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).

The gain or loss of one or more chromosomes at meiosis, including trisomy (47 chromosomes) and monosomy (45 chromosomes). Caused by chromosomal nondisjunction.

The name given to the products of a chromosome that has divided during meiosis or mitosis, before separating off as separate daughter chromosomes.

Remember that the ordinary cells of the body have 46 chromosomes in 23 pairs. The members of each of the pair don't contain the same genes (this is why you can carry a gene for recessive inheritance without being sick, but if you have both genes there's trouble). During the cell divisions that produce the germ cells (the process of meiosis), the 46 chromosomes first double to 92 before they end up with the 23 present in an egg or a sperm. Two of the four sibling chromosomes (we call them chromatids) then randomly exchange bits of themselves: there's a "cross-over". The points of junction where this happens are called chiasmata. Nature is jumbling up where the genes will end up so that in the long run you don't always have to inherit two particular genes together just because they live next door to each other on a chromosome. Cross-over does not mean that genes can end up in any chromosome they like: unless "translocated", they will remain in pairs (of alleles) within a particular pair of chromosomes. Chromosomal crossover is the origin of what geneticists call hybrid vigor.

Failure of chromatids to separate after chromosomal cross-over during meiosis. The origin of aneuploidy. More common in eggs, or oocytes, than sperm, and increases with age in women.

Entry of a sperm cell into an egg: their "marriage". The egg is activated by this event, so that: (1) "cortical granules" are expelled that stop further sperm binding to the egg; (2) the second division of meiosis is completed, with expulsion of the second polar body; and (3) the machinery of the egg is got going, which will form pronuclei of the male and female chromosomes prior to syngamy.

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.

The third stage (of four stages) of mitosis (or meiosis), at which the chromosomes, attached in a plate-like formation to a structure in the cell called a spindle, are easily distinguished with a microscope, and can be photographed to construct a karyotype.

The form of the ovum, or egg, which is undergoing a halving of the number of chromosomes through the process of meiosis;

The earliest recognisable form of the ovum, or egg; present only in the ovaries of fetuses. Multiplies by the process of mitosis before developing into oocytes by the process of meiosis. The male equivalent (the spermatogonium) normally persists in the testicles till old age. Plural, oogonia.

The form of the ovum, or egg, produced in the ovaries of fetuses by oogonia that have begun the first part of the cell division known as meiosis (by which the chromosomes will eventually halve in number). Persists into childhood and adult life by containment in follicles. Gives rise to a secondary oocyte and the first polar body just before ovulation.

The form of the sperm cell (or male gamete) at the first stage of spermatogenesis, by which spermatogonia enter meiosis to start to reduce the number of chromosomes for the more mature sperm cells (the spermatozoa) that will eventuate.

The form of the sperm cell in the second stage of spermatogenesis (through which the sperm cells are formed in the testes), produced from primary spermatocytes in the first cell division of meiosis, and giving rise to spermatids, which have just half the normal cell's complement of chromosomes, through the second division of meiosis; enveloped by Sertoli cells in the testicular tubules.

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 replicating phase of the sperm cell in the testis, equivalent to the oogonium in the ovaries, but, unlike the oogonia, spermatogonia normally persist until old age. Located among the supporting cells (Sertoli cells) in the testicular tubules. Divides by the process of mitosis until it begins to undergo meiosis by changing into the primary spermatocyte, the first step in the process of sperm cell formation, or spermatogenesis. Plural: spermatogonia.

A trisomy for chromosome number 21, or an extra chromosome-21. This is the most common of the trisomies and gives rise to Down syndrome. Research has shown that the commonest source of the extra chromosome is a mistake in the first division of meiosis in the egg cell (during the many years it rests as a primary oocyte).