Many species such as humans maintain diploid cells but produce haploid gametes.A diploid cell has two sets of chromosomes with the same genes in the same loci or place. As a result they have two copies of each gene.The haploid gametes fuse during fertilisation to form the diploid zygote that gives rise to all cells. Therefore the nucleus of all cells except gametes contains matching chromosomes called homologous chromosomes.Alleles are different versions of the same gene. Homologous chromosomes may carry different alleles.