Negative effect of shared ancestors
Having ancestors that are too closely related may negatively influence an individual's height and thinking skills, a genetic study has shown.
The effect of inheriting pairs of identical genes means children of first cousins are likely to be 1.2 centimetres (0.5 in) shorter than those whose DNA is more mixed, scientists found.
It was also predicted to have a significant impact on mental ability and educational attainment, as well as lung function.
But surprisingly, factors such as blood pressure and cholesterol that influence the risk of heart disease, diabetes and other conditions were not affected by genetic diversity.
The findings, based on genetic data of more than 300,000 individuals from around the world, suggest that natural selection is driving people to become cleverer and taller.
In contrast, risk factors for complex diseases that occur late in life do not appear to be major players in the human evolutionary story.
Dr Peter Joshi, from the University of Edinburgh's Usher Institute, said: "Our research answers questions first posed by Darwin as to the benefits of genetic diversity. Our next step will be to hone in on the specific parts of the genome that most benefit from diversity."
Genes come in pairs, one of which is inherited from each parent. The scientists looked at the entire genetic make-up of 354,224 individuals to pinpoint instances where identical copies of genes had been inherited from both a person's mother and father.
Where large amounts of the genome, or genetic code, was "homozygous" - containing identical gene pairs - this was an indication of more closely related ancestors.
The information was used to estimate the probability of being homozygous at any site in the genome, and the effect this had on 16 biomedical traits.
The traits included height, lung function, general cognitive ability, and educational attainment as well as 12 health-related factors such as cholesterol level and blood pressure.
Statistically significant correlations were seen between increased homozygosity and height, mental test performance, educational attainment and lung function.
Among the effects were a 1.2 centimetre (0.5 in) reduction in height and 10 months less educational attainment for the offspring of first cousins, who were also likely to experience impaired lung function and general thinking ability.
The researchers wrote in the journal Nature: "Since directional dominance is predicted for traits under directional evolu tionary selection, this study provides evidence that increased stature and cognitive function have been positively selected in human evolution, whereas many important risk factors for late-onset complex diseases may not have been."
Interbreeding has long been taboo in most sections of human society, with the notable exception of ruling dynasties.
Charles II of Spain, the last of the Spanish Hapsburgs, is often cited as the ultimate example of its potentially tragic consequences.
Kings of the Spanish Hapsburg dynasty, which ruled from 1516 to 1700, frequently married close relatives including nieces and first cousins.
At the end of the line was Charles II who suffered from an appalling array of physical and mental handicaps. He had a large misshapen head, his jaw stood out so much that he was unable to chew, and his swollen tongue meant he was barely able to speak. He was mentally retarded, and did not learn to speak until the age of four or walk until eight. In addition he was w racked by epileptic seizures.
The hapless Hapsburg died at the age of 39 having fathered no children despite being married twice.
Interbreeding has also been a feature of British royal dynasties, especially the House of Hanover. Kings George III, George IV, and William IV all married first, second or third cousins. The last Hanovarian monarch, Queen Victoria, famously married her first cousin Albert.
A gene for haemophilia carried by Queen Victoria led to the premature death of a number of her descendants .