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Codominance – Definition, Examples, Importance

What is Codominance?

Codominance is a genetic phenomenon where two different alleles for a single trait are both expressed simultaneously in an organism’s phenotype. This results in a phenotype that clearly exhibits both alleles’ characteristics, rather than a blend of the two. In codominance, neither allele is recessive or masked by the other; instead, both are fully and equally expressed.

A classic example of codominance is the blood type system in humans, specifically the AB blood type. In this system:

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  • The “A” allele codes for A-type antigens on the surface of red blood cells.
  • The “B” allele codes for B-type antigens on the surface of red blood cells.

When an individual inherits one A allele from one parent and one B allele from the other parent, both alleles are expressed on the surface of the red blood cells. As a result, the individual has blood type AB, with both A and B antigens present. This is different from incomplete dominance, where the phenotype is an intermediate blend of the two parental traits.

In summary, codominance is a form of inheritance where both alleles of a gene are fully and equally expressed in the phenotype of an organism, resulting in a distinct expression of both traits.

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Definition of Codominance

Codominance is a form of genetic inheritance where both alleles of a gene are equally and distinctly expressed in an organism’s phenotype, rather than blending together.

Symbolism for codominant alleles

In genetics, alleles are typically represented by letters. For codominant alleles, capital letters are often used for both alleles to indicate that neither is recessive. The choice of letters can be arbitrary, but once chosen, it’s consistent within a given context or study.

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For example, in the case of the AB blood type system:

  • The allele for A-type antigens might be represented by the letter �A.
  • The allele for B-type antigens might be represented by the letter �B.

An individual with an AB blood type, which is codominant, would have the genotype ��AB.

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It’s important to note that the specific letters used to represent alleles can vary based on the trait being studied and the conventions of the particular field or study. However, the use of capital letters for both alleles is a common way to denote codominance.

Examples of Codominance in Animals

Codominance is a genetic phenomenon where two different alleles for a gene are both expressed, resulting in a phenotype that exhibits both traits simultaneously. This is different from incomplete dominance, where the resulting phenotype is a blend of the two parental traits. In codominance, both alleles maintain their individual characteristics when they come together.

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Shorthorn Cattle and Codominance

The Shorthorn breed of cattle provides a classic example of codominance in the animal kingdom. When a cattle with a red coat, represented by the genotype CRCR, is bred with a cattle that has a white coat, with the genotype CWCW, the offspring or the F1 heterozygote exhibits a roan coat, represented by the genotype CRCW.

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In these roan-coated cattle, the red and white hairs are distinctly present. Interestingly, each hair is either red or white; there isn’t any hair that displays a color intermediate between red and white. This distinct presence of both red and white hairs gives the coat its unique roan appearance.

Roan Gene in Horses

The concept of codominance is also evident in horses, specifically in the determination of the roan coat color. The gene responsible for this is termed the “roan gene.” There are three primary varieties of roan coats in horses:

  1. Red Roan: This is a mixture of white and red hairs, giving the horse a pinkish-red appearance.
  2. Bay Roan: This variety consists of a mix of white hairs with bay-colored hairs, resulting in a unique shade.
  3. Blue Roan: This type is characterized by a combination of black and white hairs, giving the horse a bluish-gray appearance.

In conclusion, codominance is a fascinating genetic trait observed in various animals, with the Shorthorn cattle and horses being prime examples. It results in unique and distinct phenotypes that are a direct representation of both parental alleles.

Examples of Codominance in Humans

Codominance is a genetic phenomenon where two different alleles for a gene are expressed simultaneously, resulting in a phenotype that displays characteristics of both alleles. In humans, codominance is observed in various genetic systems, including the M-N blood group system, the ABO blood group system, and the inheritance pattern of sickle-cell anemia.

  1. M-N Blood Group System:
    • The M-N blood group system in humans is governed by codominant alleles represented by the symbols LM and LN. The base letter L is attributed to its discoverers, Landsteiner and Levine.
    • Three distinct blood groups arise from these alleles: M, N, and MN. Their genotypes are LMLM, LNLN, and LMLN respectively.
    • Blood groups signify the presence of specific immunological antigens on the surface of red blood cells. Individuals with the LMLN genotype possess both antigens.
    • The following chart summarizes the reactions of these genotypes with antisera:Genotype (Phenotype)Reaction with Anti–MReaction with Anti–NBlood groupLMLM+–MLMLN++MNLNLN–+N
  2. ABO Blood Group System:
    • Codominance is also evident in the human ABO blood group system.
    • The system comprises three alleles: A, B, and O. While A and B are equally dominant over O, neither A nor B dominates the other.
    • Blood group A can have genotypes AA or AO, while blood group B can have genotypes BB or BO. Individuals with the genotype AB display phenotypic traits of both blood groups A and B.
  3. Sickle-Cell Anemia:
    • The inheritance pattern of sickle-cell anemia in humans showcases incomplete dominance at the cellular level and codominance at the molecular level, specifically concerning hemoglobin.
    • The gene pair HbA (hemoglobin A) and HbS (hemoglobin S) influences the oxygen transport molecule, hemoglobin.
    • The three genotypes manifest different phenotypes:
      • HbAHbA: Normal phenotype with red blood cells that never sickle. These cells contain only hemoglobin A.
      • HbSHbS: This genotype leads to severe, often fatal anemia. The red blood cells are sickle-shaped and contain only hemoglobin S. Homozygotes with this genotype usually succumb to fatal anemia before reaching sexual maturity.
      • HbAHbS: Individuals exhibit no anemia, and their red blood cells only sickle under abnormally low oxygen concentrations. These cells contain both types of hemoglobins, A and S. In terms of anemia, the HbA allele is dominant.

