Question

Please have a step-by-step breakdown and explanation to walk me through it. This is proving to be hard to understand.

Two pigs whose tails are exactly 25 cm in length are bred over 10 years and they produce 120 piglets with the following tail lengths:

7 piglets at 15 cm

31 at 20 cm

44 at 25 cm

29 at 20 cm

9 at 35 cm.

a. How many pairs of genes are regulating the tail length character?

b. What offspring phenotype would you expect from a mating between a 15-cm and a 30-cm pig?

Answer 1

Longest tail = 35 cm
Shortest tail = 15 cm

Assume that each dominant allele contributes a small increase in length.
All recessive alleles = 15 cm = Background length
All dominant alleles = 35 cm = Background length
Effect of all dominant alleles = 20 cm

Range difference = 20 cm
Number of phenotypic groups = 5
Phenotype caused by all recessive alleles = 1
So, remaining 4 phenotypes are caused by dominant alleles.
i.e. Number of dominant alleles = 4
i.e. two genes each with two additively acting alleles

aabb = 15 cm
Aabb = 20 cm
AAbb = 25 cm
AABb = 30 cm
AABB = 35 cm

b. 15 cm = aabb
30 cm = AABb

Parental cross: aabb X AABb
Gametes: (ab) X (AB) (Ab)
Progeny:
AaBb = 25 cm
Aabb = 20 cm