Question

Identical twins are produced from the same sperm and egg (which splits after the first mitotic division) , whereas fraternal twins are produced from separate sperm and separate egg cells. If two parents one with brown eyes (a dominant trait) and the other with blue eyes (a recessive trait) produce one twin boy with blue eyes, what is the probability that a fraternal twin will transmit the blue eye allele to his or her offspring. Please show work.

Answer 1

Ans.) Probability that a fraternal twin will transmit the blue eye allele to his or her offspring = 25%

As the brown eyes are dominant trait. Let "B" indicate allele controlling brown eye color, and "b" controlling blue eye color.

As offsprings produced have blue eyes, it means genotype of mother with brown eyes is heterozygous i.e. "Bb", while genotype of father with blue eyes is "bb".

So two type of gametes would be produced by mother are "B" and "b", while father produces "b" gamete.

Probability of getting offspring with blue eyes = 1/2

And as father produces two types of gametes with X chromosome and Y chromosome so probability of producing boy is 1/2

So net probability that fraternal twins would be produced with blue eyes is 1/4 or 25%.