Question

Why the ratio in pleiotropy is 2 : 1? Explain it with an example.

Answer 1

1. When a single gene controls two (or more ) different traits, it is called a pleiotropy gene and this phenomenon is called pleiotropy or pleiotropism.
   The ratio is 2 : 1 instead of 3 : 1.
2. 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.
3. For example, the disease, sickle cell anaemia is caused by a gene Hb^s. The normal or healthy gene is Hb^A and is dominant.
4. The carriers (heterozygotes - Hb^A/Hb^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.
5. The homozygotes with recessive gene Hb^s however, die of fatal anaemia.
6. Thus the gene for sickle-cell anaemia is lethal in homozygous condition and produces sickle cell trait in heterozygous carrier.