What is the Difference Between Complete and Incomplete Dominance

The main difference between complete and incomplete dominance is that in complete dominance, the dominant gene/allele completely masks the effect of the recessive gene/allele whereas, in incomplete dominance, neither gene/allele in the pair is dominant. Furthermore, complete dominance obeys Mendel’s law of dominance while incomplete dominance does not obey Mendel’s law of dominance.

Complete and incomplete dominance are two behaviors of alleles while producing a particular trait. Codominance is the condition in which both genes/alleles in the pair are expressed dominantly.

Key Areas Covered

1. What is Complete Dominance
     – Definition, Dominancy, Examples
2. What is Incomplete Dominance
     – Definition, Dominancy, Examples
3. What are the Similarities Between Complete and Incomplete Dominance
     – Outline of Common Features
4. What is the Difference Between Complete and Incomplete Dominance
     – Comparison of Key Differences

Key Terms

Complete Dominance, Dominant Alleles, Incomplete Dominance, Mendel’s Law of Dominance, Recessive Alleles

What is Complete Dominance

Complete dominance is a genetic condition in which the effect of the dominant allele in the pair completely masks the effect of the recessive allele. Therefore, in the heterozygous condition, when both dominant and recessive alleles are present in the pair, only the trait of the dominant allele can be seen. Hence, there is no effect from the recessive allele for the trait. This situation obeys Mendel’s law of dominance, which states that “In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.”

Figure 1: Complete Dominance

In the above example, the dominant allele for the flower color of pea plants is ‘B’, which produces the purple flower color. On the other hand, the recessive allele for the flower color is ‘b’, which produces white color flowers. The three possible genotypes for flower color are BB, Bb, and bb. Here, the heterozygous genotype, Bb produces the purple color flowers, showing the complete dominance of the B allele over the b allele.

What is Incomplete Dominance

Incomplete dominance is another genetic condition in which neither allele in the pair shows complete dominance over the other. Therefore, in the heterozygous condition, both alleles are partially expressed. Which means, neither allele masks the effect of the second allele in the pair. Thus, the heterozygous condition shows a blend trait of both alleles.

Figure 2: Incomplete Dominance

In the above example, the dominant allele for the flower color of snapdragon is ‘R’, which produces red color flowers. On the other hand, the recessive allele for the flower color is ‘r’, which produces white color flowers. In heterozygous allele combination, the R allele shows incomplete dominance over the recessive allele. Hence, the flower color is expressed as pink.

Similarities Between Complete and Incomplete Dominance

• Complete and incomplete dominance are two states which differ by the dominant behavior of genes/alleles in the pair.
• Both obey Mendel’s law of segregation.
• Also, their differential expression can be identified in the heterozygous gene/allele pair.

Difference Between Complete and Incomplete Dominance

Definition

Complete dominance refers to a form of dominance in the heterozygous condition wherein the allele that is regarded as dominant completely masks the effect of the allele that is recessive while incomplete dominance refers to a heterozygous condition in which both alleles at a gene locus are partially expressed and which often produces an intermediate phenotype. Hence, this explains the fundamental difference between complete and incomplete dominance.

Dominancy

Dominancy is the main difference between complete and incomplete dominance. In complete dominance, one allele is dominant over the other allele in the pair while, in incomplete dominance, neither allele in the pair is dominant or recessive.

Expression

Moreover, in complete dominance, the dominant allele produces its trait, completely masking the effect of the recessive allele. However, in incomplete dominance, both alleles in the pair are partially expressed, producing their traits incompletely.

Trait

Another major difference between complete and incomplete dominance is that in complete dominance, the dominant trait is expressed in the heterozygous pair, whereas, in incomplete dominance, an intermediate trait occurs with the contribution of both alleles.

F1 Hybrid When Two Homozygous Parents from Contrasting Traits are Crossed

The F1 hybrid shows the dominant trait in the complete dominance while in the incomplete dominance, the F1 hybrid shows an intermediate trait. This is another difference between complete and incomplete dominance.

Examples

The height of the pea plants is an example character for complete dominance while the flower color of snapdragon is an example character for incomplete dominance.

Conclusion

Complete dominance is the situation in which the dominant allele completely masks the effect of the recessive allele in the heterozygous condition. This allows the expression of the dominant trait. On the other hand, incomplete dominance is the situation in which neither alleles of the heterozygous condition shows complete dominance. Both of them are partially expressed to produce an intermediate trait. Moreover, only complete dominance obey Mendel’s law of dominance. Thus, the main difference between complete and incomplete dominance is the dominance of the alleles in the heterozygous condition.

Reference:

1. Ilona, Miko. “Genetic Dominance: Genotype-Phenotype Relationships.” Nature Education, Nature Publishing Group, Available Here

Image Courtesy:

1. “Punnett square mendel flowers” By Madprime – Own work (CC BY-SA 3.0) via Commons Wikimedia 
2. “Incomplete dominance” By Spencerbaron – Own work (CC BY-SA 3.0) via Commons Wikimedia