Niemann Pick disease Type A is a disease that is acquired if both the alleles or copies of the gene that control the production of the enzyme acid sphingomyelinase (ASM) are mutated in a person.
This happens only if both the parents of the child are carriers.
If the offspring's father is not a carrier of the disease and therefore has normal genes and the mother is a carrier and has one mutant allele, the child will not acquire the disease.
The Punnett square to show all the possible genotypes is:
....................Mother (N M)
Father N N N M
(N N) N N M N
(Bold: genome of offspring)
There is a 2 in 4 chance that the offspring is a carrier of the disease and a 2 in 4 chance that the offspring is healthy and does not carry any mutation of the gene that leads to the disease.