The family gave birth to a blue-eyed dark-haired child who looked like his father for these reasons.

The family gave birth to a blue-eyed dark-haired child who looked like his father for these reasons. The mother is brown-eyed dark-haired, the maternal grandmother is blue-eyed dark-haired, the grandfather is brown-eyed blonde, the grandparents on the father’s side are brown-eyed dark-haired. Determine the genotype of the parents and the likelihood of the birth of a blue-eyed blonde child in this family. Brown dominates over blue, and dark-haired over blonde.

Let’s denote the gene for blue eyes as k, then the gene for brown eyes will be K. Let’s denote the gene for light hair as l, then the gene for dark hair will be L.

The mother is brown-eyed and dark-haired, but from her blue-eyed mother (kk) she could only inherit the gene for blue eyes (k), and from the fair-haired father (ll) – only the gene for blond hair (l). Therefore, the mother is diheterozygous – KkLl. It produces four types of eggs – kl, KL, kL, Kl.

The father is blue-eyed and dark-haired, while his parents also had dark hair. Most likely, he is homozygous for the hair color gene. Let’s write the father’s genotype as kkLL, and the spermatozoa of the same type produced by him – as kL.

The offspring of this married couple will be represented by the following options:

children with blue eyes and dark hair (kkLl, kkLL) – 50%;

children with brown eyes and dark hair (KkLL, KkLl) – 50%.

The chance of having a blue-eyed, blonde-haired baby is 0%.

However, the condition of the problem does not exclude the father’s heterozygosity for the hair color gene.

Let the father have the kkLl genotype, then he will produce two types of spermatozoa – kL, kl. Offspring will include options:

children with blue eyes and dark hair (2kkLl, kkLL);

children with brown eyes and dark hair (KkLL, 2KkLl);

children with blue eyes and blond hair (kkll);

children with brown eyes and light hair (Kkll).

The likelihood of having a blue-eyed, fair-haired child in this case will be 1 out of 8 genotypes, or 12.5%.