In each case, we know which genes are dominant over which. A man's blood group is determined the instant the fertilized ovum is formed and remains the same till the day he dies. Even after death tissues can be tested for blood groups. Blood groups aren't affected by age or diet or exposure to sunlight or by any kind of chemical or medical treatment. None of the things that affect the physical characteristics usually used to determine race will affect blood groups.
Blood groups are permanent.
With one exception, which we will mention later in this chapter, blood groups have no drastic effect on the health of a man, the length of his life, when or whom he marries, or the number or health of his children. That means that the blood-group genes are all passed on from generation to generation according to pure chance. Blood groups would therefore show how human beings have mixed with one another, for there would be no confusion due to the workings of natural selection. Actually, of course, nothing like this happens at all. The various blood groups are found all over the world.