Answer:
1/8
Explanation:
Assuming the allele for sickle cell is represented by s, the alternate non-sickling allele will be S.
A ss father marries a mother that is heterozygous for the trait:
ss x Ss = Ss, Ss, ss and ss.
Thus, 50% of their children will be heterozygous normal for the trait while 50% will be homozygous recessive (affected) for the trait.
The probability of producing normal children is 1/2. Hence, the probability of their first three children having a normal phenotype is equivalent to the probability of the first being normal, the second being normal and the third being normal. Mathematically;
= 1/2 x 1/2 1/2 = 1/8
The probability that their first three children will have the normal phenotype is 1/8.