In conclusion, codominance in humans is a fascinating genetic occurrence that results in the simultaneous expression of two alleles, leading to unique phenotypic traits. This phenomenon plays a crucial role in understanding human genetics and the inheritance patterns of various traits and diseases.

Codominance in Humans- ABO Blood type.
Codominance in Humans- ABO Blood type. | Image Source: YassineMrabet.

Examples of Codominance in Plants

In plants, this phenomenon can lead to unique and visually striking results, especially in flowering plants where the simultaneous expression of different color genes can produce multi-colored flowers.

  1. Rhododendrons:
    • Rhododendrons are a prime example of plants exhibiting codominance. These plants can produce flowers that showcase two different color phenotypes.
    • Specifically, when genes responsible for red and white flower colors are both present and expressed, the result is a rhododendron flower with both red and white petals. This unique appearance is due to the simultaneous expression of both the red and white genes for flower color.
  2. Camellias:
    • Another instance of codominance in plants can be observed in Camellias. When a Camellia with white-colored petals is crossed with a Camellia having red-colored petals, the outcome is not a blend of the two colors. Instead, due to codominance, the offspring produces flowers with distinct red-colored petals and white-colored petals.
    • This phenomenon occurs because both the dominant alleles for red and white petal colors exert their influence without overshadowing the other, leading to the formation of petals of both colors on the same flower.

In summary, codominance in plants results in the simultaneous expression of two alleles, leading to unique and often beautiful phenotypic outcomes. This genetic occurrence provides a deeper understanding of plant genetics and the inheritance patterns of various traits, and it also contributes to the rich diversity and visual appeal of the plant kingdom.

Importance of Codominance

Codominance has several significant implications and applications in genetics, biology, and medicine:

  1. Genetic Diversity: Codominance contributes to genetic diversity within populations. When both alleles are expressed equally in the phenotype, it can lead to a greater variety of observable traits in a population.
  2. Medical Implications: Understanding codominance is crucial in fields like transfusion medicine. For instance, the AB blood type, resulting from codominance, is vital to know when matching donors and recipients for blood transfusions.
  3. Molecular Biology Insights: Codominance can provide insights into the molecular mechanisms of gene expression. It demonstrates that both alleles can be transcribed and translated into proteins without one masking the other.
  4. Evolutionary Significance: Traits resulting from codominance might offer evolutionary advantages in certain environments. For example, an organism that expresses two different forms of a protein might be more adaptable to changing conditions.
  5. Genetic Counseling: Understanding codominance can be important in genetic counseling, especially when predicting the potential genotypes and phenotypes of offspring based on parental genetics.
  6. Agriculture and Breeding: In animal and plant breeding, understanding codominance can help breeders predict and select for specific traits. This can be particularly important when trying to maintain or enhance certain desirable characteristics in a breed or species.
  7. Teaching Genetics: Codominance, along with other genetic patterns like dominance, incomplete dominance, and multiple alleles, provides a comprehensive way to teach and understand the complexities of genetic inheritance.

In summary, codominance plays a crucial role in our understanding of genetics and has practical implications in various fields, from medicine to agriculture.

FAQ

What is codominance?

Codominance is a genetic phenomenon where two different alleles for a gene are expressed simultaneously, resulting in a phenotype that displays characteristics of both alleles.

How is codominance different from incomplete dominance?

In codominance, both alleles are fully expressed, leading to a phenotype that shows both traits simultaneously. In incomplete dominance, neither allele is fully dominant, resulting in a blended phenotype.

Can you give an example of codominance in humans?

Yes, the M-N blood group system in humans is an example of codominance. Individuals with the genotype LMLN have both M and N antigens on their red blood cells.

Are codominance and co-dominance the same?

Yes, both terms refer to the same genetic phenomenon. The hyphenated version is just a variation in spelling.

How does codominance affect phenotype?

In codominance, the phenotype displays characteristics of both alleles. For instance, if a flower has alleles for both red and white colors and exhibits codominance, the flower will have both red and white petals.

Is the ABO blood group system an example of codominance?

Yes, the ABO blood group system in humans is an example of codominance. Individuals with the genotype AB have both A and B antigens on their red blood cells.

Can codominance be observed in plants?

Absolutely! An example is the crossing of a white-colored Camellia with a red-colored Camellia, which produces flowers with both red and white petals due to codominance.

Why doesn’t codominance result in a blended trait like incomplete dominance?

In codominance, both alleles are fully and separately expressed without blending. Each allele retains its distinct characteristic in the phenotype.

How can codominance be identified in genetic crosses?

Codominance can be identified when offspring display both parental traits simultaneously without blending. For instance, in a genetic cross involving flower color, if the offspring produce flowers with both colors present, it indicates codominance.

Is codominance common in nature?

Codominance is one of several genetic inheritance patterns observed in nature. While it might not be as common as complete dominance, it is still a significant and naturally occurring phenomenon in both plants and animals.

References

  1. Xia, X. (2013). Codominance. Brenner’s Encyclopedia of Genetics, 63–64. doi:10.1016/b978-0-12-374984-0.00278-3
  2. https://www.biologyonline.com/dictionary/codominance
  3. https://biologydictionary.net/codominance/

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