Chapter 21, Problem 20P

Tristan da Cunha is a group of small islands in the middle of the Atlantic Ocean. In 1814, a group of 15 British colonists founded a settlement on these islands. In 1885, 15 of the 19 males on the island were lost in a shipwreck. In the late 1960s, four cases of retinitis pigmentosa, which progressively leads to blindness, were found among the 240 descendants of these settlers remaining on the island. The frequency of retinitis pigmentosa in Britain is about 1 in 6000. Explain the high incidence of this disease on Tristan da Cunha relative to that seen in Britain.