The evenlined ground squirrel has 4 or 8 lines on its back a

The even-lined ground squirrel has 4 or 8 lines on its back, and the lines are white or tan. A pure line cross of squirrels with 8 white lines and squirrels with 4 tan lines produces all squirrels with 8 white lines. The F₁ cross, however, produces the following results:

12 squirrels with 8 white lines

3 squirrels with 4 tan lines

1 squirrel with 4 white lines

a) What phenotype ratio would you expect in a dihybrid cross under independent assortment? (1)

b) Fully explain the genetics of this scenario, including which traits are dominant and how they produce the observed phenotypes

Solution

Answer:

Dominant epistasis (12: 3: 1):

When dominant allele ‘A’ masks the expression of ‘B’, ‘A’ is epistatic gene of ‘B’. A can express itself only in the presence of ‘B’ or b allele. Therefore it is called dominant epistasis. B expresses only when ‘aa’ is present. Therefore, in 9: 3: 3: 1 ratio both 9 and 3 are expressing the ‘A’ gene the ratio is now 12: 3: 1.

Complete dominance at both gene pairs, but one gene, when dominant, epistatic to the other.

A_B_ or A_bb ----8 white lines

aaB_ = 4 tan lines

aabb = 4 white lines

AAbb(8 white lines) X aaBB (4 tan lines)

Ab                                   aB

                  AaBb (8 white lines)-----F1

F1 X F1

AB

Ab

aB

ab

AB

AABB(8 white lines)

AABb(8 white lines)

AaBB(8 white lines)

AaBb(8 white lines)

Ab

AABb(8 white lines)

Aabb(8 white lines)

AaBb(8 white lines)

Aabb(8 white lines)

aB

AaBB(8 white lines)

AaBb(8 white lines)

aaBB(4 tan lines)

aaBb(4 tan lines)

ab

AaBb(8 white lines)

Aabb(8 white lines)

aaBb(4 tan lines)

aabb(4 white lines)

8 white lines : 4 tan lines : 4 white lines = 12 : 3 : 1