Question

1. A couple has three children, a normal boy and a boy and girl each with hemophilia. What can you say       about the parents?

Answer 1

Hemophilia is a sex-linked recessive trait. Females have two X chromosomes and males have one X chromosome and one Y chromosome. In sex-linked recessive inheritance, one copy of the mutated gene is sufficient to cause the disease to males since they have only one X chromosome. But for females, the two copies of the mutated gene are required to cause the sex-linked recessive disorder. The females with one copy of the mutated gene are carriers. carrier females do not exhibit the trait. In sex-linked inheritance, there is no male to male transmission. The males inherit the X chromosome from the mother and the Y chromosome from the father. the females inherit one X chromosome from the mother and the other X chromosome from the father

Let H be the dominant allele on the X chromosome for normal

Let he be the recessive allele on the X chromosome for hemophilia

Among three children, one is a normal boy (X^HY), one is a hemophiliac boy (X^hY) and one is a hemophiliac girl (X^hX^h). The normal boy inherits the normal X chromosome (X^H) from the mother and the Y chromosome from the father. The hemophiliac son inherits one copy of the mutated gene (X^h) from the mother and Y chromosome from the father. So the genotype of the mother is X^HX^h. The mother is a carrier. The hemophiliac daughter inherits one copy of the mutated gene (X^h) from the mother and one copy of the mutated gene (X^h) from the father. So the genotype of the father is X^hY. The father is a hemophiliac.

Father has hemophilia (X^hY) and the mother is a carrier (X^HX^h)

Parents : X^hY (hemophiliac father) * X^HX^h (carrier mother)

Gametes of the male parent: X^h, Y

Gametes of the female parent: X^H, X^h

Offspring : X^HX^h (carrier female) , X^hX^h (hemophiliac female), X^HY (normal son), X^hY (hemophiliac male)

Therefore, the father is hemophiliac (X^hY) and the mother is a carrier (X^HX^h)