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Pleiotropism
Uptill now we have observed that a specific gene has a specific effect upon a specific phenotypic trait or in other words, each gene (allele) has its relation with a single phenotypic trait, but, this is not the case. A single gene often influences more than one phenotypic trait. However, it may be that one gene may cause evidently well marked expression of some phenotypic trait (major effect) than the others with less evident phenotype (secondary effect). Most genes have their multiple effects and are called pleiotropic genes. The phenomenon of multiple effect (multiple phenotypic expressions) of a single gene is called pleiotropism.
Examples of Pleiotropism
1. In Drosophila the recessive gene for vestigial wings cause vestigial wings in homozygous condition. However, careful observation show that other traits 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.