Michigan Test for Teacher Certification (MTTC) Secondary Integrated Science Practice test

How can a codominant trait be described?

1. One allele is dominant over another

2. Both alleles contribute equally to the phenotype

3. Only recessive traits manifest in the phenotype

4. Character traits are influenced by sex

The correct answer is: Both alleles contribute equally to the phenotype

A codominant trait is characterized by the presence of two different alleles that both express themselves equally in the phenotype of an organism. This means that when an organism inherits one allele from each parent, neither allele is masked or overpowered by the other; instead, both traits are observable in the resulting phenotype. A common example of codominance is seen in certain blood types, such as AB blood, where both A and B alleles are expressed simultaneously. In contrast, a dominant trait would overshadow a recessive one, resulting in the phenotype displaying only the dominant characteristic. Codominance specifically refers to the scenario where both alleles are fully expressed and contribute to the phenotype without one being dominant. Recessive traits do not influence the phenotype unless both alleles are recessive, which is not pertinent to the definition of codominance. The influence of sex on character traits pertains to sex-linked traits rather than codominance, which deals specifically with the relationship between two alleles at a single genetic locus.