A person's appearance, and often how they act, is due to the genes located on segments of chromosomes. Genes are the units of hereditary information that are responsible for many of a person's characteristics. These genes for a specific trait, such as eye color, may exist in different forms known as alleles. Humans, along with other living things, also have genes or alleles which are responsible for many characteristics.
For a flower, an example of an allele or gene may be for its color. The alleles may make the flower red, pink, violet, etc. The alleles that always appear as a characteristic and are present are known as dominant alleles, and those that are hidden by the dominant alleles are called recessive alleles.
Finally, when both alleles are expressed or appear in some situations it is called codominance. In general, it is a genetic situation where there is neither a dominant or a recessive allele and both are expressed equally. Both traits or characteristics are visible at the same time. Note, codominance is not the reason some people have two different colored eyes.
1. 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.
2. 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.
3. 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.
4. Flower colors
If two plants were crossed to produce a yellow and blue flower, and if the alleles of the gene responsible for petal color were dominant in nature, the flower produced by the progeny plant would either be yellow with blue spots or blue with yellow spots.