Final answer:
The frequency of an X-linked recessive disease is equal to its frequency in female carriers because the condition manifests in males with one copy of the gene, and in females only if they have two copies.
Step-by-step explanation:
The gene frequency for an X-linked recessive disease is best described as equal to its frequency in female carriers (Option C). This is because males have only one X chromosome, so the presence of a recessive gene on the X chromosome will result in the disease. Females have two X chromosomes; they can be carriers if only one chromosome carries the gene, or they may have the disease if both chromosomes carry the gene. Therefore, the frequency of the gene in the population is the same as the frequency of female carriers.
When discussing the likelihood of inheriting such a disease, male offspring are more likely to inherit the disease because they need only one copy of the recessive gene, which they would inherit from their mother if she is a carrier or affected. On the other hand, a daughter would only be affected if she inherited two copies of the gene, one from each parent. Thus, the condition appears more frequently in males. The chance of a son being affected is 50% if the mother is a carrier. Females, however, can either be carriers or affected depending on the genetic makeup of the parents.