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Incomplete dominance is an exception to Mendelian principles of genetics. The pink flowers of a petunia plant result from incomplete dominance and this has been experimentally determined.
The crossing between petunia plants shows an exception to Mendel’s principles. As a result of crossing the first generation homozygous petunia plants, some alleles of the first cross generation of the petunia flowers were in between the two dominant alleles which meant they were neither dominant nor recessive to the characteristics.
The F1 generation produced by a crossing the red-flowered (RR) plants and the white-flowered (WW) petunia plants consisted of pink-coloured flowers (RW) as the first progeny. Neither of the allele was dominant here. The cases where one allele does not completely dominate another are known as incomplete dominance. The heterozygous phenotype is supposed to occur between the two homozygous phenotypes in incomplete dominance. Phenotype refers to the colour here and genotype is a representation of alleles.
The representation of the genotypes is as follows:
White coloured dominant parent petunia plant: WW
Red coloured dominant parent petunia plant: RR
The colours white and red are the phenotypes and WW or RR is the genotype of parental alleles.
When the red and white flowered petunia plants were true breaded which means the red and white flowered petunia plants had red and white colour as their dominant characteristic and they were homozygous.
The result that was seen of this true breeding was heterozygous pink flowered petunia plants in the F1 generation. The pink colour phenotype of the flowers was an intermediate between the two dominant red and white coloured petunia flowers. This meant that the allele for the red flowers were incompletely dominant over the white flowers giving rise to pink flowers.
The genotype of the pink coloured petunia flowers as well as the corresponding phenotype can be represented by the Punnet squares.
