Question

Identical twins are produced from the same sperm and egg (which splits after the first mitotic...

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 (the 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?

Let the allele for the dominant brown color be B and the recessive blue colour be b. One of the fraternal twins has blue eyes (bb) which is possible if he inherits a blue allele from each of the parent. So the parental combination must be Bb and bb. Punnett square of the parental combination is:

 B b b Bb bb b Bb bb

So for the faternal twin chance of having a blue eye will be 1/2 if his genotype is Bb. His genotype is bb so he will transmit one of the blue allele to his offspring and probability will be 1.

So the total probability that a child to have a blue eyed b allele = 1/2 * 1 = 1/2 = 0.5

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