Most genetic disorder are autosomal recessive, they can be hidden in the genome without causing any severe health problems for generations. Examples are Sickle cell anemia, Cystic fibrosis, Lysosomal acid lipase deficiency, Tay-Sachs disease, Phenylketonuria, Mucopolysaccharidoses, Glycogen storage diseases, Galactosemia.
Two mutated SNPs of the same allele are needed to get the symptoms of these disorders; one is inherited from the mother and one from the father who both act as carriers.
The chance of such an unfortunate coincidence increases if the parents are closely related or if both belong to a relatively small community that practices endogamy and is based on a small founder population. The chance of a genetic disorder can be roughly estimated by the average length of shared DNA segments (IBD=Identical by descent) within a community.
Recently, Dienekes analyzed a few populations and determined the IBD length within the community (and between different communities, not shown here).
IBD score in cM (centiMorgan):