What is Genetic Dominance and How Does It Work?

All of the features such as eye color and hair type that individuals have are due to gene transfer. These traits, as discovered by Gregor Mendel, are inherited by passing genes from parents to their children. Genes are segments of DNA found on chromosomes and are passed down from one generation to another through sexual reproduction. The gene for a particular trait can exist in more than one form or allele. For each trait or trait, animal cells typically inherit two alleles. Paired alleles can be homozygous (with the same alleles) or heterozygous (with different alleles) for a particular trait.

When allele pairs are the same, the genotype of this trait is the same and is determined by the observed phenotype or characteristic homozygous alleles. When the paired alleles for a trait are different or heterozygous, several possibilities may arise. Heterozygous dominance relationships typically seen in animal cells include complete dominance, incomplete dominance, and codominance.

Gene transmission explains why certain properties are possessed, such as eye or hair color. Traits are inherited by children based on gene transmission from their parents. The gene for a particular trait can exist in more than one form called allele. For a particular trait, animal cells usually have two alleles. One allele can mask the other allele in a complete dominance relationship. The dominant allele is completely masked by the recessive allele. Similarly, in an incomplete dominance relationship, one allele does not completely mask the other. The result is a third phenotype that is a mixture. Co-dominance relationships occur when none of the alleles is dominant and both alleles are fully expressed. The result is a third phenotype in which more than one phenotype is observed.

Full Domination

In total 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 and the phenotype is determined by the dominant allele. For example, genes for seed shape in pea plants exist in two forms; one for the round seed shape (R) or one form or allele and the other for the wrinkled seed shape (r). In pea plants that are heterozygous for the seed shape, the rounded seed shape is dominated by the wrinkled seed shape, and the genotype is (Rr).

Missing Domination

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

Spouse Domination

In codominance relationships neither alleles are dominant, but both alleles are fully expressed for a particular trait. This results in a third phenotype in which more than one phenotype is observed. An example of co-dominance is seen in individuals with sickle cell traits. Sickle cell disorder is caused by the development of abnormally shaped red blood cells. Normal red blood cells have a double concave, disc-like shape and contain large amounts of a protein called hemoglobin. Hemoglobin helps red blood cells carry oxygen to body cells and tissues. The sickle cell is the result of a mutation in the hemoglobin gene. This hemoglobin condition is abnormal and causes the blood cells to take a sickle shape. Sickle-shaped cells are usually trapped in blood vessels, preventing normal blood flow. Sickle cell traits are heterozygous for the sickle hemoglobin gene, inheriting a normal hemoglobin gene and a sickle hemoglobin gene. Since the sickle hemoglobin allele and the normal hemoglobin allele are co-dominant in terms of cell shape, they do not have the disease. 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 Domination and Codominance

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

Allele Expression

In incomplete dominance, an allele is not expressed on the fully matched allele and this is specifically for one specific trait. Taking the flower color as an example in tulips, the allele for the red color (R) does not completely mask the allele for the white color (r). In codominance, both alleles are fully expressed for a particular trait. The allele for the red color (R) and the allele for the white color (r) are both expressed and seen in the hybrid.

Allele Dependence

The effect of an incomplete dominance allele depends on its paired allele for a particular trait. In codominance, the effect of an allele is independent of the paired allele for a particular trait.

Phenotype

The hybrid phenotype in deficient dominance is a mixture of the expression of both alleles and results in a third intermediate phenotype. Example: Red flower (RR) X White flower (rr) = Pink flower (Rr) If co-dominant, the hybrid phenotype is a combination of the expressed alleles resulting in a third phenotype containing both phenotypes. (Example: Red flower (RR) X White flower (rr) = Red and white flower (Rr)

Observable Properties

In deficient dominance, the phenotype can be expressed to varying degrees in the hybrid. (Example: A pink flower may have lighter or darker colors depending on the quantitative expression of one allele versus the other.) In co-dominance, both phenotypes are fully expressed in the hybrid genotype.

In summary, incomplete dominance relationships are that one allele specific trait does not completely dominate the other allele. This results in a third phenotype in which the traits observed are a mixture of dominant and recessive phenotypes. As the codominance relationships are expressed when the allele is dominant, two genes must always be present for a specific trait. And this results in a third phenotype in which more than one phenotype is observed.