Types of Dominance Relations: 4. Codominance  
Ben G. Bareja, Nov. 16, 2013

4. Codominance. This is a type of dominance in which the heterozygote exhibits a phenotype that reflects both characters carried by the two alleles making up the heterozygous genotype. In this case both alleles are equally dominant and are thus visible in the heterozygote.

As to our hypothetical cross between parents possessing homozygous genotypes (p1p1 and p2p2), it is the dominance relation in which the heterozygote (p1p2) exhibits both paternal characters. The heterozygote (p1p2) will thus have pods each of which may be colored with patches, spots, or mosaic of green and yellow.

Similar to incomplete dominance and overdominance, the genotypes p1p1, p1p2, and p2p2 will be expressed in different ways. Therefore, the F2 progeny will consist of three distinct phenotypes with a ratio that is identical to the genotypic ratio, that is, 1:2:1. Likewise, codominance forms a new phenotype in the heterozygote that is distinct from either parental types.

The following expected observations will further illustrate the concept of this intraallelic interaction:

• F1 genotype: p1p2
• F1 phenotype: pods with patches, spots, or mosaic of green and yellow
• F2 genotypes and ratio: 1:2:1 (1p1p1:2p1p2:1p2p2)
• F2 phenotypes and ratio: 1:2:1 green, green and yellow, and yellow (1p1p1:2p1p2:1p2p2)

Comparison of Codominance and Other Dominance Relations in a Hypothetical Cross

Now let us have some fun and apply the concepts of codominance and the other types of dominance to ourselves. Let us breed, or rather cross, ourselves using purely hypothetical parents that differ in skin color but are identically homozygous in all other characters. 

Although it is impossible, let us assume anyway that we have two experimental adult, fertile humans of opposite sexes. They differ only in skin color which character is controlled only by a single diallelic gene as if the other tens of thousands of genes in the human genome are absent. Assume further that reciprocal crosses will yield the same result and that the genotype will be expressed fully regardless of environmental conditions.

One of our experimental humans is colored pure black while the other is pure white. Now, let’s cross them! What are your expected results as to skin color of the heterozygote-children?

Here are my expected results: 

          1. If the dominance relation is complete dominance, all children will be either pure black or pure white, exhibiting the skin color of either that of the father or that of the mother depending on which is dominant. If the dominant color is black, then all children will be black-skinned; otherwise, all will be white-skinned.

          2. If incomplete dominance, all children will have skin color that is halfway or a blend of black and white which may be gray, or brown, or cream. The skin color of each child is therefore partly paternal and partly maternal in visual intensity.

          3. If codominance, all children will be spotted black and white like the Dalmatian, or the skin has separate patches of black and white like that in a roan animal, or with separate stripes of black and white as in zebra, or some other pattern of color in which both black and white are visible. Unlike in incomplete dominance, there is no dilution of both parental colors. Both colors are visible on the skin of each child, but in separate spaces.

          4. If overdominance, the color of the offspring should be something that is clearly in excess in visual intensity compared to either black or white. But there seems none. Personally, I suggest that this dominance relation does not apply in this hypothetical cross where the phenotype cannot be measured quantitatively. 

LITERATURE CITED 

BORROR DJ. 1988. Dictionary of Word Roots and Combining Forms. Mountain View, CA: Mayfield Publishing Company. 134 p.

MENDEL G. 1865. Experiments in Plant Hybridisation. (Translated by the Royal Horticultural Society of London). Retrieved Nov. 2, 2013 from https://ia600409.us.archive.org/15/items/experimentsinpla00mend/experimentsinpla00mend.pdf.

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