Polymerism or Additive factor

What is Polymerism or Additive factor? – Definition

  • Involves two pairs of non-allelic genes
  • Affect the same character
  • When dominant forms of both the genes are present together they produce double effect.
  • Such genes are called as polymeric or additive genes.
  • Polymerism or additive factor observed in case of pericarp color of wheat (deep red, light red and colorless)

When wheat with deep red pericarp is crossed with colorless progeny has deep red pericarp.

In F2 segregation occurs in the ratio of 9 deep red: 6 light red: 1 colorless. This behavior is based on ratio of dihybrid cross, so the trait must be governed by two pairs of genes.

Suppose genes R1 and R2 produce color of pericarp.

R1 and R2 alone produce light red color pericarp, but in combination give rise to deep red pericarp. This deep red color is due to the double / additive effect.

Both genes in recessive homozygous condition produce colorless pericarp.

Cross between F1 F2
R1R1R2R2 X r1r1r2r2
deep red X colorless
deep red
9 deep red: 6 light red: 1 colorless

Polymerism is one of the type of gene interaction, to know other types of gene interactions go to Gene Interaction – Types.