Questions 186-188 of the GRE Biology Practice Test are prefaced with this genetic pedigree:
Questions 186-188 refer to the following pedigree for the inheritance of a very rare human disease over three generations.
It looks like the allele that caused the "affected" phenotype was dominant because Generation II showed a 3/4 probability of having the phenotype, and it looks like it was autosomal because it affected males and females roughly equally. This is confirmed by the answer to Question 186 ("Which of the following is the most likely mode of inheritance for the disease trait?") which is "(D) Autosomal dominant".
However, why don't the offspring 1, 2, 3, 4, and 5 in Generation III show a similar rate of expression?