2. A man with type B blood mamies a woman with type A blood. Their first child has type o blood. The couple says this is
impossible, and requests that the family be retested for their blood type.

Blood types:

AB +

AB -

A +

A -

B +

B -

O +

O -

It is possible to have a child with O blood despite the parents having different blood types.

While the man tested for B blood, he could have been a combination between type o and B whereas, the woman was a combination of type A and o. Since A/B are dominant, they would 'stand out' contrarily to o.

This means that their children could have AB or O blood.