One member of a chromosome pair is contributed by one parent and the other by the other parent. A gene in one chromosome acts on the same trait as a gene in the same position on the other chromosome. It has been found that one gene may be more powerful in its influence than the other gene that acts on the same trait. The more powerful gene is then said to be dominant, and the other gene is said to be recessive. Certain hereditary traits are known as sex-linked because they are carried on the X chromosome.
Color blindness is an example.
This condition, in which colors appear as varying shades of gray, is rare in females but appears in about 8 per cent of the male population. The genes for color vision are located on the X chromosomes, and the gene for normal vision is dominant to that for color blindness. A female having one gene for normal vision on one X chromosome and one for color blindness on the other will have normal vision, since the color blindness gene is recessive. A male, however, having only one X chromosome, will be color blind if that chromosome has the recessive gene, since there is no corresponding dominant gene to suppress it. It is possible for a female to be color blind, if she has two of the recessive genes, but it is quite rare that these two genes come together in one person.