Homozygous and Heterozygous. If an organism has two copies of the same allele, for example AA or aa, it is homozygous for that trait. If the organism has one copy of two different alleles, for example Aa, it is heterozygous. Keep in mind that an organism can't simply be 'homozygous,' period. For each of the organism's thousands of genes, it is either homozygous or heterozygous at that genetic locus.