Extensions of Mendelian Genetics (Deviation from Mendelism) - Intragenic Interactions - Pleiotropy

Pleiotropy - A single gene affects multiple traits:

• In Pleiotropy, a single gene affects multiple traits and alters the phenotype of the organism.
• The Pleiotropic gene influences a number of characters simultaneously and such genes are called pleiotropic genes were crossed with a variety of peas having white flowers, light coloured seeds and no spot on the axils of the leaves, the three traits for flower colour, seed colour and a leaf axil spot all were inherited together as a single unit.
• Example - sickle cell anemia.
  
• The underlying mechanism of pleiotropy in most cases is the effect of a gene on metabolic pathways which contribute towards different phenotypes, for example, disease phenylketonuria.
• This disease is caused by a mutation in the gene that codes for the enzyme phenylalanine hydroxylase (single gene mutation).
• The mutation in gene is exhibited through phenotypic expression characterised by mental retardation and reduction in pigmentation of hair and skin.