When a single gene controls two (or more) different traits, it is called pleiotropic gene and this phenomenon is called pleiotropy or pleiotropism. The ratio is 2:1 instead of 3:1. 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.
For example, the disease, sickle cell anaemia is caused by a gene Hbs. Normal or healthy gene is HbA and is dominant.
The carriers (heterozygotes – HbA/HbS) show signs of mild anaemia as their RBCs become sickleshaped (half-moon shaped) in oxygen deficiency. They are said to have sickle-cell trait and are normal in normal conditions. The homozygotes with recessive gene Hbs however, die of fatal anaemia.
Thus, the gene for sickle-cell anaemia is lethal in homozygous condition and produces sickle cell trait in heterozygous carrier.
Two different expressions are produced by a single gene.