Sickle cell anemia is carried on the recessive allele. Normal is dominant. Mary is a carrier for sickle cell and Mike is normal (but not a carrier). A. What are the chances of Mary and Mike having a sickle cell child?
B. What are the chances of them having a child who is a carrier?

A. 0

B. 1/2 or 50%

Explanation:

Let us assume that the sickle cell anemia trait is represented by s and the alternate normal version by S.

Mary, a carrier would have the genotype Ss.

Mike, a perfectly normal person would have the genotype SS.

If both Mary and Mike marry:

                     Ss     x     SS

offspring: SS    SS    Ss    Ss

(A) In order to have sickle cell, the two alleles must be recessive. None of the offspring from the cross has two recessive alleles. Hence, the chances of Mary and Mike having a sickle cell child is zero.

(B) From the cross, two out of four of their potential offspring have the trait in heterozygous condition. Hence, the chances of Mike and Mary having a child who is a carrier is 1/2 or 50%.

when it is at an intermediate population density