Why the ratio in pleiotropy is 2 : 1? Explain it with example.
Pleiotropic gene is a gene which controls or produces two or more traits or expressions, respectively, and the phenomenon is known as pleiotropy.
For example, the disease sickle cell anaemia is caused by this phenomenon. The disease is caused by gene HbS.
In a normal or healthy individual, the gene is HbA, which is dominant.
A heterozygote is a carrier and shows mild signs of anaemia, and the RBCs become sickle-shaped with a deficiency of oxygen.
The homozygous recessive individuals however show fatal anaemia and die.
Hence, if two carriers for sickle cell anaemia mate, the children are formed in the ratio of 1:2:1, with 1 normal, 2 carriers and 1 diseased.
Because the diseased child dies, the observed ratio of the surviving children is 1:2.
Thus, pleiotropy shows a 2:1 ratio and not the normal Mendelian 3:1 ratio.