Problem of hereditary disease in isolated island populations

Remote islands hold a special fascination for some of us. We might imagine pirates and buried treasure or simply dream of a peaceful holiday. Some of us travel to observe the unique wildlife of such places while others study their human populations. On two of the most isolated islands on earth researchers have demonstrated the effects of heredity on the prevalence of medical conditions.

Tristan da Cunha, a remote island in the South Atlantic, was only settled permanently in 1817. A small group led by Corporal William Glass elected to stay on the island when a military garrison intended to guard Napoleon on nearby St Helena was withdrawn.

The community flourished successfully while coping with isolation but then, following a volcanic eruption in 1961, the whole population was evacuated to safety in Britain. Many of them returned later, preferring isolation to crowded Britain.

The islanders were found to have a high frequency of retinitis pigmentosa, an inherited disease causing progressive blindness. Researchers discovered that one of the original settlers had been heterozygous for the retinitis pigmentosa mutation.

The original population of 15 had increased 17-fold during its period of isolation but the frequency of the allele remained constant. In 1961 there were four homozygotes with two copies each of the gene and nine heterozygotes each with a single copy.

A hereditary condition has also been found among the inhabitants of Easter Island, a tiny dot in the Pacific Ocean. A study of genotypes, blood groups and language has shown that the island was originally settled by a group of Polynesians.

The islanders believe that they were led ashore around 1,500 years ago by their king, Hotu Matu’a. Shortly before he died some years later, the king divided the island between his sons, who went on to found six family groups or clans.

The most powerful clan, the Miru, provided all the kings, priests and other leaders but remained genetically isolated from the other clans through inbreeding. Archaeologists who examined skeletons of the Miru noted an odd deformity in the shape of each “royal” patella and concluded that there was a significant genetic contribution to the effect.

The population of Easter Island was estimated to be over 10,000 people before the first Europeans arrived in 1722 but they were almost wiped out by civil war, slave traders and introduced diseases such as smallpox. By 1868 only 112 people remained on the island and the old clan system was gone.

Nowadays the resident population is almost 5,000, but no pure-blooded islanders remain. The curious royal kneecap seems to have become extinct as well.

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