7. Why is it impossible for a parent with type AB blood to have a baby with type O blood?

This is because a parent with type AB blood has both A and B antigens therefore, the child must inherit either A or B antigen from the parent.

Explanation: In ABO blood type, blood types are classified on the basis of the antigens present in the red blood cells. There are two types of antigens: A and B antigens.

A blood type is so named because it has A antigens, B blood type has B antigens, AB blood type has both A and B antigens while O blood type has no antigens.

Each of these blood types is controlled by two alleles, and children inherit one allele from each parent. If a parent has AB blood type, none of his offsprings will have O blood type because each offspring must inherit either antigen A or B from this parent.

the answer is a or when it says crossing over

answer; i believe its (ovulation);