Question

“PKU” is an inherited disease caused by a recessive allele. If a woman and a man are both carriers, where P is the normal allele and p is the PKU allele.
For the two parents described, what is the probability that they will have three normal children?
a. 1/64
b. 1/4
c. 3/64
d. 1/16

Answer 1

Answer:

Option A, 1/64

Explanation:

The genotype of a carrier female would be Pp

The genotype of a carrier male would be Pp

When these two adults are crossed , the following offspring are produced

Pp x Pp

PP, Pp, Pp, pp

Thus out of four children 3 children have normal phenotype but two are carrier.

So probability of having one normal  child (with genotype PP) is $\frac{1}{4}$

So the probability of  three normal children is

$\frac{1}{4} *\frac{1}{4} *\frac{1}{4} \\= \frac{1}{64}$

Hence, option A is correct