3.21 Polygenic Inheritance & Continuous Variation

• Genes can have varying effects on an organism's phenotype:
  • Some characteristics (i.e. the phenotype) are controlled by a single gene - these characteristics are known as monogenic
    • These characteristics usually show discontinuous variation (e.g. blood group)
    • Inheritance of these characteristics is known as monogenic inheritance
  • Other characteristics are controlled by several genes - these characteristics are known as polygenic
    • These characteristics usually show continuous variation (e.g. height, mass, skin colour)
    • Inheritance of these characteristics is known as polygenic inheritance

• Some phenotypes of individuals within a population show continuous variation
• The phenotypes do not fall into discrete categories (unlike in discontinuous variation)
• Instead, for these features, a range of values exists between two extremes, within which the phenotype will fall
  • For example, the mass or height of a human is an example of continuous variation
• The lack of categories and the presence of a range of values can be used to identify continuous variation when it is presented in a table or graph

Graph showing population variation in height: an example of continuous variation with quantitative differences

Causes of continuous variation

• Some phenotypes are affected by multiple different genes or by multiple alleles for the same gene at many different loci (polygenic inheritance) as well as the environment
  • Phenotype = genotype + environment
• This often gives rise to phenotypes that show continuous variation
• At the genetic level:
  • Different alleles at a single locus have a small effect on the phenotype
  • Different genes can have the same effect on the phenotype and these add together to have an additive effect
  • If a large number of genes have a combined effect on the phenotype they are known as polygenes

The additive effect of genes

• The height of a plant is controlled by two unlinked genes H / h and T / t
• The two genes have an additive effect
• The recessive alleles h and t contribute x cm to the height of the plant
• The dominant alleles H and T contribute 2x cm to the height of the plant
• The following genotypes will have the following phenotypes:
  • h h t t :  x + x + x + x = 4x cm
  • H H T T : 2x + 2x + 2x + 2x = 8x cm
  • H h T t : 2x + x + 2x + x = 6x cm
  • H H T t : 2x + 2x + 2x + x = 7x cm
  • H h T T : 2x + x + 2x + 2x = 7x cm
  • h h T t : x + x + 2x + x = 5x cm
  • H h t t : 2x + x + x + x = 5x cm