Sex-linked traits: why more males are colour blind

0:00In people of northern European ancestry, red-green colour blindness affects about 1 in 12 males but only about 1 in 200 females. Why?

0:11Most females have two X chromosomes, and most males have one X and one Y.

0:18A gene for red-green colour vision sits on the X but not on the Y. Genes on the sex chromosomes, X and Y, are called sex-linked.

0:28Versions of a gene are called alleles: capital B for normal vision, small b for colour blindness.

0:36Capital B is dominant: it masks small b. Small b is recessive: it only shows without capital B.

0:46A female with one of each has normal vision but is a carrier, who can pass small b on.

0:53A male with small b on his X has no copy on his Y to mask it, so he is colour blind.

1:01A Punnett square shows the possible children of this carrier mother and a father with normal vision.

1:08Each egg gets one of the mother's X chromosomes; each sperm gets the father's X or his Y.

1:15Every daughter gets her father's capital B, so she has normal vision, but may be a carrier.

1:23Every son gets his father's Y, so each son has a 1 in 2 chance of being colour blind.

1:31So a colour blind son gets small b from his mother, not from his father.

1:38A female needs small b on both X chromosomes to be colour blind, which is much rarer.

1:44Hemophilia, where blood clots poorly, follows the same pattern.

1:49Males have only one X, so one recessive allele is enough to show the trait.