Cousins share ancestors. Therefore when cousins marry, the number of ancestors of their descendants decreases; dramatically if close cousins. This because there are common areas for the fully expanded ancestral tree. This is true for first cousins as well as 10th cousins. Just more dramatic when first cousins marry. This sharing of branches (or branches appearing twice or more) is termed Pedigree Collapse or Endogamy. Populations that intermarried within the community, especially among close cousins, have this issue of pedigree collapse to a large degree. Royalty, in times of old, often had this issue. As did certain isolated populations that actually encouraged close marriages. Typical reported populations that have this collapse are Ashkenazi Jews, Acadians, Mennonites, and others,
A totally missed population in most reports of endogamous populations are the Parsi's of India. Parsi's, more than the other reported populations, have a strong demonstration of endogamy in their DNA test results. Anecdotal results seen so far show the average match for unrelated people is roughly near 2% or 150cM. Testers sometimes measure that their parents are related (higher than the average population; sometimes as high as 3%) but not more often as expected. There are no more instances of full-identical matching areas than other populations might exhibit either, it seems. Matching Segments number in the tens and the longest matching segment is often around 20cM. This all compared to the general European-descent population that has a matching floor of 0.1% and single segments of 7cM (i.e. basically at the limits of the testing methodology). Using traditional tools on traditional populations (see our Consanguinity charts), Parsi's measure a stronger match with each other when unrelated then the normal population does with a 3rd cousin. Hence, autosomal matching segment work is almost useless with them at the current time.
For those from populations or people that exhibit a high degree of endogamy (or pedigree collapse), or the appearance that their parents are related (e.g. first cousins marrying), you must use different techniques to extract results from DNA test / match information. Possibly techniques that will not be useful at all even when applied.
A totally missed population in most reports of endogamous populations are the Parsi's of India. Parsi's, more than the other reported populations, have a strong demonstration of endogamy in their DNA test results. Anecdotal results seen so far show the average match for unrelated people is roughly near 2% or 150cM. Testers sometimes measure that their parents are related (higher than the average population; sometimes as high as 3%) but not more often as expected. There are no more instances of full-identical matching areas than other populations might exhibit either, it seems. Matching Segments number in the tens and the longest matching segment is often around 20cM. This all compared to the general European-descent population that has a matching floor of 0.1% and single segments of 7cM (i.e. basically at the limits of the testing methodology). Using traditional tools on traditional populations (see our Consanguinity charts), Parsi's measure a stronger match with each other when unrelated then the normal population does with a 3rd cousin. Hence, autosomal matching segment work is almost useless with them at the current time.
For those from populations or people that exhibit a high degree of endogamy (or pedigree collapse), or the appearance that their parents are related (e.g. first cousins marrying), you must use different techniques to extract results from DNA test / match information. Possibly techniques that will not be useful at all even when applied.