A child is born to parents that both have hypophosphatemic rickets, a disease that is caused by a sex-linked dominant trait. What is the chance that the child will have the disease if the mother is heterozygous for the trait? 75%
50%
25%
100%

75%

Explanation:

Hypophosphatemic rickets is a disorder characterized by hypophosphatemia (low level of phosphate in the blood), defective intestinal absorption of calcium, and rickets (impaired mineralization of cartilaginous growth plates)  or osteomalacia (impaired mineralization of the osteoid) unresponsive to vitamin D. It can be inherited or acquired.

When it comes to inheritable hypophosphatemic rickets, the disorder is most often inherited in an X-linked dominant manner. This means that the gene is inherited through the X chromosome and that one dominant allele is enough to cause it.

As the mother is heterozygous for the trait, she has one dominant and one recessive allele (XHXh). The father has one X chromosome, which he inherited from his mother, and as he has the condition, the dominant allele will be present on it (XHY).

This means that, if the child is a girl, she will definitely have the disorder. Girls inherit one X chromosome from each of their parents. Even if she inherited the one with the recessive allele from her mother, she would get the one with the dominant allele from her father.

If the child is a boy, he has a 50% chance to inherit the condition. Boys inherit the Y chromosome from their father and the X chromosome from their mother. As the mother is heterozygous, he can either get the chromosome without the dominant allele or the one with it. This means that there is a 50% chance of inheriting the disorder.

If we say that there are equal chances of this couple having female and male children, when we combine these results, we get a 75% chance of them inheriting hypophosphatemic rickets.