diploid

A diploid cell carries its genetic information in duplicate: two complete sets of chromosomes, one set contributed by each parent. It is like owning two copies of the same reference book, so that if one copy has a misprint, the other can often still supply the correct page.

In humans, almost all body cells are diploid, with 46 chromosomes arranged as 23 homologous pairs — 22 pairs of autosomes plus one pair of sex chromosomes. Geneticists write the diploid number as 2n; for humans 2n equals 46. Because every gene is present in two copies, an individual may carry two matching alleles or two different ones at each locus.

Diploidy arises at fertilization, when a haploid egg and a haploid sperm — each with one set of chromosomes — fuse to restore the double set. The carrying of two copies of each gene underlies dominant and recessive inheritance: one functional copy can sometimes mask a faulty partner, which is why some recessive disorders appear only when both copies are affected.