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Epistasis

Epistasis
A gene at one locus affects the expression of another gene located elsewhere
Ex. Mouse coat color
B – dominant black allele b – recessive brown allele C – dominant pigment deposition allele c – recessive blocked pigment deposition allele
  • Dominant B and dominant C yields black mouse
  • Recessive b and dominant C yields brown mouse
  • Dominant B and recessive c yields white mouse
  • Recessive b and recessive c yields white mouse
Punnett Square:
  • Parents: 2 black mice with the same genotype, big B, little b, big C, little C.
    • Big B is the dominant black allele.
    • Little b is the recessive brown allele.
    • Big C is the dominant allele that allows pigment to be deposited in the coat hair.
    • Little c is the recessive allele that blocks pigment deposition in the coat hair.
  • These alleles are independently assorted, so we draw the four by four Punnett square that will show the possible offspring genotypes.
  • Draw colored squiggles in each square to represent the offspring.
  • Pay attention that the mice that have two little c alleles are unable to deposit pigment in their coat hair, no matter the color of the pigment, so they are white!
  • For those that have at least one big B allele and at least one big C allele, color
them black.
  • For those that have two little b alleles and at least one big C allele, color them brown.
  • For those that have two little c alleles, keep them white (even if they have a big B allele) – because they cannot deposit pigment into their coat hair.
This is the result of epistasis: the allele at the “c” locus affects the expression of the gene at the “b” locus.