(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.

Sampling the fluid from the amniotic cavity after about 14 or 15 weeks of pregnancy. Cells from the fetus can be set up in culture for a karyotype, or for special diagnoses can be examined more quickly by fluorescent in situ hybridisation (FISH), comparative genomic hybridisation (CGH) or by polymerase chain reaction (PCR). Other substances in the amniotic fluid (such as alpha fetoprotein, or AFP) can be measured to indicate whether the fetus is normal or not.

Congenital (present from birth) absence of the two vasa deferentia, which conduct sperm from the testes to the ejaculate; hence a cause of obstructive azoospermia. Because the vas deferens is usually affected on both sides, the word "bilateral" is often inserted, as CBAVD. Infertility is inevitable, but can be overcome using in vitro fertilisation with testicular sperm extraction. The cause, however, is usually the presence among the man's genes of one of the serious alleles for the genetic disease cystic fibrosis (which, having recessive inheritance, results when there are two such alleles present); it can also come about when there are one or two of the less serious abnormal alleles for this condition. The more common of the abnormal alleles should therefore be screened for (using a specially set up polymerase chain reaction on white blood cells): should an abnormal allele be present (delta F508 is the most common seriously abnormal one), the woman should be screened too, to predict the chance of cystic fibrosis occurring in the offspring.

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.