Questions: LEARNING OBJECTIVE: Understand how codominance works Which of the following is true about codominance? a.) More than one allele is present for a gene, but one allele is dominant over the other(s). b.) Three alleles can all be expressed at the same time, with none of them being dominant. c.) An example of codominance is a person who has an A and an O allele for blood type. d.) Two alleles can be expressed at the same time with neither being dominant.

The answer is the fourth one (or D): Two alleles can be expressed at the same time with neither being dominant.

Explanation for each option:

a.) More than one allele is present for a gene, but one allele is dominant over the other(s).

• This describes simple dominance, not codominance. In simple dominance, one allele masks the expression of another allele.

b.) Three alleles can all be expressed at the same time, with none of them being dominant.

• This is incorrect because codominance typically involves two alleles being expressed simultaneously. The concept of three alleles being expressed at the same time is more related to multiple alleles, not codominance.

c.) An example of codominance is a person who has an A and an O allele for blood type.

• This is incorrect. A person with an A and an O allele for blood type would have blood type A, as the A allele is dominant over the O allele. Codominance in blood types is exemplified by the AB blood type, where both A and B alleles are expressed equally.

d.) Two alleles can be expressed at the same time with neither being dominant.

• This is the correct definition of codominance. In codominance, both alleles in a heterozygous organism contribute to the phenotype, and neither allele is dominant over the other.

Summary: Codominance occurs when two alleles are expressed simultaneously in a heterozygous organism, with neither allele being dominant over the other. This is accurately described by option D.