Ploidy refers to the number of chromosome sets in a cell. Human cells are diploid and have two sets (two sets of 23 chromosomes, or 46 chromosomes in total). Sperm and egg cells are haploid and have only a single set (one set of 23 chromosomes). When the sperm and egg combine during fertilization, their two sets of DNA combine to form a diploid zygote, which will go on to form human diploid cells!
During fertilization, you can check the ploidy by counting the number of pronuclei (although PGT is needed to confirm):
- A zygote with one pronucleus (1PN) may be haploid, which is abnormal.
- A zygote with two pronuclei (2PN) is diploid, which is normal.
- Polyploid cells have more than two sets. A zygote with three pronuclei (3PN) is triploid, which is abnormal.
It’s important to distinguish between diploidy and euploidy: diploid means the embryo has the correct two sets of chromosomes, while euploid means the embryo has the correct total number and structure of chromosomes (46, not 45 or 47). A normal embryo should be diploid and euploid.