Abnormalities, qualities and disorders already present in the fertilized ovum, and thus handed down by one or both parents. Hereditary qualities are passed on in the genes in the nucleus of every cell, which in their turn are bound to chromosomes. The occurrence of hereditary disorders is determined by various factors. Some hereditary conditions are caused by an abnormal number of chromosomes, as in Down’s syndrome, for example. Most other abnormalities caused by an abnormal number of chromosomes are not compatible with life. A greater number of hereditary conditions are caused by an abnormality in the shape of a chromosome. Some disorders are dominant, as is the case with rhesus positive children with haemolytic disease (rhesus incompatibility). One abnormal gene from one parent is then all that is necessary to bring out the disease. Sometimes both parents are carriers of a hereditary disorder of which they show no symptoms (recessive traits), but a child receiving a pair of the affected genes contracts the disease. This is sometimes the case in mucoviscosidosis, some cases of muscular dystrophy and many metabolic disorders. Some hereditary disorders are sex-linked, such as haemophilia, colour blindness, Turner’s syndrome and Klinefelter’s syndrome. Academic research is constantly discovering new hereditary disorders. Diagnosis is by external symptoms, laboratory tests, amniocentesis and chromosome tests.