How can a child that is born with type AB blood?

A child can be born with type AB blood if one parent has type A blood and the other parent has type B blood. This is because type AB blood is the result of inheriting one A allele and one B allele, one from each parent.

The A and B blood types are determined by the presence or absence of two antigens, A and B, on the surface of red blood cells. People with type A blood have only A antigens, people with type B blood have only B antigens, and people with type AB blood have both A and B antigens.

When a person with type A blood and a person with type B blood have a child, the child will inherit one A allele and one B allele, resulting in type AB blood. This is because each parent will contribute one of their two alleles to the child, and the child will randomly inherit one allele from each parent.

Therefore, if one parent has type A blood and the other parent has type B blood, it is possible for their child to be born with type AB blood.