49. There is a one-half probability that the individual will get the disease (Father is Aa) and then there is another half probability that the disease allele will be passed onto the child. ½ x ½ = ¼. Therefore, answer choice B is correct.

51. When the genotype does not equate to the phenotype it can be described as reduced expressivity or penetrance. This eliminates answer choices C and D. The difference between reduced penetrance and reduced expressivity is that penetrance is a description of whether the disease is present or not, while expressivity is a description of the degree of the trait. Therefore, the passage deals with unaffected individuals who carry the Huntington disease allele and penetrance is the best answer. One can think of penetrance of how much the trait penetrates the population to produce affected individuals.