In pigs, several alleles exist for the gene POLR1B. A pig breeder arranged several matings between a group of pigs in his operation who were all heterozygous with two different alleles for the POLR1B gene (both were ‘Pp’). Individuals who are PP are black in color while heterozygous individuals (Pp) are grey. Homozygous recessive individuals (pp) are white. Below is the result of this cross Parental generation Pp x Pp F1: Black: 17 piglets Grey: 33 piglets Which of the following are possible explanations for the pattern we see in the F1 generation. Group of answer choices

a. Embryonic lethality: homozygous recessive individuals didn’t survive
b. Standard Mendelian ratio 3:1 (dominant to recessive)
c. Codominance (neither the black nor the white allele is completely dominant).
d. Both a and c above are supported by the evidence.

a. Embryonic lethality: homozygous recessive individuals didn’t survive

Explanation:

The alleles are incomplete dominant to each other so grey trait is produced as a blend of black and white variant.

PP = black

Pp = grey

pp = white

When Pp X Pp :

     P     p

P  PP   Pp

p  Pp   pp

Offspring should be in 1:2:1 genotypic and phenotypic ratio. However, we observe only black (PP) and grey (Pp) piglets in almost 1:2 ratio (17:33). There is not a single white piglet (pp) in the progeny. This could have been a result of embryonic lethality.

Recessive p allele must have been lethal, so when it was present in two copies, the piglet did not survive in embryonic stage. Even though grey piglet also had one p allele, it also had one dominant P allele, so it survived. Hence, 1:2:1 ratio got shortened as 1:2.

b. lithosphere, asthenosphere, mesosphere   is the answer most likely