Don't get away with the idea that inbreeding is only long ago and far away. A certain amount of endogamy is expected if you live in a village without cars or buses; or in a wider community [Hutterites, Ashkenazim] where conventions limit the available partners. My Pal Johnnie, who grew up a Protestant in Tuam Co Galway, was faced with a choice of three potential opposite sex partners from his church; none of whom was less than 40 years old. When I knew him 25 years ago, he had turned sufficiently that he was dating a very nice Catholicker girl from Sligo whom he's met in Dublin.
An extensive [exec summ] and exhaustive [full paper] study of the 456,414 genomes from the UK BioBank has been carried out by a group from U. Queensland, Brisbane, AU. They analysed the status of each of 300,000 genetic variants SNPs in both copies [maternal and paternal] of these these peoples DNA. They found that the genomes of 125 of them or "1 in 3,652" people born in the United Kingdom between 1938 and 1967 show extreme inbreeding, indicating couplings between full siblings, a parent and a child, a grandparent and a grandchild, all of which are specifically forbidden by the Church of England's Table of Kindred and Affinity. Most SNP variants have two possibilities from the choice of four nucleotides A T C G. If a string of SNPs in a particular part of a maternally inherited chromosome
A - G - G - G - C - T - T - A -T - C - C - T - C - A - Tis also found in the equivalent part of Dad's teaspoonful
A - G - G - G - C - T - T - A -T - C - C - T - C - A - Tit could have happened by chance (there are only 4 possibilities at each place after all to the odds are about 4^15 = 1bn-to-1) but, as the duplication extends, this explanation becomes increasingly unlikely. They found that this ratio of 1/3500 is about the same rate of reports to the police of incest (11,200 cases in a population if 55 million). Acknowledging that reported cases of incest are likely only a fraction of actual cases of incest (everyone knows incest is frowned upon by polite society, so the perps aren't going to bruit it abroad?) it suggests to me that motherfuckers use condoms. I never thought I'd have occasion to utter that phrase! It turns out (I'm sure you knew this already, but I live a very sheltered life) there is a brand of condom called Motherfucker Rubbers.
In a sense this is like a genetic disease and its occurrence has about the same prevalence as Duchenne Muscular Dystrophy DMD (1:3500 live births); haemophilia (1:10,000); cystic fibrosis CF (1:2,000 in Ireland, less elsewhere).