Sometimes a single gene defect produces many secondary effects. This is called

Concept-

• Gene is the basic unit of inheritance.
• Genes are hereditary markers from which control various characteristics like skin colour, height, etc.
• Gene which controls more than one character is called a pleiotropic gene.
• This gene shows multiple phenotypic effects.

Explanation-

• Phenylketonuria (PKU) is one such disorder that is caused due to a single gene mutation.
• The mutation is caused in the gene encoding phenylalanine hydroxylase enzyme.
• Affected people lack this enzyme, which can convert phenylalanine to tyrosine.
• Lack of the enzyme results in accumulation of phenylalanine, which gets converted to phenylpyruvic acid.
• Phenylpyruvic acid has adverse effects on the body:
  • Mental retardation
  • Reduction of hair
  • Skin pigmentation
• Thus, we can see that a single gene mutation is causing multiple phenotypic expressions.

Additional InformationPolygenic Inheritance -

• Polygenic inheritance was first described by Nilsson Ehle.
• It is the inheritance of characters in which one character is controlled by many genes.
• Here, the intensity of character depends upon the number of the dominant alleles.
• For example, human skin colour which is spread across a gradient and not distinct.
• It is the result of the additive effect of each allele.