In humans, the sickle-cell trait is caused by a single defective allele, but sickle-cell disease only occurs in individuals that are homozygous for the sickle-cell allele. a man and woman 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?

1/16

Explanation:

Sickle-cell disease is caused by a homozygous recessive genotype dd.

A man and a woman both heterozygous Dd will have the following offspring:

The probability of having a dd child with Sickle-cell disease is 1/4. Each time they have children the gamete combination is random, so the genotype of the second child is independent from the genotype of the first child.

According to rules of probability, if two events are independent from each other, the probability of both happening is calculated by multiplying the individual probabilities of each.

Therefore the probability of having the first two children with the disease is 1/4 x 1/4 = 1/16.

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