Final answer:
Gene variants that arise by mutation and exist at the same relative locations on homologous chromosomes are called alleles. Heterozygosity and homozygosity refer to individuals having two different or identical alleles of a gene on their homologous chromosomes, respectively.
Step-by-step explanation:
Gene variants that arise by mutation and exist at the same relative locations on homologous chromosomes are called alleles. In diploid organisms, two different versions of a gene on their two homologous chromosomes result in being heterozygous for that trait. Conversely, when an organism has two identical alleles of a gene on their two homologous chromosomes, they are considered homozygous for that trait.