Production of substances of blood groups, which we have mentioned, controls the single gene. Does the man of blood group A, B, AB or 0, depending on the nature of alleles of the gene that people inherit. Every person has two alleles of this gene, one on each pair of chromosomes. Each of us has inherited the gene from one father and one from the mother.
These blood groups are three homozygous type. A person can be homozygous but the blood group A, which means it can carry a gene in both chromosomes. Let’s call it AA. Man may have the gene in both chromosomes, or gene 0. He then would be BB or 00, respectively.
A gene is dominant with respect to gene 0. Suppose, for example, that a person who is homozygous for gene A, marries a woman who is homozygous for the gene 0. Men produce sperm cells, and they will all carry the gene A. A woman will produce an egg, and they will all carry the gene 0. Any fertilized egg would, therefore, have one gene A and one gene A. All the children of such marriage would be heterozygous. We can call them AO.
Since A is dominant with respect to 0, only A will occur when researchers checked the blood. Man with the AS genotype is classified as having blood type A. (For example, the case where the mother is blood type 0, and all its children have blood group A. The children could not give blood of their own mother, but any stranger with blood group 0 could.)
Suppose that a person has blood type A. Is there any way to know whether it is homozygous (AA) or heterozygous (AS)? The only way – to look at the blood group of children of this man.
We have already said that the marriage between AA and 00, all children will have blood type A. Suppose, however, that a man marries a woman AD 00. Half of the human sperm cell contains a gene A and half of the gene contains 0. All eggs contain the gene, women 0. As you might guess, fertilized eggs can be either AO or 00. In the first case the child would have the blood group A, in the second case, he would have the blood group 0.
So you see, if a person with type A blood marries a man with blood type 0, and has at least one child with blood group 0, then we have about him learned something. We found that this man with blood group A – AO, but not AA. If he had AA, he would not have children with blood group 0.
Of course, as we just said, marriage AO-00 can produce fertilized eggs, or AO, or 00. For example, assume that all children from such marriages are blood type A. In this case, still you are not able to verify that their parents with blood group A genotype – AA. In other words, you can be guided by blood group of children from this marriage, only sometimes, but not always.
The gene for blood group B also is dominant in relation to the gene of blood group 0. This means that people who are the BA, have blood type B. Blood tests can not detect the difference between BD and BB. The difference is found (sometimes but not always) in the blood groups of children of these people.
Neither the gene of blood group A or gene groups in the blood are not dominant in relation to each other. Here’s an example the case of incomplete dominance. If a person with the AA genotype marry a man with explosives, then all their children – AB.