Question

A spontaneous mutation results in a couple having only female progeny. When the daughter marries and the children, none of them are males. However, in the third generation there are few male offspring. What is the most likely explanation of this observation?

• A. The mutation reverses spontaneously in the third generation
• B. The mutation occurs on the X chromosome and is both recessive and lethal
• C. The mutation occurs on the X chromosome and is both recessive and dominant
• D. The mutation occurs on an autosome and is dominant

Answer 1

The correct option is B The mutation occurs on the X chromosome and is both recessive and lethal

The mutation has taken place on X chromosome. Hence all the progenies in the $F_2$ generation are females. Since none of them are males the mutation is recessive and lethal. Females are the carriers of the mutated gene hence they survive. But the males being hemizygous die. In the third generation when the carrier females marry a man with no mutated genes some progenies will be males. Thus the correct answer is option B.