- A locus is said to have multiple alleles when more than two allelic forms have been found. Each allele may cause a different phenotype.
- The existence of multiple alleles allows heterozygotes to occur representing any pairwise combination of alleles.
If a recessive mutation is produced by every change in a gene that prevents the production of an active protein, there should be a large number of such mutations in any one gene. Many amino acid replacements may change the structure of the protein sufficiently to impede its function.
Different variants of the same gene are called multiple alleles, and their existence makes it possible to create a heterozygote between mutant alleles. The relationship between these multiple alleles takes various forms.
In the simplest case, a wild-type gene codes for a protein product that is functional. Mutant allele(s) code for proteins that are nonfunctional.
But there are often cases in which a series of mutant alleles have different phenotypes. For example, wild-type function of the white locus of D. melanogaster is required for development of the normal red color of the eye. The locus is named for the effect of extreme (null) mutations, which cause the fly to have a white eye in mutant homozygotes.
To describe wild-type and mutant alleles, wild genotype is indicated by a plus superscript after the name of the locus (w+ is the wild-type allele for [red] eye color in D. melanogaster). Sometimes + is used by itself to describe the wild-type allele, and only the mutant alleles are indicated by the name of the locus.
An entirely defective form of the gene (or absence of phenotype) may be indicated by a minus superscript. To distinguish among a variety of mutant alleles with different effects, other superscripts may be introduced, such as wi or wa.The w+ allele is dominant over any other allele in heterozygotes. There are many different mutant alleles. Figure 1.29 shows a (small) sample. Although some alleles have no eye color, many alleles produce some color. Each of these mutant alleles must therefore represent a different mutation of the gene, which does not eliminate its function entirely, but leaves a residual activity that produces a characteristic phenotype. These alleles are named for the color of the eye in a homozygote. (Most w alleles affect the quantity of pigment in the eye, and the examples in the Figure are arranged in [roughly] declining amount of color, but others, such as wsp, affect the pattern in which it is deposited.)
When multiple alleles exist, an animal may be a heterozygote that carries two different mutant alleles. The phenotype of such a heterozygote depends on the nature of the residual activity of each allele. The relationship between two mutant alleles is in principle no different from that between wild-type and mutant alleles: one allele may be dominant, there may be partial dominance, or there may be codominance.