Biology STAAR (State of Texas Assessments of Academic Readiness) Practice Test

Which of the following best describes codominance?

1. One allele is completely dominant over another

2. Two different alleles are blended together in the phenotype

3. Both alleles are expressed equally in the phenotype

4. Only recessive alleles affect the phenotype

The correct answer is: Both alleles are expressed equally in the phenotype

Codominance is a genetic scenario where both alleles in a heterozygous organism are fully and equally expressed in the phenotype. This means that rather than one allele overriding the other, both contribute to the observable traits. A classic example of codominance is seen in the ABO blood group system, where individuals with one allele for type A blood and one for type B blood express both types on the surface of their red blood cells, resulting in type AB blood. This concept distinguishes codominance from other types of inheritance, such as complete dominance, where one allele masks the effect of another, and incomplete dominance, where alleles blend to create a new phenotype. In codominance, the distinct traits from both alleles are visible, allowing for a diverse representation of genetic characteristics in the organism.