The cells of the human body contain DNA (dezoxyribonucleic acid). This molecule encodes the genetic instruction that determines the development and functioning of every cell. Within cells, DNA is organised into smaller units called chromosomes. Somatic cells (i.e. cells of the body) have 46 chromosomes, and gametes (i.e. the sperm and the egg) contain 23 chromosomes. Two of the 46 chromosomes of the somatic cell are sex chromosomes, the remaining 22 chromosome pairs are called autosomes. Sex is determined by the sex chromosomes: XX in women and XY in men. A health egg contains one X chromosome and sperms contain either an X or a Y chromosome. This means that the sex of the progeny is determined by the sperm.
In PGD, different segments of the cells’ DNA content is analysed. Specific techniques are used to detect abnormalities affecting one or more genes, numerical or structural chromosomal abnormalities, and in certain justified cases, to determine the sex of the embryo. CGH, or comparative genomic hybridisation is a molecular method for analysing the entire set of chromosomes of the embryo; however, the high costs of this technology limit its application for the time being.