16.1.1—Haploid (n) and diploid (2n)

A haploid cell has one complete set of chromosomes, written n. A diploid cell has two complete sets, written 2n, so its chromosomes occur as homologous pairs: one chromosome of each pair was inherited through each parent.

• Gametes, such as egg and sperm cells, are haploid and contain one chromosome from each homologous pair.
• Most body cells are diploid and contain both homologues of each pair.
• Haploid and diploid describe the number of chromosome sets, not the total number of chromosomes; n does not mean one chromosome in every species.

During fertilisation, two haploid gamete nuclei fuse: n + n → 2n. The zygote is therefore diploid, restoring the two chromosome sets needed for the next generation. Keeping the gametes haploid prevents chromosome-set number from doubling at each fertilisation.

Chromosome-set number is not the same as DNA amount or chromatid number. This card defines ploidy and the gamete/body-cell boundary; it does not explain the stages of meiosis or sources of genetic variation.