Dominant and recessive genes

Genes and genotypes are usually symbolized by alphabetic letters. If there are two alleles at a locus in a population, we can call them A and a. The two can combine into three possible genotypes: AA, Aa, and aa. Suppose the alleles influence color:

A gene = blue a gene = yellow

AA individuals would be blue; aa individuals would be yellow. If we consider only the AA and aa genotypes and phenotypes in this example, there is a one to one relation between genotype and phenotype. But what about Aa individuals? There are two possibilities for the phenotype of the Aa genotype:

• The color of Aa individuals may be intermediate between the two homozygotes - they are green.

• The Aa heterozygotes may resemble one of the homozygotes; they might be blue, for instance. The A allele is then called dominant and the a allele recessive.

An example of dominance is seen in the Hawaiian happy-face spider Theridion grallator, pictured opposite. The gene for the patterned form is dominant, and the plain yellow form is recessive. Even if just one of the genes at this locus codes for a patterned form, the spider is patterned.