Sex Linked Traits
Color blindness is the one sex-linked trait. Color vision is controlled by a gene located on the X chromosome. The Y chromosome has no gene for color vision. A boy (XY) has only one X chromosome, and therefore, only one gene for color vision. A girl (XX) has two chromosomes, and, therefore, two genes for color vision.
If a boy inherits a gene for color blindness, he is color-blind. But, since the gene for normal vision is dominant and the gene for color blindness is recessive, a girl is color-blind only when she inherits two genes for color blindness-one from her mother and one from her father. If she inherits only one gene for color blindness, she has normal vision, but she is a carrier of color blindness.
Since a father does not pass on his X chromosomes to his sons, the sons of a color-blind father never have the disorder unless their mother is a carrier.
However, his daughters are carriers and transmit the defective X chromosome to about half their sons and half their daughters.
Another example of the sex-linked trait is haemophilia, a disease in which blood does not clot normally. Queen Victoria was just a carrier and did not have the disease. Because she was XX, the gene on her “good” X chromosome overcame the recessive haemophilia gene on the other X chromosome.
Those of queen Victoria’s male descendants who received her defective x chromosome had the disease, and those who received the good X chromosome did not. Those of Queen Victoria’s female descendants who received her defective X chromosome became carriers of the disease.
Not all hereditary diseases are caused by sex-linked genes. Some hereditary diseases, such as sickle cell anemia, always require two recessive genes.
