Pleiotropy

d. Pleiotropy :

Pleiotropy is the phenomenon in which a single gene influences two or more apparently unrelated traits simultaneously, and the gene responsible is called a pleiotropic gene. The chapter's example is sickle-cell anaemia in humans, caused by the recessive allele HbS against the normal dominant allele HbA. Heterozygous carriers (HbA HbS) show only a mild form of the condition — their red blood cells become sickle- (half-moon-) shaped only under unusually low oxygen concentration — while individuals homozygous for the recessive allele (HbS HbS) suffer full sickle-cell anaemia and, in the chapter's account, do not survive.

Because the homozygous recessive class is lost, a cross between two carriers (HbA HbS × HbA HbS) does not yield the usual 1:2:1 ratio of surviving offspring; instead the affected homozygotes die, leaving carriers and normal individuals among survivors in an altered 2:1 ratio. The gene is therefore pleiotropic in a very concrete sense — the very same allele simultaneously determines both the shape of the red blood cell and, in double dose, the severity and lethality of the resulting anaemia — showing two distinct outcomes from what is, at root, a single genetic change.

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