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Inheritance of blood groups
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What antigens are connected to the blood group A?
All humans carry one of the four main blood types; they belong to a certain blood group. The four blood groups are A, B, AB and O. The blood groups are determined by antigens on the surface of the red blood cells. Blood group A has A antigens on the surface of its red blood cells. Blood group B has B antigens.
Blood group AB has both A and B antigens. And blood group O has no antigens at all on the surface of its red blood cells. We inherit our blood group from our parents. It is determined by our genes. Our genes come in pairs, one gene from each parent.
Let’s look at an example: One of the genes in this pair results in A antigens on the red blood cells. It’s a gene for blood group A. But the gene from the other parent results in no antigens, so it’s a gene for blood group O. How do the blood cells in the child of these parents look? Well, there are A antigens, from one of the parents.
From the other parent, come no antigens. Therefore the child’s blood group is A, since A antigens are attached to the blood cells. So the gene of the parent with blood group A is the gene that decides here. It dominates over the gene for blood group O. In the same way, the gene for blood group B dominates over the gene for blood group O.
If in a pair of genes one gene gives A antigens, and the other B, both types of antigens will be present on the child’s red blood cells. A from one parent, and B from the other. The child’s blood group becomes AB. If both genes give O, there will be no antigens on the surface of the blood cells. That is when the blood group for the child becomes O.
We can create tables of how genes pair up to help us see what different blood groups a child could belong to. Here is a mother who carries the genes for A and O. So her blood group is A. The father has the genes for B and O; his blood group is B. Each parent gives one gene to its child.
These gene pairs gives four possibilities for the child’s blood group: AB, BO, AO, and OO This gene pair, AB, means that both types of antigens are present on the surface of the child’s red blood cells. The child’s blood group is AB. This gene pair, BO, means that B antigens are present. The child’s blood group is B. And in the same way, this gene pair, AO, means that A antigens are present on the surface of the cells.
The child’s blood group is A. The gene pair OO means there are no antigens on the child’s red blood cells. The child’s blood group is then, O. The probability of each of these events is a quarter, or 25 %. A table like this is called a Punnett square.
Let’s draw another Punnett square, and see what can happen when this child, with blood group O, grows up and becomes a parent herself. She has a child with a partner whose blood group is AB. What do the gene pairs look like? O and O for the mother. A and B for the father.
The possible combinations of gene pairs for their children are: AO, AO, BO, and BO So we see that the possible blood groups for their children are A or B. And there is an equal probability of both: 50%. Though the mother belongs to blood group O, none of her children with this partner will have it, because the traits for blood groups A and B dominate. And none of the children will belong to the same blood group as the father. The children belong to A or B, while the father belongs to AB.