Question

A couple has tree children, all of whom have blue eyes and blond hair. Both parents are homozygous for blue eyes(bb), and have brown hair (Rr). Which hair color allele is dominat?----------How is it possible for their three children to all have blonde hair? What is the probability that their next child will be blue eyed and have brown hair? Show how you arrived at your answers.

Answer 1

Allele (R) for brown hair is dominant and allele (r) for blonde hair is recessive.

It is possible that all three children have blonde hair because both the parents are heterozygous (Rr) and there is possibility of transfer of one recessive allele (r) from each parent to the offsprings. This way all the three children will have homozygous recessive alleles (rr) showing blonde hair color as phenotype.

All the children will have blue eyes only because both the parents are homozygous recessive (bb) for eye color and there is no possibility of dominant eye color allele to be found in children. The probability of the next child to be blue eyed and brown hair can be calculated using the punnett square:

R r

R   RR Rr

r Rr rr

RR, Rr will have brown hair color and rr will have blonde hair color.

Probability of child with brown hair = 3/4 = 0.75.