In Problem 29, we learned that for some diseases, such as sickle-cell anemia, an individual will get the disease only if he or she receives both recessive alleles. This is not always the case. For example, Huntington’s disease only requires one dominant gene for an individual to contract the disease. Suppose that a husband and wife, who both have a dominant Huntington’s disease allele (S) and a normal recessive allele (s), decide to have a child.
(a) Genes are always written with the dominant gene first. Therefore, there are two instances the offspring could have genotype Ss (one if the mother contributes the dominant allele and the father contributes the nondominant allele, and vice versa). List the possible genotypes of their offspring.
(b) What is the probability that the offspring will not have Huntington’s disease? In other words, what is the probability that the offspring will have genotype ss? Interpret this probability.
(c) What is the probability that the offspring will have Huntington’s disease?