That's parent/child, full sibling, grandparent/grandchild, and uncle/niece level. As we didn't need to be told, it's extremely bad for the offspring.

See:
https://www.nature.com/articles/s41467-019-11724-6


"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 numbers are very low, thank goodness, although I do think the actual numbers might be a bit higher. This is an upper economic group we're talking about and probably these children were adopted and had/have no idea of the circumstances of their conception, while the available data says that it is more present at lower quintiles.

When we're talking about parent/child, especially, it's really disgusting and is no doubt child abuse on the part of the biological father.

Uncle/niece marriage is still permitted in the Near East I think, and repeated first cousin marriage over hundreds of years could bring the inbreeding co-efficient up to this level. Not a good idea, imo.

See also:
https://www.newsweek.com/extreme-inb...impact-1457359

"The team found people there were a host of negative health effects associated with extreme inbreeding. They had a shorter stature, reduced cognitive ability and lower fertility. They also had reduced lung function and, overall, appeared to have a 44 percent increased risk of disease of any kind, in comparison to the general population."

"
Researchers said there are a number of limitations to their study. Generally, people who submitted their genetic information to the biobank were healthier than average and have a higher level of education: "Highly inbred individuals who suffer severe health consequences may be less likely to participate in a study such as the U.K. Biobank," the authors said in a statement. "Therefore, our estimate of the prevalence might be too low."

"
They found the prevalence of extreme inbreeding was one in 3,652 participants. Estimates for extreme inbreeding in England and Wales—gathered through police reports of incest offences—is one in 5,247."

So, as one would expect, it's not always reported. Plus, not all child sexual abuse by a relative doesn't always result in offspring.