Una mujer lleva en uno de sus cromosomas X un gen letal recesivo (l), y en el otro alelo dominante normal (L). ¿Cuál es la proporción de sexos que cabe esperara en la dependencia de esta mujer si se casa con un hombre normal?

Question in English:

A female carries a recessive lethal gene (l) on one of her X chromosomes, and a normal dominant allele (L) on the other. What is the sex ratio to be expected in this woman's dependency if she marries a normal man?

2/3 females

1/3 males

Explanation:

Females have two X chromosomes (XX) and males have an X and a Y chromosome (XY).

The genotype of the female is XLXl. The genotype of the male is XLY, since he is normal.

The possible genotypes are:

                           XL                  Xl

XL                     XLXL             XLXl

Y                       XLY               XlY

All female offspring will be normal as they will always have one normal copy of the X chromosome from their father.

50% of the male offspring will be normal, but 50% will inherit the lethal gene from their mother.

Because the allele is lethal, that means XlY males will not be born.

That means 2/3 of the children will be females, and 1/3 will be males.