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.

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.

(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.