MEDICAL MATTERS: In human society, incest is taboo and cousin marriage frowned upon. This is not just from a moral and religious viewpoint, but also because close liaisons of this nature can promote the spread of gene-linked diseases.
Some of these conditions, such as muscular dystrophy and Huntington's chorea, can be directly inherited with incomplete penetrance (ie not all those who have the gene will be affected) while others are sex-linked and are carried by one or other parent.
In haemophilia, for instance, the responsible gene is carried by the mother, who does not manifest the condition herself: half her daughters will be carriers, and a son has a 50 per cent chance of being clinically affected.
Royalty is particularly prone to the disadvantages of marriage within closed circles. Traditionally, royal matches were made primarily for dynastic and political considerations, rather than for love and affection, within a very small and closed circle of suitable aristocrats. Once the necessary business was done, and heirs - and ideally a few spares - were produced in an era of high infant-mortality, kings were free to dally elsewhere while their mare-queens, in a staggering display of hypocrisy, were expected to remain chaste and demure.
As an example of the dangers of breeding too closely, look no further than that real-life ongoing soap opera known as the British royal family. But first: a brief history lesson. The Settlement Act of 1701 had deprived the legitimate Catholic Stuart heirs of the right to the throne. With the death of Queen Anne in 1714, the succession went to the nearest Protestant claimant, George, the Elector of Hanover, a rough, uncultivated German who spoke no English and seldom visited England.
Of necessity, he and his successors could marry only Protestant princesses. For over 200 years, British monarchs sought spouses among the aristocracy of northern Germany, in a rarefied gene pool where many were already closely related.
Germany was also a favourite hunting ground for spouses for Russia's tsars, leading to complicated and not altogether healthy links between British, German, Danish and Russian royalty. Queen Victoria (of whom more later) married her own first cousin, Prince Albert, while Queen Elizabeth II and her husband Prince Philip are third cousins (twice) through both Danish and British connections.
History does not judge the Hanoverian monarchs as having been very bright. The comic portrayal of Prince Georgie in television's Blackadder as an inbred, dim-witted Teutonic twit may not be too far from the truth.
By the early 20th century, European royalty was so closely knit that Britain's George V was first cousin to both Kaiser Wilhelm II of Germany and Russian Empress Alexandra on his father's side, and first cousin to Tsar Nicholas II on his mother's. However, judging by the results of the first World War, family solidarity counted for very little.
George III, famous for losing America, was prone to intermittent bouts of mental illness which medical historians have attributed to porphyria, a rare, inherited metabolic disorder characterised by intermittent abdominal pain, abnormal urinary discoloration similar to port wine, neurological symptoms and psychological disturbance.
He once began an address to the House of Lords, "My lords, ladies and peacocks . . ." On another occasion, in Great Windsor Park, he engaged in debate on European politics with an oak tree which he mistook for the king of Prussia. His treatment, which in an era before evidence-based medicine consisted of purging, bleeding, beating and physical restraint, was unlikely to have been helpful: eventually he became totally incapacitated and his duties were carried out by his son, the Prince Regent (Blackadder's Georgie).
Queen Victoria had several children, most of whom were decanted into the other monarchies of Europe, so that she became known as the grandmother of European royalty. What was not known then, but is now well recognised, is that she was a carrier of the haemophilia gene: the best-known of her descendants to suffer haemophilia was her great-grandson, Russian heir Tsarevich Alexis. Other European dynasties experienced inherited defects, if somewhat prosaic in comparison. Many of the Habsburgs of Austria had large, unsightly jaws and hare-lips; the Medicis, medieval rulers of Florence, were apparently martyrs to gout.
Today, we know a lot more about genetic disorders. Haemophilia and some metabolic disorders are treatable or their worst effects can be mitigated. In terms of prevention, identification of faulty genes and the concept of genetic engineering to replace them sounds promising but this science is very much in its infancy. But a caveat - as Bill Bryson points out in his marvellous A Short History of Nearly Everything - if everybody has two parents, four grandparents and so on, the hypothetical number of ancestors will soon reach impossibly large numbers, far greater than the number of humans that has ever existed throughout recorded time.
Therefore we are all distantly related.
Meanwhile, choose your breeding partner carefully - but you can never be 100 per cent certain that genetic problems can be avoided.
Dr Charles Daly is a GP practising in Dungarvan, Co Waterford.