Codominance is the phenomenon where both the alleles of the same gene get expressed at the same time. As a result of this, the individual ends up obtaining multiple traits of that particular gene.
Unlike most other instances, here no one trait is dominant over the other. Here all the traits of the respective alleles are equally dominant and hence the concept of having one dominant and one recessive trait does not apply.
Generally, each of the parents contributes an allele for a given gene. When the two alleles are different than one another, the dominant one out of the two gets expressed while the weaker one remains as a recessive allele.
But in codominance both of the alleles are dominant and none of them are recessive. Hence both phenotypes are expressed in the offspring.
1. Blood type
The ABO blood type system is probably the most commonly cited example of codominance in humans.
Here both the A and B alleles get blended and the offspring shows similarities in their blood alleles to both of their parents. So the F1 generation instead of showing a dominant aspect of one of the parents shows both the parental characters.
There are specific kinds of proteins attached to each of our blood cells which further helps our body to identify these cells. Three specific alleles are responsible for the attachment of these proteins to the blood cells.
These three alleles are A, B, and O. The A allele codes for the A protein whereas the B alleles codes for the B protein. The O allele does not code any kind of a protein to the cells, so people who have this gene expressed have neither A nor B protein.
So people who obtain the A allele from one parent and the B allele from the other can sometimes showcase an AB blood type where both A and B alleles are expressed as their blood cells contain both A and B proteins. So both of these alleles are equally dominant over each other.
2. Rhododendron flower colors
Codominance is commonly found in plants as well. The flowers of rhododendron plants show multiple colored areas in the petals signifying the blending of two different alleles.
For example, the flowers of rhododendrons have a simultaneous presence of red and white flowers which further signify the dual expression of the red and white genes respectively.
If further, these two different colored flowers of the plant are crossed with each other, it can produce a flower having both white and red color patches on it.
The flower colors in some plants vary and can show up in patches while in others the petals can have contrasting colors where the distribution of the colors is more or less the same.
3. Roan Horses
When a dark-colored horse is bred with a white-colored horse, then the offspring can have both white and dark colors on their body.
This means that both the color traits of the parents are being expressed in the offspring and are equally dominant.
There are three types of roan horses commonly found in the world:
- Red Roan (They have chestnut color as the base color. Along with that they have a mixture of red and white hairs on their body)
- Bay Roan (The body is more or less covered with light-colored hair which is mostly white in nature and the tail, the mane, the head area, and the lower legs are covered with the hair color of the parental bay).
- Blue Roan (The base color is dark often of a blackish color. They are the rarest of the roan horses. The mane, head region, tail, and lower legs are generally blackish. The rest of the body is either greyish or bluish. The mixture of the white and black hairs gives them a distinct bluish appearance).
4. Speckled chickens
On farms, if white chickens breed with colored chickens then the resultant chickens can have both colored feathers and white feathers.
This happens as the young chicken got both the white allele and the colored allele from each of their parents and both of these alleles are expressed in them.
The breeding of two colored chicken variants won’t lead to a mixture of the two colors as the two traits never get intermixed with one another (for example, if white chicken breeds with a black feathered chicken, their offspring won’t be grey in color but can have both the white and black color on their feathers).
Such chickens with multiple colored feathers are termed speckled chickens.
5. Sickle-Cell Trait
Sickle Cell Anemia is a widely found inherent disorder where the red blood cells get deformed in shape and as a result of which their function of carrying oxygen to different parts of the body gets hampered.
Normally the red blood cells are circular as that helps them in faster movement through the bloodstream.
But owing to this genetic disorder, the RBC becomes sickle-shaped which not only affects their oxygen-carrying ability but the deformed shape ends up slowing down their movement as well.
But Sickle Cell Anemia is not the same as Sickle Cell Trait. Even though both the terms are very closely related to one another, the former is not an example of codominance but the latter is.
