The F1 phenotype generated by each pair of alleles defines the dominance relationship between these alleles. One allele is not always completely dominant or completely recessive to another. With incomplete dominance, the F1 hybrid phenotype resembles neither parent. With codominance, the F1 hybrid phenotype contains observable components from both parents. Many allele pairs are codominant at the level of protein production.
One gene can contribute to multiple traits (pleiotropy); for such a gene, the dominance relation between any two alleles can vary according to the particular phenotype under consideration.
A single gene may have any number of alleles, each of which can cause the appearance of different phenotypes. New alleles arise by mutation. Alleles whose frequency is greater than 1% in a population are considered wild types; those whose frequency is below 1% are considered mutants. When two or more wild-type alleles exist for a gene, the gene is considered polymorphic; a gene with only one wild-type allele is monomorphic.
Two or more genes may interact in several ways to affect the production of a single trait. It is often possible to arrive at an understanding of these interactions by observing characteristic deviations from traditional Mendelian phenotypic ratios.
In epistasis, the action of an allele at one gene can hide traits normally caused by the expression of alleles at another gene. In complementary gene action, dominant alleles of two or more genes are required to generate a particular trait. In heterogeneity, mutant alleles at any one of two or more genes are sufficient to elicit a phenotype. The complementation test can reveal whether a particular phenotype arises from mutations in the same or separate genes.
In many cases, the route from genotype to phenotype can be modified by the environment, chance, or other genes. A phenotype shows incomplete penetrance when it is expressed in fewer than 100% of individuals with the same genotype. A phenotype shows variable expressivity when it is expressed at a quantitatively different level in different individuals with the same genotype.
A continuous trait can have any value of expression between two extremes. Most traits of this type are polygenic, that is, determined by the interactions of multiple genes.
