6.2.8

• epistasis - interaction of non-linked gene loci where one masks the expression of the other
• Genes can interact

  in some cases different genes, at different loci on different chromosomes, interact to affect one phenotypic characteristic - when one gene masks or suppresses the expression of another gene, this is termed epistasis

  • the gene in question may work together antagonistically (against each other) or in a complementary fashion

  because the gene loci are not link, they assort independently during gamete formation

  • epistasis reduces the number of phenotypes produced in the F2 generation of dihybrid crosses + therefore it reduces genetic variation
• Genes work antagonistically

  Recessive epistasis

  the homozygous presence of a recessive allele at first locus prevents the expression of another allele at a second locus

  • the alleles at the first locus are epistatic to those at the second locus, which are hypostatic to those at the first locus

  an example of recessive epistasis is the inheritance of flower colour in Salvia. Two gene loci, A/a + B/b, on two different chromosomes, are involved

  figure 2

  
• Genes work antagonistically 2

  There are two types of epistatic interactions. The key difference is whether the epistatic allele needs to be recessive or dominant to exert its masking effect.
• Recessive epistasis:

  • Occurs when the epistatic gene must be homozygous recessive to block the expression of the hypostatic gene.
  • An example is homozygous recessive alleles for gene B in the coat colour of mice blocking the expression of gene A.
• Dominant epistasis:

  • Occurs when the epistatic gene is dominant and actively modifies or blocks the expression of the hypostatic gene.
  • Only one copy of the epistatic allele is needed to prevent the expression of the hypostatic gene.
• Genes working in a complementary fashion

  epistasis is more often explained in terms of the genes working to code for two enzymes that work in succession, catalysing sequential steps of a metabolic pathway
• Epistasis in biochemical pathways

  Epistasis also occurs in the interactions between genes coding for enzymes in biochemical pathways.

  For instance, the pathway might be for the production of a pigment that produces red flower colour in petals.

  If any one of the enzyme genes is not expressed, that step in the pathway is disrupted. This prevents the production of the intermediate substrate needed for the next enzyme in the sequence. As a result, the phenotype lacks the pigment, even if other genes are expressed. An early blocked step masks the activity of downstream genes in the pathway.