The stuff of which genes are made. DNA is an abbreviation of deoxyribonucleic acid, a molecule made of a variable sequence of units, the nature and order of which forms the genetic code. DNA is located chiefly in the chromosomes, which form a cell's nucleus. A small amount of DNA (coding for about 13 genes) is found in the mitochondria (for which see also mtDNA).

Testing for the presence or absence of a particular sequence of DNA using molecular methods such as the polymerase chain reaction or, increasingly, with DNA arrays, or DNA chips.

Mitochondrial DNA. The small amount of DNA found in the mitochondria. mtDNA is the genetic code for 13 proteins involved in metabolism. Because mtDNA is much less stable than the DNA in the chromosomes (where the huge bulk of genetic code is stored), mutations accumulate with time at a greater rate than that for chromosomal DNA -- and eventually limit how well a particular cell or tissue can function with increasing age. Because all of the mtDNA you have you inherited from your mother (you derive your mitochondria from the egg you came from), we could have an explanation: (1) why all eggs are formed before birth (so the mitochondria don't have to keep on dividing too often and risking genetic errors); and (2) why miscarriages and infertility get commoner with age (as the egg's several thousand mitochondria begin to succumb to genetic errors).

The mixture of a single (but double stranded) long string of genetic material (DNA) wound around supporting proteins. There are 46 chromosomes (23 pairs) in every normal human cell (other than the germ cells). Each cell therefore contains all the genetic information needed to make a human being. But it's only in the first few days of the embryo that all of a cell's DNA is accessible: once cells differentiate to have special purposes only the DNA they need remains unmasked. Chromosomes are located in the cell's nucleus and come in pairs, so that each cell has two alleles of each gene.

(CGH) A molecular DNA diagnostic technique whereby a set of chromosomes (a genome) is compared with a standard set using different colored dyes (typically red and green), so that any areas that do not exactly match will appear green or red instead of brown; a sensitive technique that can be applied to preimplantation genetic diagnosis as well as to chorionic villous sampling, or CVS, and to amniocentesis for prenatal diagnosis, yielding results much faster than is possible with a formal karyotype and more comprehensively than is possible with fluorescent in situ hybridisation.

The part of a cell that is not the nucleus (the nucleus contains the chromosomes). The cytoplasm is contained by the cell's plasma membrane and contains all the other cellular structures, including the mitochondria. Genetic inheritance is mostly by way of the nucleus (with a contribution from mother and father); a small part is by way of the cytoplasm (with a contribution only from the mother). It is the cytoplasm of the egg (as a secondary oocyte) into which a sperm cell (spermatozoon) is injected in the process of intracytoplasmic sperm insertion. See also mtDNA.

Known as "FISH". A form of DNA testing for genetic diagnosis in which a special region of a chromosome is stained with a dye that emits colored light when exposed to ultraviolet light. For example, a marker for chromosome 21 will normally show two spots of light, whereas three spots of light would indicate trisomy 21 (Down syndrome). Useful because the technique is accurate with just one cell, making diagnosis possible in an IVF embryo before transfer (see preimplantation genetic diagnosis), and speeding up prenatal diagnosis while awaiting a full karyotype. Being replaced in some labs by comparative genomic hybridisation.

The commonest cause of severe mental retardation in males, caused by a long "triple repeat sequence" in a part of a gene for intelligence located on an X-chromosome, rendering this sex chromosome susceptible to breakage, which can be revealed either in a karyotype performed under special laboratory conditions or using a special PCR DNA test; females have two X-chromosomes and so may be carriers for this devastating condition, which will affect one-in-two of their male children; the carrier state in females can have clinical effects, namely a susceptibility to reduced numbers of ovarian follicles and hence primary ovarian failure and premature menopause, so should be looked for before assisted conception attempts are made in these clinical circumstances.

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.

A full set of DNA code (a full set of genes) for a whole animal or organism, or for a defined component of an organism, such as a mitochondrion.

(PCR) A method of amplifying (increasing in number) a single piece of DNA (the stuff of genes) to get enough of it to analyze physically or chemically for DNA testing. A machine is used that uses alternate cycles of high temperature (to separate DNA's two strands) and medium temperatures (to combine each of the two single strands with free nucleic acids to make new complementary strands), doubling the amount of DNA in the machine's soup each cycle. A few hours in the machine automatically creates millions of identical DNA molecules from just one specimen. Needless to say, it's extremely important that you start with the right bit of DNA, so you're not inadvertently amplifying a bit of contamination! In the film Jurassic Park, PCR was used to amplify bits of dinosaur DNA recovered from the bellies of contemporary insects, which had been trapped and preserved for millions of years inside pieces of amber. A biopsy of a single cell from an embryo after IVF can, with PCR, produce enough DNA to test it for certain genes that cause serious genetic disease.

(SCSA) A measurement of the proportion of sperm cells that have damaged DNA. Fewer than 15% is consistent with normal fertility; 15%-30% can result in subfertility; and more than 30% may cause sterility or recurrent miscarriage.