The Punnett square shows a cross between two parents. The allele (H) represents the mutated gene that causes Huntington’s disease while the allele (h) represents an unaffected state.

One parent is affected by Huntington’s disease while the other parent is unaffected by the disease. What can be concluded about the nature of Huntington’s disease?

The disease is sex-linked recessive.
The disease is sex-linked dominant.
The disease is autosomal recessive.
The disease is autosomal dominant.

Huntigton's disease is an autosomal dominant disease because the allele for this disease is present on an autosomal chromosome and the person with even one mutated allele (H) can develop the disease even if he has one normal allele (h) too. If a person is affected with Huntigton's disease, there are 50 percent chances that the children will also suffer from the disease.

For example: A father is suffering from Huntigton's disease but mother is normal. Let us see how it will be passed to kids.

P1: Hh : hh

Gametes: H : h: h: h

Offspring: Hh: Hh: hh: hh

50% : 50%

Therefore, 50 percent chances are there that the kids will have disease even if only parent suffers from it.