All of this raises the intriguing prospect of genetic testing that could tell us if we, or our children, are at high risk of developing one of over 3,000 known hereditary diseases. But it is not quite that simple.

▲ Prenatal testing for numerous inherited diseases is becoming possible, as “markers” for those diseases are found in human DNA. A highly sensitive and very rapid prenatal test for sickle-cell anemia has been introduced.