In a family where the parents could hear well and had one smooth hair and the other curly, a deaf child with smooth hair was born. Their second child heard well and had curly hair. What is the likelihood of further appearance of deaf children with curly hair in a family if it is known that the curly hair gene dominates the gene for smooth hair, deafness is a recessive trait, and the genes that determine the development of these traits are located on different chromosomes?
Let’s designate the gene that causes the development of curly hair as B, then the gene for straight hair will be b.
Let’s denote the gene that causes deafness as c, then the gene for normal hearing will be C.
A deaf child with smooth hair has the bbcc genotype. He inherits all recessive genes (2 b and 2 c) from his parents, therefore, normally hearing parents are heterozygous for the hearing gene – Cc, and the parent with curly hair is heterozygous for the hair shape gene – Bb.
Let’s write down the genotypes of the parents.
The parent with normal hearing and curly hair is BbCc, it produces sex cells bc, BC, bC and Bc.
The parent with normal hearing and straight hair is bbCc, it produces two types of germ cells – bC and bc.
All possible offspring of this married couple are represented by the following options:
children with normal hearing and straight hair (2bbСс, bbСС);
children with normal hearing and curly hair (BbCC, 2BbCc);
children with deafness and straight hair (bbcc);
children with deafness and curly hair (Bbcc).
Probability of deaf children with curly hair: 1 genotype in 8, or 12.5%.