Final answer:
Alleles are alternative versions of genes that reside on chromosomes at a specific locus. They can be either identical or different on a pair of homologous chromosomes, leading to homozygous or heterozygous genotypes and contributing to the diversity of traits in organisms.
Step-by-step explanation:
Alleles, which are alternative versions of a gene, reside on chromosomes. Specifically, alleles are located on the same locus, the designated location on a chromosome. Each chromosome may carry identical (homozygous) or different (heterozygous) alleles at a given locus. This variation in alleles contributes to the traits and characteristics we see in organisms. An example of this genetic diversity is the presence of the various blood types, which are determined by different alleles at the blood type gene locus on human chromosomes.
Diploid organisms, like humans, have two sets of chromosomes, one inherited from each parent, which means there are two alleles for each gene. These two alleles can either be the same version (homozygous) or two different versions (heterozygous), which in turn affects the trait that is expressed. Hence, it is possible to inherit identical alleles from both parents or a combination of different alleles from each one.