Different Types of Dominance in Genetics

The concept of dominance in genetics is important in understanding why we show particular characteristics and not others. 

To understand dominance in genetics, let’s first define some key terms:

1. Alleles: Alleles are alternative forms or variants of a gene. They occupy the same position (locus) on homologous chromosomes and determine a specific trait. For example, the gene responsible for blood type in humans has three alleles: A, B, and O.
2. Genotype: The genotype refers to the specific combination of alleles an organism carries for a particular gene. For example, a person with blood type AB has the genotype AB, carrying both the A and B alleles.
3. Phenotype: The phenotype is the observable physical or biochemical characteristic resulting from the expression of the genotype. For example, the phenotype associated with blood type includes the presence of specific antigens on the surface of red blood cells.

Incomplete dominance and complete dominance are two patterns of inheritance that describe how alleles, or alternative forms of a gene, interact with each other.

The differences between these two types of dominance are:

Complete dominance

In complete dominance, one allele is fully dominant over the other, and the phenotype of the heterozygous individual is determined solely by the dominant allele. The recessive allele is not expressed in the phenotype of heterozygotes. For example, in a flower color trait with a red allele (R) and a white allele (W), in complete dominance, a heterozygous individual (RW) would display the red phenotype, as the red allele is fully dominant over the white allele.

Incomplete dominance

In incomplete dominance, neither allele is completely dominant over the other, resulting in an intermediate phenotype in heterozygous individuals. This means that when an organism carries two different alleles for a trait, the phenotype expressed is a blend or combination of the two alleles. For example, if we consider a flower color trait with a red allele (R) and a white allele (W), in incomplete dominance, a heterozygous individual (RW) may exhibit a pink phenotype, which is a blend of the red and white colors.

It’s worth noting that complete dominance is more common and easier to observe since the dominant allele masks the presence of the recessive allele. In contrast, incomplete dominance reveals a blending effect between alleles, resulting in an intermediate phenotype.

It’s important to remember that these patterns are simplified explanations of genetic inheritance and that traits are often influenced by multiple genes and environmental factors, leading to more complex inheritance patterns.

Codominance

Codominance is a concept in genetics that describes a situation in which both alleles of a gene pair are fully expressed in the phenotype of an organism. In other words, neither allele is dominant or recessive over the other, and both alleles contribute to the observable traits or characteristics of an individual. This is in contrast to complete dominance, where one allele masks the expression of the other.

In codominance, both alleles of a gene are expressed independently and contribute equally to the phenotype. This means that neither allele is dominant or recessive to the other, and the traits associated with each allele are simultaneously visible in the individual’s phenotype.

Let’s take the example of the ABO blood group system to illustrate codominance. In this system, there are three alleles: A, B, and O. The A allele produces the A antigen, the B allele produces the B antigen, and the O allele does not produce any antigens.

• If an individual has two A alleles (genotype AA), they will have blood type A and express only the A antigen.
• If an individual has two B alleles (genotype BB), they will have blood type B and express only the B antigen.
• If an individual has one A allele and one B allele (genotype AB), they will have blood type AB and express both the A and B antigens simultaneously.

In the case of blood type AB, neither the A nor the B allele is dominant over the other. Both alleles are codominant, resulting in the expression of both antigens on the surface of red blood cells.

Codominance can also be observed in other genetic traits. For example, in certain coat color patterns in cattle, the alleles for red and white coloration are codominant. When a heterozygous individual (Rr) with one red allele and one white allele is crossed with another heterozygous individual, the offspring will have a coat color that shows both red and white patches, rather than blending the two colors.

In summary, codominance is a genetic phenomenon where both alleles of a gene are fully expressed in the phenotype, without one allele dominating the other. This leads to the simultaneous expression of both alleles and the presence of multiple traits or characteristics in the organism’s phenotype.