Albinism

Albinism is not sex-linked. The gene sits on an autosome, so it behaves exactly like Mendel's pea characters — which makes it the clearest example of a recessive condition in humans.

Albinism

A condition in which the body cannot make melanin, the dark pigment of skin, hair and eyes. It is caused by a recessive allele on an autosome, so a person shows it only when they inherit that allele from both parents.

Why both parents must contribute

Write the normal allele A and the recessive allele a. Then:

AA and Aa both produce melanin. The Aa person is a carrier and looks no different from AA.
aa produces no melanin, and the person has albinism.
Two Aa parents, neither of whom has albinism, have a one in four chance of an aa child at each birth. It is the F₂ Punnett square again, unchanged.

Figure to be added

Figures 20.25 and 20.26 — albinism in people and in other animals: a child with the very pale skin, white hair and light-coloured eyes of albinism, and a white peacock displaying beside a peacock of the usual colouring.

It appears in every kind of animal

Melanin is made the same way across the animal kingdom, so the same kind of mutation gives the same result — white peacocks, white buffalo, white squirrels. The cause is not a separate mystery in each species; it is one missing step in one shared pathway.

Living with albinism

Melanin protects skin from ultraviolet light and helps the eye work in bright conditions, so people with albinism sunburn easily and often have reduced or unusually sensitive vision. Shade, covering clothing and eye care deal with most of that. It is an inherited variation with practical consequences, not an illness and not a reason to treat anyone as less able.

Two recessive alleles, one on each chromosome of the pair

Albinism is autosomal recessive. Carriers look unaffected; a child has the condition only when it inherits the recessive allele from both parents, a one in four chance when both are carriers.