What is the main characteristic of homozygous alleles?

Homozygous alleles are characterized by having two identical alleles for a particular gene. This means that both alleles in a pair, located at the same locus on homologous chromosomes, are the same. For example, an individual may have two alleles for a gene that are both dominant (like AA) or both recessive (like aa). This genetic arrangement can lead to the expression of certain traits, as either the dominant allele will determine the phenotype if present, or the recessive will express only in the absence of a dominant allele.

The other options highlight misunderstandings about the nature of homozygous alleles. Some alleles can be dominant or recessive, but being homozygous does not guarantee that they are always dominant; they can also be recessive. The notion of different forms aligns with heterozygous alleles, which consist of two distinct variants at a gene locus. Lastly, occupying different loci refers to the situation with alleles that are not homozygous, as these would be at separate locations on chromosomes. Thus, the core concept behind the definition of homozygous alleles hinges on their sameness at a single locus.