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

Explanation

(1) When a single gene controls two or more different traits, it is called a pleiotropic gene and the phenomenon is known as pleiotropy or pleiotropism.
( 2) The inheritance of sickle-cell anaemia is the best example of pleotropy.
(3) The sickle-cell anaemia is caused by the gene `Hb^(s)`. The healthy or normal gene which is dominant is `Hb^(A)`.
(4) The heterozygotes or carriers, i.e., `Hb^(A)//Hb^(s)` show anaemia.
(5 ) The homozygote possessing the recessive gene `Hb^(s)` die because of fatal anaemia. Owing to this the ratio is 2 : 1 instead of 3 : 1. This is evident from the graphic representation of a cross given below :

Reason: Owing to lethal gene, one offspring dies making the ratio 2 : 1 instead of 3 : 1 .