The frequency of one form of recessive Xlinked colorblindnes

The frequency of one form of recessive X-linked color-blindness is 5% among European males. What is the expected frequency of this form of color blindness among females? What fraction of females would be heterohygous carriers?

Solution

The frequency of

In sex-linked diseases, the defective allele is present in X-chromosome. In this case both the parents are said to be normal. Means the father is not having the defective allele. X- linked or sex-linked diseases could be developed in females only when they inherit two X-linked recessive alleles, one from the mother and another from the father. Whereas in males, only one defective allele will result in expression of the disease phenotype.

Thus, in case of X-linked diseases, males contain only either p or q. Affected males contain q, thus, the frequency of affected males = frequency of q in the population.

In the given example, the frequency of affected males, q = 5% or 0.05.

Affected females are always homozygous recessive, i.e. q2 = 0.0025 or 2.5%

The frequency of dominant allele = 1- frequency of recessive allele = 1-0.05 = 0.95

The frequency of heterozygotes (all are females because males carry only one X allele) is = 2*0.05*0.95 = 0.095 or 9.5%.