Write a short note on Pleiotropy

When a single gene·controls two (or more) different traits, it is called pleiotropic gene.

This phenomenon is called pleiotropy or pleiotropism. The ratio is 2:1 instead of 3:1.

According to Mendel's principle of unit character, one gene (factor) controls one character (trait), but sometimes single gene produces two related or unrelated phenotypic expressions.

For example, the disease, sickle-cell anaemia, is caused by a gene Hb^S.

Normal or healthy gene HbA, is dominant.

The carriers (heterozygotes- Hb^AHb^s) show signs of mild anaemia as their RBCs become sickle-shaped (half-moon-shaped) in oxygen deficiency.

They are said to have sickle-cell trait and are normal in normal conditions.

The homozygotes with recessive gene Hb^s however, die of fatal anaemia.

Thus the gene for sickle-cell anaemia is lethal in homozygous condition and produces sicklecell trait in heterozygous carrier.

Two different expressions are produced by a single gene and this is example of pleiotropy.

A gene which causes death of the bearer is called lethal gene.

A marriage between two carriers will produce normal, carriers and sickle-cell anaemic children in 1:2:1 ratio.

Sickle-cell anaemics die leaving carriers and normals in the ratio 2:1.

The heterozygotes (carriers) can be identified by microscopic examination of blood.

Arranged marriages between two heterozygotes can be discouraged to avoid birth of children with fatal sickle-cell anaemia.