Question

Mia and Kevin’s first child (Ethan) developed problems shortly after birth and was eventually diagnosed with Duchenne muscular dystrophy (DMD). Ethan is now four years old and doing well on a carefully-planned therapy program although DMD cannot currently be cured. Both Mia and Kevin are healthy. Mia just gave birth to a little girl named Daisy. What is the probability that Daisy will be diagnosed with Duchenne muscular dystrophy? Explain. Duchenne muscular dystrophy is a serious condition caused by a recessive allele of a gene on the human X chromosome. Patients have muscles that weaken over time because they have absent or decreased dystrophin, a muscle protein.

Answer 1

A - healthy
a - DMD

PP: X(A)X(a) x X(A)Y
G: X(A), X(a); X(A), Y.
F1:
X(A)X(A) - healthy (girls, 1/4)
X(A)Y - healthy (boys, 1/4)
X(A)X(a) - healthy (girls, 1/4)
X(a)Y - DMD (boys - Ethan's genotype, 1/4).

25% of girls in F1 are X(A)X(A), healthy, 25% - X(A)X(a), also healthy.
Probability that Daisy will be diagnosed with DMD = 0%.