Suppose that Rosie and Ross are both carriers for PKU (a rare autosomal recessive disease.) If they have eight children, what is the probability that all of eight children will have the same phenotype?

1/65,536 = 0.001523 %

Explanation:

A: normal allele

a: recessive allele causing PKU

The probability that Rosie and Ross have one child with PKU will be >>  

Aa x Aa = 1/4 AA; 1/2 Aa; 1/4 aa

In consequence, the probability that Rosie and Ross have eight children with PKU will be >>  

(1/4)⁸ = 1/65,536 = 0.001523 %