The state of a cell with 23 chromosomes (half the normal diploid chromosome state), found normally only in the spermatozoon and the egg (as the secondary oocyte). The noun form of this adjective is haploidy (the state of being haploid).

The smallest unit of inheritance coded by DNA. Generally, a single gene codes for a single protein. Genes come in pairs of alleles (one inherited from each parent) in all tissues except the haploid cells, particularly the spermatozoon.

Distinct from the somatic cells that compose most of the body's tissues and organs, germ cells form the eggs and sperm that will form the next generation. Early germ cells are diploid and replicate by mitosis, but during their development they enter meiosis (for eggs this occurs before birth, for sperm it occurs after puberty), so that at the time of fertilisation the contributing sperm and egg are haploid.

A process similar to mitosis in which two successive divisions of a diploid cell's nucleus result in four 'daughter' cells, each with a haploid number of chromosomes. Unlike mitosis, each chromosome therefore duplicates just once (before the beginning of meiosis). Meiosis in humans (and other higher animals) takes place only among the germ cells (oogonia and spermatogonia, which will have been multiplying by mitosis. By differentiating into, respectively, primary oocytes or primary spermatocytes, each with 92 chromatids, meiosis commences. With completion of the first meiotic division the products (including secondary oocytes and spermatocytes) each contain 46 chromosomes. With completion of the second meiotic division the haploid number (23) of chromosomes, suitable for fertilisation, is reached. In the testis, meiosis and the production of new sperm cells (spermatozoa) can continue throughout life, but in the ovary all egg cells that survive commence meiosis about 20 weeks before birth, spending the remaining time (up to 50 years or more) locked up in primordial follicles as primary oocytes. Whereas a primary spermatocyte gives rise to four haploid sperm cells, a primary oocyte produces just one secondary oocyte (the spare 46 chromosomes are dumped into the first polar body just before ovulation), and then one egg cell (the spare 23 chromosomes are dumped into the second polar body after fertilisation).

Can occur when an egg (as a secondary oocyte) undergoes activation (by itself, with some non-specific stimulus, or through fertilisation by a sperm) and starts to divide (it undergoes cleavage), but the male chromosomes are not incorporated and the egg remains haploid; its further development will soon stop, probably well before implantation. Can be the cause of apparent (but false) late fertilisation in the IVF lab.

A multiple of the haploid number of chromosomes in a cell other than the normal diploid state. Includes triploid (three times the haploid number, or 69 chromosomes) and tetraploid (four times the haploid number, or 92 chromosomes). The noun form of this adjective is polyploidy (the state of being polyploid).

Replication of a diploid organism after combination of haploid male and female gametes, each of which has been produced through the process of meiosis, thus assuring a reassortment of the alleles present among the two parents, in the form of a new, unique, individual genome. Preserved with in vitro fertilisation despite the absence of sexual intercourse. The opposite to cloning.

The product of the spermatocyte in the testis when it has completed meiosis (by which the number of chromosomes is halved, so it's in the haploid state), and also enclosed in the testicular tubules by the Sertoli cells. Early spermatids are round-shaped, whereas late spermatids closely resemble mature spermatozoa.

The final stage of development of the maturing sperm cell, as it leaves the tubules of the testis the testicular tubules, to mature in the epididymis. Composed of a head (including the acrosome), a midpiece loaded with mitochondria to provide energy, and a tail, or flagellum, for propulsion. Packed into the head of the spermatozoon is a haploid number of chromosomes (there are normally 23, including either an X-chromosome or a Y-chromosome).

The final part of the formation of a mature sperm cell, in which the round, early spermatid (already with a haploid number of chromosomes) loses its round shape to acquire the features of the mature, elongated spermatozoon. Contrast with spermatogenesis, which encompasses spermiogenesis as it's completed.

If fertilisation is the 'marriage' between egg and sperm then syngamy is its 'consummation', as the male and female pronuclei come together for the respective haploid sets of chromosomes to combine into a diploid set. An event of legislative importance in the conservative Australian state of Victoria, after which permission for embryo research becomes much harder to obtain, whatever the views of the couple concerned or an ethics committee might be.

A state of 69 chromosomes, or three times the haploid number, most commonly caused by fertilisation of the egg with two sperm; the embryo that results can develop as far as a fetus, but is doomed to miscarry. The trophoblast of the placenta often undergoes partial changes of a hydatidiform mole. The noun form for this adjective is triploidy (the state of being triploid).