Pleiotropism and Examples | Simple Genetics Notes

A single gene may have two or more phenotypic expressions. The multiple phenotypic effect of a single gene is called pleiotropism.

Examples of Pleiotropism

1. The allele ‘Ay’ in mice has two phenotypic effects

• Controlling lethality
• controlling yellow coat colour.

2. In human beings, Marfan’s syndrome, characterized by long limbs, slender body, hypermobility of joints, lens dislocation and susceptibility to cardiac diseases are caused by single pleiotropic gene.

3. Sickle cell disease

• It is a genetically transmitted haemolytic disease.
• Homozygotes have two copies of abnormal gene (beta haemoglobin) leading to formation crescent shaped or sickle shaped hemoglobin (Hbs). These cells have very low oxygen carrying capacity. Thus impair circulation. This often led to renal failure, cardiac failure and thrombosis. Thus a single mutation in a gene has multiple consequences in numerous tissues. That is, a single gen has multiple effects on different sites or tissues referred as pleiotropism.
• But in heterozygotes (having one abnormal copy and one normal copy), there is an advantage called heterozygote advantage. These heterozygotes are protected against most lethal form of malaria.

4. Phenylketonuria

• One of the most widely cited examples in pleiotropy in humans 
• Inborn metabolic disorder in which the homozygous recessive individual lacks the enzyme phenylalaine hydroxylase needed to change phenylalanine (amino acid) to tyrosine (amino acid).
• Lack of the enzyme phenylalanine hydroxylase is due to the the abnormal autosomal recessive gene on chromosome 12.
• In children, symptoms are mental retardation, decrease pigmentation of hair and skin and eczema.
• The heterozygous individuals are normal but carriers.
• It occurs in about 1 in 18000 births among Europeans.