Which type of inheritance involves traits expressed in both homozygous and heterozygous individuals?

The type of inheritance that involves traits expressed in both homozygous and heterozygous individuals is codominance. In codominance, alleles are neither dominant nor recessive; instead, both alleles in a heterozygous individual contribute equally to the phenotype. This results in a situation where both traits are visible simultaneously.

For example, in human blood types, individuals who inherit an A allele from one parent and a B allele from another exhibit type AB blood, where both A and B antigens are expressed on the surface of red blood cells. In contrast to recessive inheritance, where the trait only appears in homozygous recessive individuals, or dominant inheritance, where the trait is expressed in both homozygous dominant and heterozygous individuals but not as a combination, codominance showcases the distinct influence of both alleles.

Polygenic inheritance involves multiple genes influencing a trait, leading to a range of phenotypes rather than clear-cut expression seen in codominance. Hence, codominance is the accurate choice for this question.