Diploid ($2n$)

Diploid biological cells have two sets of chromosomes. In the ordinary one-locus inheritance model, a diploid genotype therefore contains two allele copies at each genetic locus.

Two homologous chromosomes of a given type are distinct inherited members of the two sets; the two sister chromatids of one replicated chromosome are its copied members. In the stipulated $2n=6$ example, a diploid G2 phase cell nucleus has six replicated chromosomes, not twelve sets or twelve independently counted chromosomes.