Answer:
The mother can carry a full-term baby with A blood type because the mother's blood does not mix with the baby's blood, so the mother's immune system does not activate attacking the baby.
Explanation:
The reason why a mother with B blood type and A antibodies in her system can carry a full-term pregnancy is thanks to the placenta. The placenta is a shared organ between the mother and the baby. Its function is to protect the baby and produce the necessary exchanges of nutrients and wastes between the maternal blood and the baby's blood. As the two types of blood are separated, and they never get in contact during pregnancy, the immune system of the mother does not recognize the baby as a treat. The mother can have A antibodies in her plasma due to a previous pregnancy where during delivery, the two types of blood mixed, also it can be due to any contact with the A blood type. When the foreign blood enters the body, the immune system forms antibodies for it.
This family trait that skips a generation comes from an autosomal recessive trait or as Mendel called as hidden non-dominant trait. Offsprings have a dominant and recessive trait which comes from both parents. Recessive trait appears only when two offspring with same recessive trait blends. This happens in self-fertilization. In the human population, marriage is prohibited between offsprings, thus having recessive trait is only imminent when cousins are married.
