A diploid individual having two different alleles for a specific trait is called heterozygous. An allele is the alternative form of the same gene. The cells in diploid organisms contain sets of homologous chromosomes, which are paired chromosomes. Each chromosome of a homologous pair has the same genes on the same positions. Although homologous chromosomes have the same genes, they may have different alleles for those genes.
Example: The gene for seed color trait in a pea plant exists in pair. One gene or allele for yellow seed (R) and the other for green seed color (r). A heterozygous plant would contain alleles for seed color (Rr). Gene R is present one chromosome and its allele (r) is present on the other chromosome of the homologous pair.
