What does it mean when two alleles are homozygous?

When two alleles are described as homozygous, it means that both alleles for a specific gene are identical. This means that the organism carries two copies of the same allele, whether they are both dominant or both recessive. For example, if an organism has two alleles for a trait that code for blue flowers, it is homozygous for that flower color trait.

Homozygosity can influence how traits are expressed in an organism. If an individual is homozygous dominant (e.g., AA), the dominant trait will be expressed; if homozygous recessive (e.g., aa), the recessive trait will be expressed. The key point is that homozygous refers specifically to the genetic makeup being the same, leading to a consistent trait expression.

The other options refer to various genetic concepts, such as heterozygosity (which involves having different alleles), mutations that can alter allele forms, or only aspects of gene expression. The term 'homozygous' directly pertains to having identical alleles, hence confirming the accuracy of the chosen answer.