Question

Which phoenomenon gives 2 : 1 ratio instead of 3 : 1 ratio? Describe with graphical representation.

Answer 1

In Pleiotropy, the ratio is 2 : 1 instead of 3 : 1.
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 heterozygous carrier. Two different expressions are produced by a single gene, and this is an example of pleiotropy. (A gene which causes the death of the bearer is called a lethal gene) A marriage between two carriers will produce normal, carriers and sickle-cell anaemia children in 1 : 2 : 1 ratio.

                                 Pleiotropy - a cross between two sickle cell anaemics carriers.

Sickle cell anaemics die leaving carriers and normals in the ratio 2 : 1.