Haploid

Haploid describes a cell or organism that has only one set of chromosomes. In humans, the haploid number (n) is 23. Only gametes (sperm and egg cells) are haploid in humans - each carries 23 unpaired chromosomes, which is half the full (diploid) set.

• We use n to denote the haploid number of chromosomes. In humans, n = 23. Thus, when fertilization occurs (sperm 23 + egg 23), the resulting zygote is 2n = 46.
• Haploid cells are produced from diploid cells by meiosis. This reduction ensures that when two gametes fuse, the species' chromosome number stays constant across generations.
• Be aware: not all species have the same haploid number. (e.g., a fruit fly's haploid number is 4, so 2n=8). The concept of haploid just means "one complete set," whatever that number is for the organism.

• Typical question: "Which cells in humans are haploid?" Answer: gametes (sperm and egg). Recognize that haploid cells have half the chromosome count of somatic cells.
• Another angle: A question might give an organism's diploid number and ask for the haploid number (or vice versa). For example, "If 2n = 16, what is n?" You should answer 8 (haploid number is 8).