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
- 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.