Incomplete dominance - SS3 Biology Lesson Note

Incomplete dominance is a pattern of inheritance in which the heterozygous genotype results in an intermediate phenotype that is a blend or mixture of the two homozygous phenotypes. In other words, neither allele is completely dominant over the other, leading to an expression of the trait that is different from the typical dominant-recessive relationship.

In the case of incomplete dominance, when a heterozygous individual carries one dominant allele and one recessive allele for a particular trait, the phenotype exhibited is a combination or intermediate between the phenotypes of the two homozygous genotypes. This intermediate phenotype is often represented as a third distinct phenotype, different from both the dominant and recessive phenotypes.

A classic example of incomplete dominance is the flower colour in snapdragons. The alleles for flower colour in snapdragons are red (dominant allele, denoted as R) and white (recessive allele, denoted as r). When a plant with red flowers (RR) is crossed with a plant with white flowers (rr), the resulting F1 generation (Rr) exhibits pink flowers. The pink colour is the result of the incomplete dominance of the red allele over the white allele, leading to an intermediate phenotype.

It is important to note that incomplete dominance does not mean that the alleles themselves are partially dominant or blended. Instead, it refers to the expression of the phenotype resulting from the interaction between the alleles. The alleles are still inherited as discrete units, following Mendelian inheritance principles, but their interaction produces an intermediate phenotype in heterozygous individuals.

Incomplete dominance provides a nuanced understanding of genetic inheritance, adding complexity to the simple dominant-recessive relationship. It demonstrates how traits can exhibit a range of phenotypes and highlights the continuous nature of gene expression and inheritance.