Question

Familial vitamin-D-resistant rickets is an X-linked dominant condition in humans. If a man is afflicted with this condition and his wife is normal, it is expected that among their children, all the daughters would be affected and all the sons would be normal. In families where the husband is affected and the wife is normal, this is almost always the outcome among their children when such families have been studied. Very rarely an unexpected result occurs in such families where a boy is born with the disorder. If the chromosomes of such unusual boys are examined, what might be expected to be found?

a) Two X chromosomes

b) One X and one Y chromosome

c) Two Y chromosomes

d) An extra copy of the X chromosome

Answer 1

If the chromosomes of a boy born with familial vitamin-D-resistant rickets are examined, it might be expected to find an extra copy of the X chromosome, suggesting a condition like Klinefelter syndrome, where the male has an extra X chromosome (XXY).

Step-by-step explanation:

Familial vitamin-D-resistant rickets is an X-linked dominant condition. In cases where the father carries the disorder and the mother is normal, all daughters will inherit the condition, and no sons will inherit it, because sons receive the Y chromosome from their father.

However, if a boy is born with the disorder unexpectedly, the chromosomes of such boys might reveal an abnormality. The most likely explanation for a boy with an X-linked dominant disorder is that he has an abnormality in his X chromosome, such as Klinefelter syndrome, where a male has an extra X chromosome (XXY).

Therefore, if the chromosomes of such unusual boys are examined, it might be expected to find an extra copy of the X chromosome (option d).