A man in 30s has 3 children discovers that his mother Huntington disease father is unaffected. What probability first child will develop

The probability of the first child developing Huntington's disease is 25%.

Huntington's disease is an autosomal dominant genetic disorder, which means that only one copy of the mutated gene is needed to cause the disease. The man's father is unaffected, which means that he does not carry the mutated gene. The man's mother has Huntington's disease, which means that she carries the mutated gene. Therefore, the man has a 50% chance of carrying the mutated gene. If he does carry the mutated gene, then each of his children will have a 50% chance of inheriting it. Therefore, the probability of the first child developing Huntington's disease is 0.5 * 0.5 = 0.25, or 25%.