Last Updated on October 8, 2020 by Sagar Aryal
Incomplete dominance definition
Incomplete dominance is a mechanism of dominance in heterozygotes, where the dominant allele does not entirely overcome the phenotypic expression of the recessive allele, and there occurs an intermediate phenotype in the heterozygote.
- Incomplete dominance is also called partial dominance or semi-dominance as the phenotype resulting from the genotype is a blend of dominant and recessive alleles.
- An example of this is observed in flowers where the dominant allele is red, and the recessive is white. However, the heterozygous flowers from these alleles might appear pink due to incomplete dominance.
- In incomplete dominance, the dominant allele cannot completely dominate the recessive allele, as a result of which, the resulting phenotype becomes a mix of both.
- Incomplete dominance is important as it explains the existence of a mix of two alleles that are not described by Mendel in his experiment.
- Mendel explained the Law of dominance to indicate that one of the two alleles is dominant as it always dominates the recessive character.
- Mendel couldn’t study incomplete dominance as the pea plant he selected for his experiment didn’t show incomplete dominance.
- However, his model can still be used to determine the results of crosses of alleles by incomplete dominance. According to his model, the resulting F1 generation will be in the genotypic ratio of 1:2:1 and the phenotypic ratio will be red: pink: white.
- This result indicates that the alleles are still inherited according to Mendel’s rule even with incomplete dominance.
- In quantitative genetics, if the phenotype of heterozygous alleles is exactly between (numerically) that of the two homozygotes, this is considered as no dominance. That is, for dominance to occur, the phenotype of heterozygote must lie closer to one of the homozygotes.
Co-dominance is the mechanism of dominance seen in some alleles where both alleles of a gene in a heterozygote lack the dominant and recessive relationship, and each allele is capable of some degree of phenotypic expression.
- Co-dominance is sometimes considered as no dominance at all as the heterozygote shows the phenotypes of both homozygotes.
- Thus, heterozygote genotype gives rise to a phenotype distinctly different from either of the homozygous genotypes.
- For codominant alleles, all upper case base symbols with different superscripts are used. The upper case letters indicate that each allele can express itself to some degree even when in the presence of its alternative allele.
- An example of co-dominance can be observed in plants where the dominant phenotype is red, and the recessive phenotype is white, and the heterozygote will have flowers with pink and white spots.
- Like incomplete dominance, co-dominance was also not explained by Mendel as the model he chose didn’t express co-dominance.
- However, his model can still be used to determine the results of crosses of alleles by co-complete dominance. According to his model, the resulting F1 generation will be in the genotypic ratio of 1:2:1 and the phenotypic ratio will be red: spotted: white.
- Co-dominance can usually be easily detected in plants and animals with two different colors, but it might also occur in some less visible traits like the blood type.
- Thus, co-dominance is different from incomplete dominance as in co-dominance both the alleles co-exist but separately but in incomplete dominance, the phenotype will be a blend of the two alleles.
Key Differences (Incomplete dominance vs Co-dominance)
Basis for Comparison
|Definition||Incomplete dominance is a mechanism of dominance in heterozygotes, where the dominant allele does not entirely overcome the phenotypic expression of the recessive allele, and there occurs an intermediate phenotype in the heterozygote.||Co-dominance is the mechanism of dominance seen in some alleles where both alleles of a gene in a heterozygote lack the dominant and recessive relationship, and each allele is capable of some degree of phenotypic expression.|
|Phenotype of the Hybrid||The resulting phenotype of the hybrid is a mix or blend or intermediate of the two homozygotes.||Both the homozygous phenotypes are expressed independently.|
|Expression of alleles||In incomplete dominance, the effect of one of the two alleles is more conspicuous than the other.||In co-dominance, the effect of both the alleles is equally conspicuous.|
|The effect of the two parental alleles is an intermediate on the offspring.||Both the parent allele will produce their effect independently.|
|Phenotype||The resulting phenotype is a novel one by the mixing of the two phenotypes from both the parents.||The resulting phenotype is not a novel one as the two parental phenotypes do not mix together.|
|None of the parental phenotypes can be observed in the hybrid.||Both parental phenotypes can be observed in the hybrid.|
|The new phenotype doesn’t have its’ own allele.||The new phenotype is expressed as a combination of two phenotypes of two alleles.|
|Dominance||One of the two alleles is incompletely dominant over the other.||Neither of the alleles acts as dominant or recessive over the other.|
|Quantitative approach||Incomplete dominance allows the quantitative analysis of both incompletely dominant alleles.||Co-dominance only allows the quantitative analysis of the gene expression.|
|Examples||Examples of incomplete dominance are the pink flowers of Mirabilis jalapa and the hair structure in humans.||Examples of co-dominance include the blood type in humans and livestock with spotted colors on their hair or feathers.|
Examples of incomplete dominance
Wavy hair in humans
- Curly hair is the dominant trait in humans, whereas straight hair is the recessive trait.
- In heterozygous species, the resulting phenotype is wavy hair which is an intermediate between straight and curly.
- Thus, wavy hair results from incomplete dominance where the phenotype results due to the mixing of the two traits.
- Wavy hair, thus, represents a novel phenotype different from straight or curly hair.
- Offsprings formed from two parents with homozygous genotypes will have a genotypic ratio of 1:2:1 with the phenotypic ratio or curly: wavy: straight.
Pink flowers in Mirabilis jalapa
- Incomplete dominance is also demonstrated when a pure line or homozygous with red petals (C1C1) is crossed to a pure line with white petals (C2C2), the F1 progeny has no red petals but pink petals (C1C2).
- If an F2 is produced, its progeny exhibits the following results :
¼ red petals – 1 C1C1
½ pink petals – 2 C1C2
¼ white petals – 1 C2C2
- The dominant trait for the color of the flowers is red, and the recessive trait is white. Thus the phenotype resulting from incomplete dominance is pink.
- The phenotype thus formed is a novel one that didn’t exist in the generation with the homozygous parents.
Examples of co-dominance
Blood type in humans
- Blood type in humans is determined on the basis of the gene for the proteins that appear on the outside of the blood cells.
- The alleles present are A, B, and O, where A and B represent two different proteins, but O represents the absence of any proteins.
- The existence of A and B proteins, like two colors in flowers, can occur together as a result of co-dominance.
- Thus, if both the proteins A and B are inherited to the offsprings, and both are expressed, AB blood type might occur in the offsprings.
- However, the blood type O represents a dominant/recessive relationship where if A and B genes are expressed, then O doesn’t get expressed.
- Different animals have different colors on their skin and feathers as a result of co-dominance.
- When a chicken with white feathers breeds with a chicken with black feathers, the offsprings have both white and black feathers as a result of co-dominance.
- During co-dominance, both the traits are expressed independently of each other.
- A similar phenomenon is also observed in cows where the breeding of black and white cows results in cows with the spotting of white and black.
- As a result of co-dominance, both the traits are expressed independently of each other.
- The mixed coat of different colored hair in cattle is termed as roan, which is also a result of co-dominance.
References and Sources
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