What Is Genetic Dominance and How Does It Work?

Complete Dominance

In complete dominance relationships, one allele is dominant and the other is recessive. The dominant allele for a trait completely masks the recessive allele for that trait. The phenotype is determined by the dominant allele. For example, the genes for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r). In pea plants that are heterozygous for seed shape, the round seed shape is dominant over the wrinkled seed shape and the genotype is (Rr).

Incomplete Dominance

In incomplete dominance relationships, one allele for a specific trait is not completely dominant over the other allele. This results in a third phenotype in which the observed characteristics are a mixture of the dominant and recessive phenotypes. An example of incomplete dominance is seen in hair type inheritance. Curly hair type (CC) is dominant to straight hair type (cc). An individual who is heterozygous for this trait will have wavy hair (Cc). The dominant curly characteristic is not fully expressed over the straight characteristic, producing the intermediate characteristic of wavy hair. In incomplete dominance, one characteristic may be slightly more observable than another for a given trait. For example, an individual with wavy hair may have more or fewer waves than another with wavy hair. This indicates that the allele for one phenotype is expressed slightly more than the allele for the other phenotype.

Co-dominance

In co-dominance relationships, neither allele is dominant, but both alleles for a specific trait are completely expressed. This results in a third phenotype in which more than one phenotype is observed. An example of co-dominance is seen in individuals with the sickle cell trait. Sickle cell disorder results from the development of abnormally shaped red blood cells. Normal red blood cells have a biconcave, disc-like shape and contain enormous amounts of a protein called hemoglobin. Hemoglobin helps red blood cells bind to and transport oxygen to cells and tissues of the body. Sickle cell is a result of a mutation in the hemoglobin gene. This hemoglobin is abnormal and causes blood cells to take on a sickle shape. Sickle-shaped cells often become stuck in blood vessels blocking normal blood flow. Those that carry the sickle cell trait are heterozygous for the sickle hemoglobin gene, inheriting one normal hemoglobin gene and one sickle hemoglobin gene. They do not have the disease because the sickle hemoglobin allele and normal hemoglobin allele are co-dominant with regard to cell shape. This means that both normal red blood cells and sickle-shaped cells are produced in carriers of the sickle cell trait. Individuals with sickle cell anemia are homozygous recessive for the sickle hemoglobin gene and have the disease.

Differences Between Incomplete Dominance and Co-dominance

Incomplete Dominance vs. Co-dominance

People tend to confuse incomplete dominance and co-dominance relationships. While they are both patterns of inheritance, they differ in gene expression. Some differences between the two are listed below:

1. Allele Expression

• Incomplete Dominance: One allele for a specific trait is not completely expressed over its paired allele. Using flower color in tulips as an example, the allele for red color (R) does not totally mask the allele for white color (r).
• Co-dominance: Both alleles for a specific trait are completely expressed. The allele for red color (R) and the allele for white color (r) are both expressed and seen in the hybrid.

2. Allele Dependence

• Incomplete Dominance: The effect of one allele is dependent upon its paired allele for a given trait.
• Co-dominance: The effect of one allele is independent of its paired allele for a given trait.

3. Phenotype

• Incomplete Dominance: The hybrid phenotype is a mixture of the expression of both alleles, resulting in a third intermediate phenotype. Example: Red flower (RR) X White flower (rr) = Pink flower (Rr)
• Co-dominance: The hybrid phenotype is a combination of the expressed alleles, resulting in a third phenotype that includes both phenotypes. (Example: Red flower (RR) X White flower (rr) = Red and white flower (Rr)

4. Observable Characteristics

• Incomplete Dominance: The phenotype may be expressed to varying degrees in the hybrid. (Example: A pink flower may have lighter or darker coloration depending on the quantitative expression of one allele versus the other.)
• Co-dominance: Both phenotypes are fully expressed in the hybrid genotype.

Summary

In incomplete dominance relationships, one allele for a specific trait is not completely dominant over the other allele. This results in a third phenotype in which the observed characteristics are a mixture of the dominant and recessive phenotypes. In co-dominance relationships, neither allele is dominant but both alleles for a specific trait are completely expressed. This results in a third phenotype in which more than one phenotype is observed.

Sources

• Reece, Jane B., and Neil A. Campbell. Campbell Biology. Benjamin Cummings, 2011.