or traits of an individual that are the consequence of a given genotype. In practice, however, the term phenotype is usually used to denote the consequences that result from the particular alleles that are under experimental study. Readily observable phenotypic characteristics are critical in the genetic analysis of mutations.