Each pregnancy is an independent event. One does not influence the other. Assuming each parent is heterozygous for the recessive gene (otherwise they would have dies at 20 months), the chances for the disorder is 25% for every pregnancy.
If the events are independent, the probability of both/all events occurring is determined by multiplying the probabilities of the individual events.
For the next three to be healthy = .75^3 = .42.
A couple has a child with an autosomal recessive form of glycogen storage disease, a deficiency of a lysosomal enzyme. An enlarged liver and heart and weak muscles cause the baby to die from heart and lung failure at 20 months. Now the woman is pregnant again. Her doctor offers her and her husband genetic counseling, but they decline, claiming familiarity with the laws of inheritance- since they have already had an affected child, they reason, the next three will be healthy. Why is their reasoning incorrect?
1 answer