What could be the genotype of a healthy person in his forties. Wheras his family history shows no signs of Niemann-Pick disease type A. Assuming That ......the person is not a carrier.
Your question is a little vague here. Niemann-Pick disease type A affects infants in the early stages of life. The prognosis is not good, most die before the age of two. Niemann-Pick disease types B and C affect juveniles, the prognosis is better, with life spans reaching into young adulthood. You are asking about a mature adult forty years of age.
If you are asking about a 40 year old male that has fathered a child with Niemann-Pick type A, he would have had to be a carrier of the disease. For the disease to express itself, two recessive alleles must pair together. If he is a carrier, and the female he fathered the child with is a carrier, there is a 25% chance the child will have the disease.
The only other conceivable way this could have occurred with no background of the disease would be if the alleles representing for the presence of this disease mutated to code for the allele that produces the disease. Then, if he fathered a child with a person who is a carrier for the disease, the above-mentioned probability for producing a diseased child falls into place.
The genotype for a carrier would be Nn, where one allele would be dominant for non-Niemann production, masking the recessive n, which would be for Niemann production.