Introduction

DNA-DNA in situ hybridisation is widely used by geneticists as a tool for mapping genes on chromosomes. More recently, the development of chromosome-specific probes has enabled in situ hybridisation to be used to detect the presence of specific chromosomes in interphase nuclei and to determine the number of copies of chromosomes present. This has been applied to clinical prenatal diagnosis of human genetic diseases that involve numerical chromosome aberrations and in cases where only boys are at risk for X-linked disorders. This approach has also been useful as an experimental tool for detecting polyploid interphase nuclei. My own work in this field has been confined to in situ hybridisation with a Y chromosome DNA probe to sex the human fetus and pre-embryo. These studies illustrate the possibilities of using DNA-DNA in situ hybridisation both for prenatal or preimplantation diagnosis and for experimental investigations of ploidy.

Prenatal diagnosis

At present, samples of cells for prenatal diagnosis may be obtained by amniocentesis, chorionic villus sampling or fetal blood sampling. In the future it may also be possible to sample cells of a preimplantation stage embryo (or pre-embryo) for genetic tests before the pre-embryo is returned to the uterus to continue development. This possibility of preimplantation diagnosis is discussed in a later section.

Amniocentesis involves the aspiration of 10–20 ml of amniotic fluid at 16–20 weeks gestation. At 18–19 weeks, 20 ml of amniotic fluid contains about 1 million cells (Gosden & Gosden, 1985) of heterogeneous origin and yields approximately 7 µg of DNA (Old, 1986).