For a given gene there are two copies of alleles present in humans. So humans can have one of the following three variations of genotypes
- HbA-HbA (Normal condition)
- HbS-HbS (Individual born with Sickle Cell Anemia)
- HbA-HbS (Heterozygous condition where the condition of Sickle Cell Trait arises)
People with Sickle cell anemia have two copies of abnormal alleles of the hemoglobin beta gene (HbS-HbS).
People with Sickle cell trait only have one copy of the abnormal allele while the other copy is normal. These individuals do not suffer from any symptoms of sickle cell anemia.
This is an example of codominance as both the normal and the abnormal allele are codominant in these individuals (the HbA and the HbS).
People with sickle cell anemia can face an intense deficiency of blood regularly. This is because the lifespan of a normal RBC is around 120 days but the lifespan of these deformed blood cells is only 10 to 20 days.
So this shortage of blood cells in their body leads to oxygen shortage in all parts of the body and can lead to intense cycles of body pain. This further makes the individuals having this disease very susceptible to infections, injuries, vision problems, and even partial paralysis.
On the contrary, in people with sickle cell trait, the patients do not showcase any symptoms of this dreadful disease and their blood cell levels also remain normal under most circumstances.
6. Roan cattle
When a brown cow mates with a white bull, the resultant offspring can have both brown and white spots on their body.
When this mixed coat pattern where multiple color spots are present on the body of a cattle offspring occurs, then the cattle are referred to as “roan”.
Under such circumstances, the individual hair colors of the different color spots of the cattle will be of that color variety i.e, the brown spots will be totally brown, it won’t be light brown.
7. Agouti animals
The agouti gene controls the amount and distribution of yellow/red and black/white pigmentation on the coat of a mammal.
So in individuals where this gene gets expressed, multiple bands of pigmentation of different colors are observed on their fur. The agouti gene regulates the melanin production by the body hair of some mammals.
The Alpha-Melanocyte-Stimulating hormone is responsible for the pigmentation of the hair and skin. In animals with an active agouti gene, this hormone helps in regulating the brown to black eumelanin.
On the other hand, the agouti gene helps in the production of red to yellow pheomelanin. This ultimately ends up giving the agouti animals their characteristic fur coloration.
Such distinct codominant gene expression of the fur coloration is observed in many animals including dogs, wolves, cats, rabbits, mice, squirrels, hamsters, etc. Animals devoid of such gene expression tend to have darker-colored coats instead of multi-colored coats.
8. Tabby cats
Tabby cats are the cats that have got the agouti gene expressed in them. They are born when a black cat mates with a cat that has tan-colored fur. The agouti gene plays a huge role in controlling the coding of the agouti signaling protein.
This leads to the expression of the pigmented colors in form of bands on the cat fur in a tabby pattern. As a result, they have got an even distribution of colored strips on their body.
Generally, tabby cats display traits that are very similar to one another.
- A darker pigment will border their light-colored nose.
- The darker pigments will form a M shape on the forehead
- Lined, dotted, or bold stripes on the chest, abdomen, back, etc.
- There will be a dark pencil line present on the face, especially around the eyes which will be surrounded with light-colored fur.
- The pigmentation in the form of banding will be prominent on the legs and tail.
Generally, tabby cats can further be categorized into particular groups based on the color combination of their fur:
- Mackerel tabby
- Classic tabby
- Ticked tabby
- Spotted tabby
- Orange tabby
9. Camellia flowers
When two Camellia plants are crossed, one producing white flowers and the other producing red flowers, then the chances of obtaining a resultant plant that produces flowers with both white and red patches on the flowers is highly possible.
So if the resultant plant demonstrates codominance then the flowers will be white with red spots on them or red with white spots on them. This happens as both alleles are dominant and hence both colors are expressed in the flowers.
Often in such instances, the resultant plant flowers can follow incomplete dominance so, in those circumstances, the flowers produced by this plant will be pink in color.
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