A woman with a medium-sized nose and brown eyes marries a blue-eyed man with a large nose.

A woman with a medium-sized nose and brown eyes marries a blue-eyed man with a large nose. the first son was born blue-eyed and with a large nose. what is the probability of having brown-eyed children with a small nose? genotypes of parents and children.

In the problem statement, three nose sizes are mentioned – small, medium and large. It should be concluded that the size of the nose is inherited according to the principle of incomplete dominance: heterozygotes for the gene for the size of the nose have medium-sized noses.

Let’s designate the gene that causes a small nose as c, then the gene that causes a large nose in a homozygous state will be designated as C, and people with a medium-sized nose will be heterozygous – Cc.

As you know, the gene for brown eyes dominates over the gene for blue eyes. Let’s designate the gene for blue eyes as p, then the gene for brown eyes will be designated as P.

Blue-eyed man with a big nose – Сcpp. It produces spermatozoa Cp and cp.

The first son with a large nose and blue eyes is CCpp. Since the second recessive gene for blue eyes (p) he inherits from the brown-eyed mother, it should be concluded that the woman is heterozygous for eye color. Thus, a woman possesses the CcPp genotype and produces four types of germ cells – CP, cp, Cp, and cP.

All possible variants of the offspring of this married couple:

children with a large nose and brown eyes (CCPp) – 12.5%;

children with a middle nose and blue eyes (2Ссpp) – 25%;

children with a large nose and blue eyes (CCpp) – 12.5%;

children with a medium nose and brown eyes (2CcPp) – 25%;

children with small noses and blue eyes (ccpp) – 12.5%;

children with small noses and brown eyes (ccPp) – 12.5%.

Answer: The probability of having a baby with a small nose and brown eyes is 12.5%.