Sex-linked and extrachromosomal inheritance

Haemophilia, which ran through Britain's royal family, was one of the first known sex-linked traits. In 1910 Morgan found a single white-eyed male fly among normal red-eyed Drosophila.

Crossing the white-eyed male with a red-eyed female gave an F1 that was all red-eyed, so white (w) is recessive. In the F2, red and white eyes appeared in a 3:1 ratio, but there was a twist: every white-eyed fly was male, and all the females were red-eyed.

Morgan concluded that the eye-colour gene lies on the X chromosome: sex-linked inheritance. A female has two X chromosomes, so a recessive allele on one X can be hidden by a normal allele on the other. A male has only one X, so a single recessive allele shows. Colour blindness and haemophilia are inherited the same way in humans, which is why they are much more common in males.

Some genes are not in the nucleus at all. Mitochondria and plastids carry their own DNA. The sperm brings almost no cytoplasm into the egg, so these organelles, and their genes, come from the mother. This is extrachromosomal, cytoplasmic or maternal inheritance, and it does not follow Mendel's ratios.

Example: in the four o'clock plant, genes in the plastids make leaves green, white or variegated (patchy). The offspring's leaves follow the mother plant, whatever the father is like.