If your ancestors married their relatives, you may appear more closely related to your matches than you actually are. This is not uncommon. If you are part of an endogamous population, such as Ashkenazi Jews, you will likely be more related. Endogamous means that a group intermarried either due to geography or society more often with one another than with other groups. The impact of this on Family Finder results varies. It depends on how closely related the family members were, the frequency of marriages between your ancestors, and how recently the marriages occurred.
There are two factors in sharing between genetic cousins with multiple lines of descent from a common ancestor or multiple common ancestral lines. The first one is increased overall sharing.
The amount of increased average sharing is additive. It is calculated by adding together the amounts of expected sharing for each known relationship.
|4th Cousin Once Removed||0.098%|
Thus, if you share two lines of descent with a cousin at the 4th cousin level, one line at the 4th cousin once removed level, and one line at the 7th cousin level, your expected sharing would be 0.195% + 0.195% + 0.098% + 0.003% = 0.491% rather than the 0.195% of a 4th cousin who is related in only one way. Keep in mind that the expected percentages are averaged. Any one relationship could have significantly more or less sharing.
The second factor when more than one line of descent in common is many smaller common identical by descent segments rather than the expected levels of some large, medium, and small segments. Over time and depending on the degree of intermarriage in the population. These blocks may become common to a significant percentage of the population group.
Note: Beginning on April 21, 2011, our bioinformatics team has implemented a set of calculations that detects and compensates for interrelatedness in the Ashkenazi Jewish population.