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

Incomplete dominance - No blending of genes:

• The German Botanist Carl Correns’s (1905) Experiment - In 4 O’ clock plant, Mirabilis jalapa when the pure breeding homozygous red (R¹R¹) parent is crossed with homozygous white (R²R²), the phenotype of the F₁ hybrid is heterozygous pink (R¹R²). The F₁ heterozygous phenotype differs from both the parental homozygous phenotype. This cross did not exhibit the character of the dominant parent but an intermediate colour pink.
• When one allele is not completely dominant to another allele it shows incomplete dominance. Such allelic interaction is known as incomplete dominance.
• For example: The inheritance of flower colour in Antirrhinum sp. or snapdragon (dog flower).
  

  Incomplete dominance in 4 O’ clock plant

• Phenotypic ratio: 1: 2: 1 (1 Red: 2 Pink: 1 White)
  Genotypic ratio: 1: 2: 1 (1 R¹R¹: 2 R¹R²: 1 R²R²)
• F₁ generation produces intermediate phenotype pink coloured flower. When pink-coloured plants of F₁ generation were interbred in F₂ both phenotypic and genotypic ratios were found to be identical as 1: 2: 1(1 red: 2 pink: 1 white). From this, we conclude that the alleles themselves remain discrete and unaltered proving Mendel’s Law of Segregation.
• In the F₂ generation, R¹ and R² genes segregate and recombine to produce red, pink, and white in the ratio of 1: 2: 1. R¹ allele codes for an enzyme responsible for the formation of red pigment. R² allele codes for a defective enzyme. R¹ and R² genotypes produce only enough red pigments to make the flower pink.

This indicates the following facts:

1. Pink is the phenotype of the heterozygous genotype (R¹R²).
2. The phenotypic and genotypic ratios are the same.
3. The fact that 1/4^th of the F₂ progeny have red flowers and another 1/4^th have white flowers - the parental combinations - shows that this pattern of inheritance is not the result of character blending.
4. Here, one allele is not completely dominant to another allele it shows incomplete dominance.
5. If blending had taken place, the original pure traits would not have appeared and all F₂ plants would have pink flowers. It is very clear that Mendel’s particulate inheritance takes place in this cross which is confirmed by the reappearance of the original phenotype in F₂.