PLEIOTROPIC GENES

 PLEIOTROPIC GENES

The ability of a gene to have multiple phenotypic effect because it influences a number of characters simultaneously is known as pleiotropy. The gene having a multiple phenotypic effect because of its ability to control expression of a number of characters is called pleiotropic gene. It is not essential that all the traits are equally influenced. Sometimes the effect of a pleiotropic gene is more evident in case of one trait (major effect) and less evident in case of others (secondary effect). Occasionally a number of related changes are caused by a gene. They are together called syndrome.

In human, the gene for disease phenylketonuria has pleiotropic effect and produces various abnormal phenotypic traits, collectively called syndrome. For example, the affected individuals secrete excessive quantity of amino acid phenylalanine in their urine, cerebrospinal fluid and blood. They become short stature, mentally deficient, with widely spaced incisors, with pigmented patches on skin, with excessive sweating, and with non-pigmented hairs and eyes.

The AY allele for yellow coat in mice is also a good example of pleiotropic gene. It affects two characters : coat colour and survival. It is most probable that both effects of the AY allele are the result of same basic cause which promotes the yellowness of coat in a single dose and death in double dose. Genetic analysis has revealed that lethal pleiotropic AY allele basically affects the cartilage of mice and cause death.

In Drosophila the recessive gene for vestigial wings cause vestigial wings in homozygous condition. However, careful observations show that other traits as well are affected—(i) the tiny wing- like balancer behind the wings; (ii) certain bristles; (iii) the structure of the reproductive organs; (iv) egg production is lowered, and, (v) longevity is reduced.

Hereditary disease sickle-cell anaemia is also an example of pleiotropy