Amit is a healthy man in his forties . his family history shows no sigh of Niemann pick disease type A . Thus you can assume that Amit is not a carrier. What is Amit's genotype?
Nieman-Pick disease type A is an autosomal recessive trait. That means its is represented by the recessive allele in the gene, and it can be masked (showing no physical effects) if there is a dominant allele for that same gene. Here are the possible genotypes for this gene if we choose to represent it with the alphabet "A":
Homozygous Dominant: AA (Both are dominant alleles that code for all the normal physical features of a normal human.)
Heterozygous Dominant: Aa (Carries one of each allele. The dominant allele masks the effects of the recessive allele in this individual so they will not show any signs of the disorder, but there is still a possibility that the disorder can be passed on to future generations.)
Homozygous Recessive: aa (Both are the recessive allele, which code for this disorder. Anyone who has this genotype will exhibit features of Nieman-Pick disease because there is no dominant allele to mask its effects.)
Since the question states that Amit is a healthy man who is not a carrier, the only possible genotype left for him is "AA."