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.