Incomplete Dominance vs. Codominance: Videos & Practice Problems

Incomplete dominance is a genetic inheritance pattern where heterozygous individuals exhibit a blended phenotype, which is an intermediate expression of the traits from two alleles. For instance, when crossing red and white flowers, represented by the alleles r₁ (red) and r₂ (white), the resulting offspring display a pink flower phenotype instead of either red or white. This occurs because the heterozygous genotype r₁r₂ does not express the traits of either parent fully but rather combines them into a new, intermediate form.

In this example, the red flower is homozygous for the r₁ allele, while the white flower is homozygous for the r₂ allele. When these two are crossed, the offspring inherit one allele from each parent, resulting in the heterozygous genotype r₁r₂. The key takeaway is that in incomplete dominance, the phenotype of the heterozygous individual is a blend of the two parental phenotypes, leading to the unique pink flower, which is not explicitly dictated by either allele. This pattern of inheritance is crucial for understanding how traits can be expressed in a blended manner rather than in a dominant-recessive relationship.

Codominance is a genetic inheritance pattern where heterozygotes express both phenotypes from two different alleles equally, without one allele masking the other. The term "codominance" derives from the prefix "co," meaning together, indicating that both alleles dominate simultaneously. This concept can be illustrated through the example of flower color in plants.

Consider a cross between a homozygous red flower (with two R₁ alleles) and a homozygous white flower (with two R₂ alleles). The resulting offspring, which are heterozygous with one R₁ and one R₂ allele, will not produce a blended phenotype, such as pink flowers, as seen in incomplete dominance. Instead, the flowers will exhibit distinct patches of red and white, showcasing both phenotypes equally. This characteristic expression of both traits in patches is the hallmark of codominance.

In summary, codominance allows for the simultaneous expression of both alleles in heterozygotes, leading to a phenotype that displays both parental traits distinctly, rather than blending them into an intermediate form. Understanding this concept is crucial for distinguishing it from other inheritance patterns, such as incomplete dominance.

Codominance is a genetic phenomenon where two different alleles are expressed equally in a heterozygote, and human blood type serves as a classic example of this concept. Blood type is determined by three possible alleles: I_A, I_B, and i. The alleles I_A and I_B are codominant, meaning that neither allele masks the other when both are present. In contrast, the allele i is recessive to both I_A and I_B.

To understand how these alleles combine to form different blood types, consider the following genotypes and their corresponding phenotypes:

1. **Type A Blood**: This can occur with the genotypes I_AI_A or I_Ai. In both cases, only the I_A allele is expressed, resulting in A molecules on the surface of red blood cells.

2. **Type B Blood**: This can occur with the genotypes I_BI_B or I_Bi. Similar to Type A, only the I_B allele is expressed, leading to B molecules on the red blood cell surface.

3. **Type AB Blood**: An individual with the genotype I_AI_B expresses both A and B molecules on the surface of their red blood cells. This is a clear example of codominance, as both alleles are equally represented.

4. **Type O Blood**: The genotype ii results in no A or B molecules being present on the red blood cells, leading to Type O blood, which can be visualized as lacking any surface molecules.

Understanding these combinations helps clarify how blood types are inherited and expressed, showcasing the principles of codominance and dominance in genetics. As you continue your studies, applying these concepts will deepen your comprehension of genetic inheritance patterns.