Question

in humans, the sickle-cell trait is caused by a single mutant allele, but sickle-cell disease only occurs in individuals that are homozygous for the sickle-cell allele. a male and female each carry the trait, but do not have sickle-cell disease. what is the probability that their first two children will both have sickle-cell disease?
multiple choice
3/8
1/16
1/8
1/4
1/2

Explanation:

Step1: Identify parent genotypes

Both parents are carriers (heterozygous: Ss, where S=normal, s=sickle-cell allele).

Step2: Probability of one child with disease

A child needs ss (homozygous recessive). From Ss×Ss cross, probability of ss is $\frac{1}{4}$.

Step3: Probability of two children with disease

Multiply probabilities for independent events: $\frac{1}{4} \times \frac{1}{4} = \frac{1}{16}$.

Answer:

1/16