Pleiotropy example and definitions

Aug 09, 2022, 16:45 IST

Pleiotropy (Pleiotropic genes):

  The ability of a gene having a multiple phenotypic effect because to control expression of a number of characters is called pleiotropic gene.

The effect of pleiotropic gene is more evident in case of one trait (major effect) and less evident in case of others (secondary effect). 

Pleiotropy examples are

• Sickle cell anaemia : It is an autosomal hereditary disorder in which the erythrocytes become sickle shaped under oxygen deficiency as during strenuous exercise and at high altitudes.

The disorder is caused by formation of an abnormal haemoglobin (haemoglobin – S). Haemoglobin – S differs from normal haemoglobin – A in only one amino acid i.e. 6th amino acid of - chain, glutamic acid, is replaced by valine (major effect)

During oxygen deficiency 6 – valine forms hydrophobic bonds with complementary sites of other globin molecules which causes distortion of haemoglobin configuration. As a result, erythrocytes having haemoglobin – S become sickle shaped (Secondary effects)

Other secondary effects result from the sickle – shaped erythrocytes. The cells cannot pass through narrow capillaries. They have a tendency to clot and degenerate. They, therefore, clog blood capillaries. Blood circulation and oxygen supply are disturbed. Spleen and brain get damaged. The patient feels acute physical weakness.

The homozygotes having only haemoglobin – S usually die before reaching maturity because erythrocyte distortion and degeneration occur even under normal oxygen tension.

Despite having harmful effect, the allele for sickle – cell anaemia continues to persist in human population because it has survival value in malaria infected areas like tropical Africa. Malarial parasite is unable to penetrate the erythrocyte membrane and cause any harm.

When two sickle cell carriers or heterozygotes marry they produce three types of children, - homozygous normal, heterozygous carriers and homozygous sickle celled in the ratio of 1 : 2 : 1. However, homozygous sickle – celled individuals (HbS HbS) die in childhood due to actue anaemia.

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