One of two major forms of a human condition called neurofibromatosis is inherited as an autosomal dominant gene, although it may range from mild to very severely expressed. Since symptoms of the disease can vary for individuals who have this allele a allele is said to show variable expressivity.
Neurofibromatosis affect a variety of organ systems and can have neurological, musculoskeletal as well as other symptoms.
A man with mild symptoms of Neurofibromatosis (he is heterozygous for the Neurofibromatosis allele) has son with a women who does not have Neurofibromatosis. What is the probability that his son will also develop some form of Neurofibromatosis.
The man's genotype is Aa and the woman's genotype is aa. The affected son must have genotype Aa. The probability of such genotype is 1/2.