An example of codominance in humans would be hair texture, and let's assume a father with curly hair has children with a mother with straight hair:
The children that are a result can inherit straight, wavy, or curly hair.
Straight and curly haired children are homozygous bevause they are of the parental phenotype, and wavy haired children are heterozygous because they have a independent phenotype that represents both parents in a way.
