How do dominant alleles mask the effects of recessive alleles, and what is the effect of co-dominant alleles?

Sourav PanNovember 8, 2024

Answered

Dominant and recessive alleles play crucial roles in determining phenotypes, with dominant alleles masking the effects of recessive alleles, while co-dominant alleles express both traits simultaneously. Here’s a detailed explanation of these concepts:

Dominance and Recessiveness

1. Dominant Alleles:
   • A dominant allele is one that expresses its phenotype even when only one copy is present (heterozygous condition). This means that if an individual has at least one dominant allele, the trait associated with that allele will be visible in the phenotype. For example, in pea plants, the allele for round seeds (R) is dominant over the allele for wrinkled seeds (r). Thus, both homozygous dominant (RR) and heterozygous (Rr) individuals display round seeds, while only homozygous recessive (rr) individuals show wrinkled seeds.
2. Recessive Alleles:
   • A recessive allele only expresses its phenotype when two copies are present (homozygous condition). In the case of the pea plant example, the wrinkled seed phenotype appears only in individuals with the genotype rr. The presence of a dominant allele (R) masks the effect of the recessive allele (r), preventing it from being expressed in the phenotype.
3. Mechanism of Masking:
   • The masking effect occurs because dominant alleles often encode functional proteins or enzymes that produce a specific trait, while recessive alleles may encode non-functional proteins or no protein at all. For instance, a dominant allele might result in a functional enzyme that contributes to a trait, whereas a recessive allele might lead to a lack of that enzyme, thus not contributing to the phenotype when paired with a dominant allele .

Co-Dominance

1. Definition:
   • Co-dominance occurs when both alleles in a heterozygous genotype are fully expressed, resulting in offspring that display traits from both parents without one masking the other. This leads to a phenotype that shows characteristics of both alleles simultaneously.
2. Examples:
   • A classic example of co-dominance is found in the ABO blood group system. In this system, alleles IA and IB are co-dominant to each other while being dominant over the recessive allele i. Individuals with genotype IAIB express both A and B antigens on their red blood cells, resulting in blood type AB .
   • Another example is seen in certain flower colors where a plant with one red allele and one white allele may produce flowers that are red and white spotted rather than blending into pink (which would be an example of incomplete dominance)