Pleiotropy refers to a genetic phenomenon where a single gene influences multiple, seemingly unrelated traits or characteristics.
The phenotypic ratio is 1:2 instead of 3:1 because of the death of recessive homozygote. 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 i.e. half- moon- shaped only under abnormally low O2 concentration.