You are the staff of a medical genetic consultation. A couple has come to you who want to have a baby.

You are the staff of a medical genetic consultation. A couple has come to you who want to have a baby. The wife’s father was color blind, and the husband did not have such a disease among the relatives. What are the odds of having a color blind son?

It is well known that such a pathology of vision as color blindness is inherited through the transfer of a gene linked to the X chromosome.

Let’s designate the chromosome containing the color blindness gene as Xd. The chromosome containing the gene for normal color discrimination will be designated XD.

The wife’s father suffered from color blindness, therefore, he had the Xd Y genotype and produced two types of sperm – Xd and Y. He passed on to his daughter a chromosome containing a pathological gene and responsible for the development of the female sex – Xd.

Since a woman is healthy, she is a heterozygote who inherited a healthy gene from her mother. Thus, the wife’s genotype is XD Xd. It produces two types of eggs – XD and Xd.

The husband is healthy and there were no cases of color blindness among his relatives. He has the genotype

XD Y and produces two types of sperm – XD and Y.

The following offspring options should be expected from this married couple:

heterozygous girls with normal vision (XD Xd) – 25%;

homozygous girls with normal vision (XD XD) – 25%;

boys with normal vision (XD Y) – 25%;

boys with color blindness (Xd Y) – 25%.

Answer: The chance of having a son with color blindness is 25%.