Question

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

Answer 1

When a single gene controls two (or more) different traits, it is called pleiotropic gene and this phenomenon is called pleiotropy or pleiotropism.

a. The ratio is 2:1 instead of 3:1.

b. 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.

c. For example, the disease, sickle cell anaemia is caused by a gene Hb^s . Normal or healthy gene is Hb^A and is dominant.

d. 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.

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

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

g. Two different expressions are produced by a single gene.

h. A marriage between two carriers will produce normal, carriers and sickle-cell anaemic children in 1:2:1 ratio. But, sickle-cell anaemics who are homozygous for gene Hb^S will die, as Hb^S is a lethal gene causing death of the bearer.

Thus the sickle cell anemics die leaving carriers and normals in the ratio 2:1.