Module 6 Lesson 3 - 3

Codominance

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Another pattern of inheritance is codominance. In codominance, both alleles are equally dominant and are fully expressed.

An excellent example of codominance is a roan horse. When a pure breeding red horse is crossed with a pure breeding white horse, the resulting offspring is red roan in colour. From far away, the offspring looks as if it has a brown greyish coat colour. The roan coat is actually a mixture of red and white hairs, each individual hair being either red or white.

P1 Generation Phenotype = Red Coat x White Coat

P1 Generation Genotype = C^RC^R x C^WC^W

	C R	CR
C W	CRCW	CRCW
CW	CRCW	CRCW

F1 Generation Phenotype = 100% Red Roan Coat

F1 Generation Genotype = 100% C^RC^W

The F1 generation expresses fully both parental colours. This is different from incomplete dominance in which the parental phenotypes are blended to produce an intermediate type.

When two F1 red roan horses are crossed, their offspring have red, red roan, and white coats.

F1 Generation Phenotype = Red Roan Coat

F1 Generation Genotype = C^RC^W x C^RC^W

	CR	CW
CR	CRCR	CRCW
CW	CRCW	CWCW

F2 Generation Phenotype = 1 Red Coat : 2 Red Roan : 1 White Coat

F2 Generation Genotype = 1 C^RC^R : 2 C^RC^W : 1 C^WC^W

Codominance occurs when both alleles are equally dominant and neither allele can conceal completely the presence of the other. Unlike incomplete dominance in which the cross results in a blend of the two alleles to yield an intermediate type, both alleles are expressed fully in codominance. However, the genotypic and phenotypic ratio of the F2 generation in codominance is the same as incomplete dominance - 1:2:1.

Example: Sickle Cell Anemia

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Sickle cell anemia is a blood cell disorder that results in abnormal hemoglobins. Hemoglobins are the proteins that carry oxygen molecules within the red blood cells. In the presence of abnormal hemoglobin, the shape of the red blood cells changes into a stiff, crescent shape. Sickle shaped red blood cells have trouble carrying oxygen to tissues and organs.

Hb ^A is the normal hemoglobin allele and Hb^S is the sickle cell hemoglobin allele.

• Individuals with Hb^AHb^A genotype produce normal hemoglobin and have normal red blood cells. 

• Individuals with Hb^AHb^S genotype produce both normal and "sickle" hemoglobin and have both normal and sickled red blood cells.  These heterozygotes do not develop sickle cell anemia and are described as having the sickle cell trait.
  

• Individuals with Hb^SHb^S genotype produce only "sickle" hemoglobin so their red blood cells have the sickle shape and they have sickle cell anemia.
  

The highest frequency of the sickle cell anemia is found in tropical regions. The disease occurs more commonly in people whose ancestors are from the sub-Saharan regions. Why is this disease still prevalent in the world?

Individuals who are heterozygous for the sickle cell anemia allele are more resistant to malaria. Malaria is a common, mosquito-borne disease that can lead to death. The distribution pattern of sickle cell anemia and malaria match closely. In other words, individuals who have slightly sickle-shaped red blood cells are more likely to survive malaria and pass their genes to future generations. This is called heterozygote advantage.

Sickle cell inheritance is a great example of how the terms recessive, dominant, codominant (and incomplete dominant) describe the phenotype, not the genotype.  Depending on the trait, the sickle allele can be recessive, dominant or codominant.

Phenotype	Dominance
Blood cell shape	Codominant - heterozygote has both normal and sickled red blood cells
Malaria Resistance	Dominant -  one copy of sickle allele to become resistant
Sickle Cell Anemia	Recessive - need two copies of sickle allele to be anemic

Note about the Symbols

You may have noticed that the symbols used in incomplete dominance and codominance are different than those used in previous monohybrid questions. The symbols to indicate equally dominant alleles can be written several ways.

1. Change the symbols, such as RW (pink)
   
2. Use superscripts, such as C^RC^W (red roan)
   
3. Use subscripts with numbers, such as R₁R₁ (red) : R₁R₂ (pink) : R₂R₂ (white)

All these notations are acceptable for incomplete dominance, codominance, and multiple alleles.