A woman is a carrier for an X-linked disease. The disease frequency in the population is 1 in 2,000. What fraction of women in this population are carriers for this particular disease?
**Core Concept**
The question involves understanding the genetic basis of X-linked diseases and how they manifest in different sexes. X-linked diseases are caused by mutations in genes on the X chromosome, and since women have two X chromosomes, they are less likely to express the disease but can be carriers. The frequency of the disease in the population is given as 1 in 2,000.
**Why the Correct Answer is Right**
For an X-linked disease, the frequency of carriers in women can be estimated using the Hardy-Weinberg principle. Let's denote the frequency of the disease allele as `q`. Since the disease frequency is 1 in 2,000, the frequency of the disease allele is `q = 1/2000`. The frequency of carriers in women can be calculated using the formula: `2 * q * (1 - q)`, since they can inherit either the disease allele or the normal allele from their mother. Plugging in the value of `q`, we get: `2 * (1/2000) * (1999/2000) = 1998/4000000`. This simplifies to `0.0004995` or approximately `0.05%`.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not related to the calculation of carrier frequency in women and is therefore incorrect.
**Option B:** This option does not take into account the Hardy-Weinberg principle and the fact that women can be carriers even if they do not express the disease.
**Option C:** This option is incorrect because it does not accurately represent the calculation of carrier frequency in women.
**Option D:** This option is also incorrect because it does not accurately represent the calculation of carrier frequency in women.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that X-linked diseases have a higher frequency in males because they have only one X chromosome, and any mutation in the disease gene will lead to expression of the disease. In contrast, females are less likely to express the disease but can be carriers.
**Correct Answer:** B. 1/200.