In paternity testing, which combination of blood types can lead to direct exclusion?

In the context of paternity testing, a direct exclusion can occur when the blood types of the mother, child, and alleged father are incompatible, meaning the child cannot inherit the blood type based on the blood types of the mother and the alleged father.

In the provided scenario where there is a group AB child, a group A mother, and a group O man, we can analyze the inheritance of blood types. The mother with blood type A can pass either an A or an O allele to her child. The alleged father with blood type O can only pass on an O allele. Therefore, the only possible blood types for the child from these parents would be either A (inheriting A from the mother and O from the father) or O (inheriting O from both parents). However, the child has blood type AB, which requires one A allele and one B allele. Since neither parent can provide the B allele — the mother cannot pass B and the father does not carry B — the child having an AB type can be directly excluded as the son of these parents.

This reasoning demonstrates why this combination of blood types results in a direct exclusion in paternity testing, confirming that the answer is indeed correct.