Questions: If one parent is affected by an autosomal dominant disorder, which outcome would explain this mode of inheritance? 50% chance of having an affected child

The answer is the fourth one: $50 \%$ chance of having an affected child.

Explanation for each option:

1. $50 \%$ chance of passing multiple abnormal genes: This is incorrect because an autosomal dominant disorder typically involves a single abnormal gene being passed on, not multiple genes. Each child has a 50% chance of inheriting the single abnormal gene from the affected parent.

2. $100 \%$ chance of passing on abnormal gene: This is incorrect because if one parent is affected by an autosomal dominant disorder, they have one normal allele and one abnormal allele. There is a 50% chance of passing on the abnormal allele and a 50% chance of passing on the normal allele to each child.

3. $25 \%$ chance of having affected child: This is incorrect because a 25% chance is typically associated with autosomal recessive disorders, where both parents are carriers. In autosomal dominant disorders, the chance of having an affected child is 50%.

4. $50 \%$ chance of having an affected child: This is correct. In autosomal dominant inheritance, if one parent is affected, there is a 50% chance that each child will inherit the abnormal gene and be affected by the disorder.

In summary, for an autosomal dominant disorder, each child has a 50% chance of being affected if one parent carries the abnormal gene.