If you read a book like Principles of Population Genetics, or know a little animal breeding, you know inbreeding has some serious consequences. The UK Biobank turns out to have about ~100 individuals who are the products of extreme inbreeding (EI). That is, they are the offspring of parent-child pairings or full-sibling pairings, as inferred from the runs of homozygosity in their genomes (there are lots).
Intuition, theory, and a few results tell us that these individuals will have issues. Genomics confirms. Extreme inbreeding in a European ancestry sample from the contemporary UK population:
In most human societies, there are taboos and laws banning mating between first- and second-degree relatives, but actual prevalence and effects on health and fitness are poorly quantified. Here, we leverage a large observational study of ~450,000 participants of European ancestry from the UK Biobank (UKB) to quantify extreme inbreeding (EI) and its consequences. We use genotyped SNPs to detect large runs of homozygosity (ROH) and call EI when >10% of an individual’s genome comprise ROHs. We estimate a prevalence of EI of ~0.03%, i.e., ~1/3652. EI cases have phenotypic means between 0.3 and 0.7 standard deviation below the population mean for 7 traits, including stature and cognitive ability, consistent with inbreeding depression estimated from individuals with low levels of inbreeding. Our study provides DNA-based quantification of the prevalence of EI in a European ancestry sample from the UK and measures its effects on health and fitness traits.
The two major caveats are I’d put out there is that UK Biobank sample is a bit healthier and better educated than the average British person, and, the rates of individuals who were adopted is considerably higher in people who are products of EI than is the norm. In other words, these people are from an atypical sample, and they are themselves somewhat atypical (since they were given up for adoption they likely had no idea they were the products of EI).
While reading the first lines my first thought was: Which kind of people are more likely for extreme inbreeding in a Western cultural environment?
Well, definitely not above average people. Its more likely in an environment which is per se lower in stature and intelligence on average. The value of the whole approach is therefore very limited.