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 HbA , is dominant. The carriers (heterozygotes- HbA / HbS ) 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. 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 homozygous carrier. Two different expressions are produced by a single gene and this is example of pleiotropy. (A gene which causes death of the bearer is called lethal gene).
A marriage between two carriers will produce noemal, carriers and sickle-cell anarmic children in 1:2:1 ratio.
Sickle-cell anaemics die leaving carriers and normals in the ratio 2:1. The heterozygotes (carriers) can be identified by microscope examination of blood. Arranged marriages between two heterozygotes can be discouraged to avoid birth of children with fatal sickle-cell anarmia.