Describe codominance as a situation in which both alleles in heterozygous organisms contribute to the phenotype

Codominance is a genetic phenomenon where both alleles in a heterozygous organism are fully expressed, resulting in a phenotype that displays characteristics of both alleles simultaneously. This situation contrasts with complete dominance, where one allele masks the expression of another.

1. Definition of Codominance

In codominance, neither allele is dominant or recessive. Instead, both alleles contribute equally and visibly to the organism’s phenotype. This means that individuals with two different alleles for a trait (heterozygous) will exhibit traits from both alleles rather than showing a blended or masked phenotype.

2. Examples of Codominance

ABO Blood Group System

One of the most well-known examples of codominance occurs in the human ABO blood group system:

• Alleles:
  • IA: Allele for blood type A
  • IB: Allele for blood type B
  • i: Allele for blood type O (recessive)

In individuals with genotype IAIB, both A and B antigens are expressed on the surface of red blood cells, resulting in blood type AB. This is a classic example of codominance because both alleles contribute to the phenotype without blending.

Flower Color in Certain Plants

Another example can be seen in some flowering plants, such as snapdragons:

• When a red-flowered plant (RR) is crossed with a white-flowered plant (WW), the resulting offspring (RW) exhibit flowers that are red and white striped or spotted, rather than being pink (which would be an example of incomplete dominance).

3. Genetic Implications of Codominance

Codominance has several important implications in genetics:

• Genetic Diversity: Codominant traits can enhance genetic diversity within a population, as they allow for multiple phenotypes to coexist.
• Blood Type Inheritance: Understanding codominance is crucial for determining blood types and compatibility for transfusions.
• Plant Breeding: In agriculture, recognizing codominant traits can aid in breeding programs aimed at producing specific flower colors or patterns.

4. Punnett Square Example of Codominance

To illustrate codominance using a Punnett square, consider the following example with flower color:

• Let’s say we have:
  • R = allele for red flowers
  • W = allele for white flowers

If we cross a red-flowered plant (RR) with a white-flowered plant (WW):Punnett Square Setup:

Results:

• All offspring (RW) will have flowers that express both red and white traits, resulting in a striped or spotted appearance.