Incomplete Dominance

“Incomplete dominance” is a key concept within the Principles of Inheritance and Variation in genetics. It refers to a situation where neither allele of a gene completely masks the other in a heterozygous condition. This results in a phenotype that is a blend or intermediate between the phenotypes of the two alleles. Here are some important aspects of incomplete dominance:

1. Intermediate Phenotype: In incomplete dominance, the heterozygous phenotype is a mix or intermediate of the phenotypes of the two homozygous forms. This is different from complete dominance, where the dominant allele completely masks the effect of the recessive allele.

2. Example: A classic example is the flower color in snapdragons. When a red-flowered plant (RR) is crossed with a white-flowered plant (rr), the F₁ generation plants (Rr) have pink flowers—an intermediate phenotype.

3. Genotypic and Phenotypic Ratios: In incomplete dominance, the genotypic and phenotypic ratios are the same. For example, when two pink-flowered snapdragons are crossed (Rr x Rr), the F₂ generation will have a genotypic and phenotypic ratio of 1 red: 2 pink: 1 white.

4. No Dominant or Recessive Alleles: In this type of inheritance, neither allele is dominant or recessive. Instead, both alleles contribute to the phenotype in the heterozygous condition.

5. Molecular Basis: Incomplete dominance often occurs because the heterozygote produces a reduced amount of functional protein compared to the homozygote for the dominant allele, leading to an intermediate phenotype.

6. Inheritance Patterns: This pattern of inheritance challenges the classical Mendelian inheritance where traits are classified into distinct categories of dominant and recessive. Incomplete dominance demonstrates that inheritance can be more complex and not always ’either-or'.

7. Pedigree Analysis: In human genetics, incomplete dominance can be observed in the analysis of certain traits in pedigree charts, where the heterozygous individuals can be identified by their intermediate phenotype.

8. Educational Importance: Understanding incomplete dominance helps students and researchers grasp the complexity and diversity of genetic inheritance patterns beyond the simple Mendelian concepts.