Question

Short Answer Question.

Write a note on pleiotropy.

Give an account of pleiotropy with a suitable example.

Answer 1

1. When a single gene controls two (or more) different traits it is called pleiotropic gene and the phenomenon is called pleiotropy or pleiotropism.
2. The phenotypic ratio is 1: 2 instead of 3: 1 because of the death of recessive homozygote.
3. For example, the disease, sickle-cell anaemia, is caused by a gene Hb^S. Normal or healthy gene Hb^A is dominant.
4. The carriers (heterozygotes Hb^A /Hb^S) show signs of mild anaemia as their RBCs become sickle-shaped i.e. half-moon-shaped only under abnormally low O₂ concentration.
5. The homozygotes with recessive gene Hb^S die of fatal anaemia.
6. Thus, the gene for sickle-cell anaemia is lethal in homozygous conditions and produces sickle cell trait in the heterozygous carrier. Two different expressions are produced by a single gene.
   

   Graphical representation of pleiotropy

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