Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

Genetic interaction is the dependence of the degree or character of the manifestation of one allele on the presence of other alleles. If these are alleles of a single gene, only several basic possibilities can occur in diploid organisms that bear two alleles from each gene. If allele A completely suppresses the manifestation of allele B and an individual with two A alleles then looks the same as an individual with one A allele and one B allele, then allele A is denoted as dominant with respect to allele B, while allele B is denoted as recessive with respect to allele A. For example, this is the case of the allele for brown and blue eye colour – a homozygote with two alleles for brown eye colour does not differ from a heterozygote that bears one allele for brown colour and one allele for blue colour (simplified somewhat – I hope experts will forgive me). If the expression of alleles A and B are averaged out and an individual with a pair of alleles AB (heterozygote) has traits somewhere between the traits of individuals with two A alleles and the traits of individuals with two B alleles, this is called semi-dominance (incomplete dominance) – the allele of the pair that is manifested more strongly in the heterozygote is denoted as semi-dominant. For example, semi-dominance is exhibited by the S allele responsible for the detrimental manifestations of sickle cell anemia – a homozygote with two alleles is much worse off than a heterozygote with one normal and one S allele. The case when a heterozygote with a pair of alleles AB exhibits the relevant trait to a greater degree than either homozygotes AA or BB is termed super-dominance. Only a limited number of types of gene interaction can occur between the alleles of a single gene in diploid organisms. There are a much greater number of types of gene interactions between the alleles of various genes, called epistatic interactions, because the number of interacting alleles can attain any number. For example, a particular allele of one gene can directly affect the manifestations of alleles A and B of a different gene and can also affect whether allele A will be dominant or recessive towards allele B.