Sex determination
In humans, there are 23 pairs of chromosomes. 22 of the pairs are called autosomes. 1 pair of chromosomes is called the sex chromosomes. Sex chromosomes contain all the genes that determine an individual’s sex characteristics (plus many other genes that have nothing to do with sex characteristics.)
Males have XY as their sex chromosomes; females have XX.
Sex-linked Traits in Humans
Because the sex chromosomes are a matched pair in females and an unmatched pair in males, traits on these genes show unusual patterns of inheritance. Traits that have their genes on the X chromosome are called X-linked traits.
Recessive traits controlled by genes on the X chromosome affect far more men than women. WHY?
If a baby boy is born with a recessive trait on his X chromosome, the trait will always show because there is only one X chromosome. The boy’s other sex chromosome is a Y.
Ø Red-green color blindness
One gene (but possibly more) on the X chromosome controls red and green color vision. Normal color vision is dominant; color blind is recessive. Color blindness affects ten times as many boys as girls (remember boys only get one X gene). Color blindness is also unusual in that the severity of the blindness is variable. Colorblindness for green only is more common than for red. Total colorblindness (vision is black, white and gray only) is very rare.
Ø Hemophilia
Blood normally has the ability to clot, and stop bleeding from minor cuts quickly. The gene for blood clotting enzyme is on the X chromosome. About 1 out of 10,000 males has hemophilia, but only about 1 out of 1,000,000 females has it. Descendents of Queen Victoria of England carried the gene in a very high percentage. Through marriages, the gene was passed into the Spanish and Russian royal families.