In humans, the sickle-cell trait is caused by a single detective allele, but sickle-cell disease only occurs in individuals that are homozygous for the sickle-cell allele. Two people carry the trait but do not have sickle-cell disease. What is the probability that they will have two children with sickle-cell disease? 1/16 1/2 3/8 1/4 Solution Answer is 1/2. Parents are carriers. Ss x Ss. The children will be of genotypes SS, Ss, sS, ss..