Understanding Incomplete Dominance: Phenotype Expressions and Examples

In the field of genetics, incomplete dominance refers to a fascinating phenomenon where the heterozygous form of a gene shows a blended phenotype that is intermediary between the two homozygous forms. This concept challenges the notion that offspring will simply blend parental traits, offering a more complex and dynamic understanding of inheritance.

What is Incomplete Dominance?

In genetics, incomplete dominance arises when the heterozygous form of a gene (heterozygote) expresses a phenotype that is intermediate between the phenotypes of the homozygous dominant and recessive genotypes. Instead of blending or averaging the traits, the heterozygote produces an outcome that is a combination of both parental traits.

Examples of Incomplete Dominance

40 Clock Plant and Snapdragon

One of the most celebrated examples of incomplete dominance is seen in the 40 clock plant and the Antirrhinum majus (snapdragon) plant. In these plants, the alleles for flower color manifest in an intermediate phenotype.

40 Clock Plant

In 40 clock plants, when a red allele and a white allele are present together (heterozygous condition), the resulting flower color is a lovely pink, illustrating an intermediary phenotype. This phenomenon can be observed when a plant with the genotype CRCW (red allele CR and white allele CW) exhibits flowers of this pink color, a combination of the red and white.

Snapdragon (Antirrhinum majus)

The Antirrhinum majus or snapdragon is another excellent example. If one crosses a homozygous red-flowered plant, with the genotype CRCR, and a homozygous white-flowered plant with the genotype CWCW, the offspring will display pink flowers, with the genotype CRCW. This illustrates the intermediate phenotype resulting from the heterozygous combination.

Mendel’s Discoveries and Incomplete Dominance

Gregor Mendel’s experiments were groundbreaking in part because they challenged the prevailing theory that parental traits were blended in offspring. However, incomplete dominance is an exception where this blending does not occur. In cases of incomplete dominance, the heterozygous phenotype may indeed be a blend, as seen in the snapdragon example where the pink flowers result from the combination of red and white alleles.

Genetic Prediction and Inheritance

Even when dealing with incomplete dominance, the fundamental rules of Mendel’s genetics still hold true. When a pink-flowered plant with the genotype CRCW is self-fertilized, the expected genotype ratio and phenotype ratio can be predicted. The offspring will have a similar genotypic ratio of 1 CRCR : 2 CRCW : 1 CWCW and a phenotypic ratio of 1 red : 2 pink : 1 white.

These results demonstrate that while the phenotype is intermediate, the genetic information is still being transmitted according to Mendel’s laws of inheritance.

Conclusion

Incomplete dominance is a crucial concept in understanding genetic inheritance beyond the simple blending of traits. It offers valuable insights into how genetic information is expressed in offspring, providing a richer and more nuanced view of genetic inheritance.

Learn More

For a more detailed understanding, you can watch the video below or explore additional resources on incomplete dominance in genetics.