Question

a parent has a single copy of a dominant allele (a) for the dominant disorder huntingtons disease. the children have a 50% chance of inheriting the disorder. what is most likely the other parents genotype?
a aa
b aa
c aa
d aa

Explanation:

Huntington's disease is a dominant disorder, so the allele \( A \) is dominant and \( a \) is recessive. One parent has genotype \( Aa \) (since they have a single dominant allele \( A \)). To get a 50% chance of the child inheriting the disorder, we analyze Punnett squares:

• If the other parent is \( aa \): The cross \( Aa \times aa \) gives \( Aa \) (affected, 50%) and \( aa \) (unaffected, 50%), matching the 50% chance.
• If the other parent is \( AA \): All children would get \( A \) from this parent and either \( A \) or \( a \) from \( Aa \) parent, so all would be affected (100% chance), which doesn't match.
• If the other parent is \( Aa \): The cross \( Aa \times Aa \) gives 75% chance of \( A\_ \) (affected), which doesn't match.
• Genotype \( aA \) is same as \( Aa \), so not correct. So the other parent's genotype is \( aa \).

Answer:

A. \( aa \)