21. A sex-linked recessive gene c produces red-green color blindness. A normal woman whose father was color blind marries a color blind man. (a) What genotypes are possible for the mother of the color blind man? (b) What are the chances that the first child from this marriage will be a color blind boy? (c) Of all the girls produced by these parents, what percentage is expected to be color blind? (d) Of all the children (sex unspecified) from these parents, what proportion is expected to have normal vision?

Unless otherwise stated, sex-linked is commonly interpreted as meaning X-linked. These are alleles found only on the X chromosome and not on the Y-chromosome. This means that women can be homozygous or heterozygous allowing women to become carriers for recessive alleles. Men are "hemizygous", and therefore cannot be carriers. Alleles on the X-chromosome which do not have a counterpart on the Y-chromosome are ALWAYS expressed. Another point that needs to be made somewhere, is the use of the word "normal". Normal often slips into the vocabulary with the understanding that it means "common", and if refers to the phenotype. Therefore "Normal Woman" means a woman who has the common phenotype, and not a woman who is socially acceptable!

Let X^c refer to the x-linked allele that causes red-green colorblindness. Note, c is recessive.
Let X⁺ refer to the wild type version of the allele, i.e. "normal". Note,  + is dominant.

The woman had a color-blind father. He was therefore: X^cY. Herself, she has normal vision, meaning she is X⁺X^?. She certainly didn't inherit the X⁺ from her dad! Yet her dad must have given her something!! We conclude she got the X^c from her father and that she is X⁺X^c. Our problem is half solved....

Her husband is colorblind. Since he is hemizygous, his genotype must be: X^cY
Part (a) of the question asks what this man's mother must have been like? Since men always get their X chromosome from their mothers, she must have had an X^c chromosome. Consequently she was X^cX^? which means she was definitely a carrier, but could also have been affected.

Okay, let's look at the children this couple can produce...
X^cY x X⁺X^c

(b) The chances that the first child from this marriage is a color-blind boy is ... 25%

(c) Of all the girls (this means we assume the child's sex is no longer an issue of probability), we have a 50% chance that they be color-blind.

(d) Of all the children (sex unspecified), there is a 50% chance of having normal vision.