what is an example of codominance?
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what is an example of codominance?
codominance, in genetics, phenomenon in which two alleles (different versions of the same gene) are expressed to an equal degree within an organism. As a result, traits associated with each allele are displayed simultaneously.
An example of codominance is seen in the MN blood group system of humans. MN blood type is governed by two alleles, M and N. Individuals who are homozygous for the M allele have a surface molecule (called the M antigen) on their red blood cells. Similarly, those homozygous for the N allele
have the N antigen on their red blood cells. Heterozygotes—those with both alleles—carry both antigens. An example of codominance for a gene with multiple alleles is seen in the human ABO blood group system. Persons with type AB blood have one allele for A and one for B; the O allele is recessive (its expression is masked by the other alleles).
Examples of codominance in animals include speckled chickens,
which have alleles for both black and white feathers, and roan cattle, which express alleles for both red hair and white hair. Codominance is also seen in plants. For example, rhododendrons with simultaneous expression of red and white genes for flower colour display flowers with both red and white petals.
Definition of Codominance
We look and act the way we do because of our genes. Genes are units of hereditary information that are located on segments of chromosomes. The genes for a specific trait may exist in different forms known as alleles.
An example of an allele or a gene is a flower color. A flower may have alleles that make it red, or pink, and so on. Alleles that always show up when they are present are known as dominant alleles. Alleles that are masked or hidden by dominant alleles are known as recessive alleles. In some situations, both alleles are expressed equally. A genetic scenario where neither allele is dominant or recessive and both get expressed is known as codominance.
Examples of Codominance:
AB Blood Type
People with this blood type have A and B proteins at the same time. The ABO gene determine what blood type a person has, and everyone has two copies of this gene, one from each parent. There are several combinations of blood types that can result, but when a person has both an A and a B allele, it will lead to blood types visible in the blood, AB.
You can work out the various genotype-phenotype combinations here for yourself, a fun exercise when you know your blood type and are curious about either your parents’ possible genotypes or those of any children you may have.
For example, if you have the blood type O, both of your parents must have donated a “blank” to your genome (the sum of all of your genes). This does not mean, however, that either of your parents necessarily has O as a blood type, because either or both could have the genotype AO, OO or BO.
Thus the only certainty here is that neither of your parents could have type AB blood.
Sickle-Cell Anemia
Sickle cell anemia is a disease where red blood cells become thin and stretched out. If a person has a single copy of the sickle cell allele, then half their red blood cells become abnormally shaped. If this happens, codominance occurs because both normal and sickled shapes are mixed and seen in the blood.
Rhododendron
Rhododendrons and other flowers may also exhibit codominance. In the case of rhododendrons, the crossing of a red and white flower may yield a flower that has both red and white patches. Many flowers show similar patterns of codominance, where both of the parental flower colors show up in different parts of the plant.
Black and white coat
In other animals, codominance is exemplified by a mix of coat colors in a progeny of parents with different coat colors. For example, a cross between a black-furred male dog and a white-furred female dog could produce offspring with a black-and-white coat. This means that the color coat traits of the parents are codominant as both were expressed together in the progeny.
Livestock
When a chicken with white feathers breeds with a chicken with black feathers, the result is an offspring chicken that grows up to have both black and white feathers. Likewise, when a red cow breeds with a red cow, the resulting offspring may show both red and white hairs, resulting in a mixed coat pattern called “roan.”
At first look, a “roan” coat may look like incomplete dominance because it looks like a full mix of red and white to produce a much lighter red coat color. However, if you look at individual hairs they are either entirely red or entirely white. Incomplete dominance would produce a coat of a solid color, just much lighter than a full red cow. Each hair would be the same, lighter color.
Horse color
The roan coat color of a horse is due to codominance. Roan is the result when a color appears in conjugation with white. It is the graying out of a color, and in horses there are actually three types of roans: red, bay, and blue. All of the colors follow similar co-dominance patterns.
Codominance in Plants
It can be observed very distinctly in case of plants, and can be easily discerned from a case of incomplete dominance. The most commonly observed examples are that of the color of the flowers. Hypothetically, if one were to cross two plants that produce a yellow and a blue flower, and if the alleles of the gene responsible for petal color followed incomplete dominance, the resultant flower would be green in color. But if the alleles were codominant in nature, the flower produced by the progeny plant would either be yellow with blue spots or blue with yellow spots.
In the cross shown in the above image two camellia plants are crossed, where one produces white flowers and the other produces red flowers. Since the alleles for both red and white petal color are dominant, the petal color in the resulting plant is decided by codominance. Here, both alleles express themselves and produce their respective red and white colors in the petals, resulting in a flower with alternating patches of red and white. If the alleles followed incomplete dominance, the flower would be an intermediate color, i.e. pink.
Another such plant that exhibits such a pattern of in the color of the flower is the four o’clock plant.
Understanding the patterns of dominant and recessive traits and their inheritability are crucial in the study of gene behavior and genetic interactions. This understanding can be practically applied by people involved in the selective breeding of animals and the crossing of plants to produce new and better varieties that have superior traits as compared to their parental organisms.
Codominant Traits
Codominant traits are traits that are expressed by codominant genes. A codominant trait is expressed when both alleles for a gene are expressed equally, and thus both protein products are made. The proteins made by genes during gene expression are what creates the observable phenotype of an organism. Thus, codominant alleles produce a phenotype that is not blended, nor does it favor one trait or the other, but it instead has the protein product of both alleles.
Alleles and Genes
An allele is a variant of a gene. Alleles come in pairs. Each pair has own spot (locus) in the chromosome and controls the same trait. The allele that is expressed is described as dominant whereas the allele that is masked is described as recessive.
In order to understand codominance, an overview of the basic concepts is essential. A gene is the fundamental, physical, and functional unit of heredity. It is comprised of nucleotides responsible for
the physical and heritable characteristics of an organism. It specifies the structure of a particular protein or of an RNA molecule.
A variant of a gene is referred to as an allele. Alleles come in pairs. The pair of alleles occupies a similar locus (I.e. a spot in the chromosome) and they control the same trait. The allele that is expressed or masks the effect of the other allele is described as dominant. Conversely, the allele whose effects are not manifested or are masked by the dominant allele is described as recessive.
Codominance versus Incomplete Dominance
Codominance is often confused with Incomplete Dominance. The difference between codominance and incomplete dominance is subtle. While codominance and incomplete dominance both result in a different phenotype than complete dominance would produce, they are not the same.
Codominance results in two phenotypes being expressed in different parts of an animal. For example,
a Holstein cow has black and white spots. This results from a “white” allele and a “black” allele that are each fully expressed in different parts of the organism.
By contrast, incomplete dominance is seen when the two alleles mix together to create an entirely different phenotype. The flowers below are an example of incomplete dominance because the “red”
and “white” alleles mix together in certain individuals to create a pink phenotype. The pink phenotype is a mixture of both alleles being expressed at the same time in every cell. If a Holstein cow expressed incomplete dominance, the entire cow would be grey instead of having distinct spots.
How Is It Different from Incomplete Dominance?
Remember, codominance is different from incomplete dominance. In the case of incomplete dominance, a heterozygous organism would have a blend of the phenotypes produced by each allele. If this applied to the example above, the heterozygous bird would have uniform lighter blue feathers
throughout (a blend of the blue and white phenotypes). With a codominant trait, the original phenotypes do not blend. You can still see patches of blue and patches of white.
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