Epistasis

• • Epistasis is used to describe situations where a phenotype is influenced by the presence of two or more different genes
  • this means that the allele of one gene can mask or block the expression of another gene
• • In epistasis genes are on 2 different chromosomes and are not linked
• • They will assort independently during gamete formation
• • Epistasis reduces genetic variation because crossing over does not occur as the loci for the genes are different
• How is epistasis achieved?

  • Often two or more different genes will code for two or more different enzymes which control the same reaction. Therefore 2 different genes can affect the same characteristic
  • The genes you inherit can interact to produce different phenotypes
• • The gene masking or complementing another gene is Epistatic
  • The gene which is masked or complemented is Hypostatic.
• Recessive epistasis

  • presence of homozygous recessive genotype prevents the expression of the second gene
• Dominant epistasis

  • homozygous dominant or heterozygous genotype prevents the expression of the second gene
• example of epistasis

  • Genes A/a determine the distribution of melanin in the hairs

  -allele A= grey brown banding called agouti

  -allele a= uniform black colour

  • Gene C/c determines production of melanin

  -allele C= allows colour to develop

  -allele c= no melanin made

  • Gene C is epistatic to gene A and gene A is hypostatic to gene C. This is because gene C can alter the effects of gene A