Which term describes a situation where both alleles of a gene are fully expressed in the phenotype?

The term that describes a situation where both alleles of a gene are fully expressed in the phenotype is codominance. In codominance, neither allele is recessive, and both contribute to the organism's observable traits. This means that when an individual has two different alleles for a particular gene, both are expressed simultaneously without blending into a new phenotype.

A classic example of codominance is seen in certain blood types, where a person with an A allele and a B allele expresses both A and B antigens on the surface of their red blood cells, resulting in blood type AB. This clear expression of both alleles is what distinguishes codominance from other forms of inheritance, such as incomplete dominance, where the phenotype may show a mix or blend of the traits instead of fully expressing both.

Heterozygous refers to the genetic composition of an individual carrying two different alleles for a gene but does not specify how those alleles are expressed in the phenotype. Dominant describes an allele that can mask the expression of a recessive allele but does not inherently indicate both alleles are actively represented in the phenotype. Understanding these distinctions is crucial in genetics and helps clarify why codominance specifically applies in this context.