Husband and wife have curly (A) and dark (B) hair. They have a child with curly (A) and light (B) hair.

Husband and wife have curly (A) and dark (B) hair. They have a child with curly (A) and light (B) hair. What are the possible genotypes of parents and their children, if it is known that the genes responsible for their hair color structure are on different chromosomes?

A child born with blond hair has a genotype cc homozygous for the recessive blond hair gene, because homozygosity is a necessary condition for the phenotypic realization of this trait. Since he inherits recessive genes from both parents, therefore, both dark-haired parents are heterozygous for hair color – Bb.

According to the first sign (curly / straight hair), the parents can be both homo- and heterozygous. Let’s consider all possible options.

Let both parents be diheterozygous. They have genotypes AaBb and produce four types of germ cells – AB, AB, AB and AB. The offspring are represented by the following possible options:

children with dark curly hair (4 AaBb, AABB, 2AaBB, 2AABb) – 56.25%;

children with light-colored curly hair (2Aab, AAbb) – 18.75%;

children with light straight hair (aabb) – 6.25%;

children with dark straight hair (aaBB, 2aaBb) – 18.75%;

Let one of the parents be diheterozygous AaBb, it produces four types of germ cells – AB, AB, AB and AB. Let the second parent be homozygous for the first trait – AABb. It is capable of producing two types of gametes – AB and Ab. The offspring will include possible options:

children with dark curly hair (2AaBb, AABB, AaBB, 2AABb) – 75%;

children with blond curly hair (Aabb, AAbb) – 25%;

Let both parents be homozygous for the first trait – AABb. They produce two types of germ cells – AB and Ab. The offspring are represented by options:

children with dark curly hair (AABB) – 25%

children with dark curly hair (AABb) – 50%;

children with light curly hair (AAbb) – 25%.