If the gene on one member of the pair exerts its effect regardless of the same gene on the other member of the pair, it is said to be dominant, and the other gene is said to be recessive. If both genes are active in a cell, then there may be no dominance, or partial dominance. If a person inherits two recessive genes, one from each parent, then that person will show the action of those genes, which may simply be a physical trait, such as blue eyes, or may be a disease, such as Tay-Sachs disease. One chromosome pair is different from all the others, the XX or XY pair that determines sex. All women are XX, meaning they received one X chromosome from each parent.
All men are XY, meaning that they received an X from their mother, and a Y from their father.
The Y chromosome is much smaller than the X, and carries very a few genes. Therefore genes on the X chromosome from the mother usually are the only ones active in a male child on that chromosome pair, and if the mother's X chromosome carries the gene for color blindness, which is very common, then her sons may be color blind, since the Y chromosome from their father carries no gene for color vision. When a woman carrying the gene for color blindness conceives a girl, the child will usually not be color blind, since she will inherit another X gene from her father, and the normal color vision gene on his chromosome will be dominant over the color blindness gene on the mother's chromosome. Similar X-linked recessive inheritance is seen in several other conditions, including hemophilia A, which affected the Romanoff line of Russian czars, Duchenne's muscular dystrophy, G6PD deficiency, and others. In the case of color blindness the genes make it difficult to impossible for the person to distinguish reds from greens and affected men are often unaware of their condition until they are tested, as was the writer of today's letter, since they have never really appreciated those colors.