6.2.8 - Epistasis

• The interaction of different gene loci, where one gene masks or suppresses the expression of another gene in a metabolic pathways ('Epi - Static' to stand over).
• 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 genes in question may work together antagonistically or in a complementary fashion.
• As the gene loci are not linked, they assort independently during gamete formation:

  • Epistasis reduces the number of phenotypes produce in the F2 generation of dihybrid crosses, and therefore reduces genetic variation.
• Epistasis also involves two unlinked genes, like dihybrid inheritance, but in epistasis, the two genes affect only one phenotypic trait.
• The epistatic gene masks the expression of the hypostatic gene:

  • Either one dominant epistatic allele masks both hypostatic alleles
  • Or, homozygous recessive epistatic alleles mask both hypostatic alleles.
• Approach questions involving epistasis (interacting genes) in the same way as a dihybrid cross, but interpret the genotypes according to the epistatic gene and how it affects the hypostatic gene.
• Antagonistic - genes work against each other to mask expression, either:

  • Recessive antagonistic epistasis
  • Dominant antagonistic epistasis.
• Complementary - genes work with each other to express a phenotype.
• Epistatic gene loci are not linked, they are not on different chromosomes and assort independently during meiosis.
• Epistasis reduces the number of phenotypes produced in the f2 generation of a dihybrid cross, reducing genetic variation.
• The homozygous presence of a recessive allele at the 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:

  • The alleles at the second locus are hypostatic to those at the first locus.
• Recessive Antagonistic Epistasis Example:

  Two gene loci on different chromosomes control flower colour:

  • Epistatic gene controls colour/no colour, A/a.
  • Hypostatic gene controls purple/pink, B/b.
  • This means aa masks colour, so flowers would be white.
• If purebred AAbb (pink) are bred with purebred aaBB (white), all the resulting offspring will have genotype AaBb, purple.

  Interbreeding the F1 generation will produce purple, pink and white flowers in the ratio of 9:3:4, a modified dihybrid ratio.
• Homozygous aa is epistatic to both alleles of B/b, so neither B/b allele is expressed if no dominant A allele is present.
• Dominant Epistasis:

  A dominant allele at a first gene locus masks the expression of alleles at a second locus

  For example: The hypostatic gene C/c codes for coloured feathers in chickens, and the I allele of the epistatic gene I/i prevents the formation of colour, even with a C allele present.
• Individuals carrying at least one dominant allele, I, have white feathers even if there is a C allele present:

  • Birds that are homozygous for the recessive allele, c, also have white feathers as this mutated allele does not cause pigment to be made.
• Purebreeding White Leghorn chickens (IICC) crossed with Purebreeding White Wyandotte chickens (iicc)

  All the offspring are IiCc, and the offspring of the F2 generation will be 13 White: 3 Coloured.
• The epistatic genotype produces pigment, but the hypostatic homozygous recessive genotype is white.
• Genes working together to encode 2 enzymes that work in succession, catalysing sequential steps of a metabolic pathway.
• At least one dominant allele of both genes is needed to get one phenotype - all other combinations give another phenotype
• Homozygous recessive allele at either locus masks the expression of the dominant allele at the other locus.
• Genes working together in Complementary Fashion - Flower Colour:

  • A/a determines whether the flower will have colour - it catalyses a colourless substrate into a colourless precursor molecule.
  • B/b catalyses the precursor into purple pigment.
  • Crossing parents AAbb x aaBB creates and F1 generation that are all AaBb.
  • The F1 generation creates the F2 generation that have a phenotypic ration of 9 purple : 7 white.

  It is also possible to produce a ratio of 9:3:4 or 9:3:3:1.