Sickle cell anemia is a recessive trait in humans. In a cross between a father who has sickle cell anemia and a mother who is heterozygous for the sickle cell allele, what is the probability that all of their first three children will be unaffected?

75% probability

Explanation:

The father have the trait of sickle cell anemia which means he is a sickle cell dominant with a trait of dominant SS ( meaning he has displays the severe symptoms of sickle cell disease that occur in a person who has two copies of that allele i.e homozygous). While the mother is heterogygous which means that she has one abnormal allele of the hemoglobin beta gene represented as As.

Based on this notion, upon crossing of the traits the probabilities are 75% Heterogygous trait and 25% homoxygous traits (i.e As, As, As, SS). Meaning 75% of the children will be unaffected by the Sickle cell anemia, while 25% will be affected by the anemia.