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NK19191
07-12-2013, 02:05 PM
How accurate is the information below? What is the best way to calculate your relationship to your relatives?


Match outcomes:

10 cMs or higher indicates a match usually within 5 generations (4th or more recent cousins)* - better chance to find a match with a highly developed genealogy

5-9 cMs can cover a range of 6 to 11 generations (5th to distant cousins)* later generations may include low level matches that show you probably share a common ancestor but may not be able to effectively trace genealogically

Below 5 cMs (Mb) gives ancient ancestry

A generation can run an average of every 2535 years.


Ranges of total centimorgans of IBD segments expected, based on family relationship

Parent/child: 3539-3748 centimorgans (cMs)
1st cousins: 548-1034 cMs
1st cousins once removed: 248-638 cMs
2nd cousins: 101-378 cMs
2nd cousins once removed: 43-191 cMs
3rd cousins: 43-ca 150 cMs
3rd cousins once removed: 11.5-99 cMs
4th and more distant cousins: 5-ca 50 cMs


Ranges of the number of shared IBD segments based on family relationship

Parent/child: 23-29
1st cousins: 17-32
1st cousins once removed: 12-23
2nd cousins: 10-18
2nd cousins once removed: 4-12
3rd cousins: 2-6?
3rd cousins once removed: 1-4
4th and more distant cousins: 0-2

According to an article , The Spittoon, under a simple model where a family has 2-3 children, you would have about 190 third cousins, 940 fourth cousins and a about 4,700 fifth cousins. Therefore, it would not be surprising that among your 4,700 fifth cousins, some of them and their families happened to travel to other continents.

ilmari
07-13-2013, 09:22 PM
The examples are accurate, but if you come from a population that has quite a bit of pedigree collapse, as I do, this doesn't work as well.

To explain, many of my cousins have large amounts of DNA in common with me, but we are much more distantly related in real life. The reason is because we share so many common ancestors and sometimes have connections on my mother's side as well as my father's side that it throws this off.

IF you are matching with someone that ONLY has one of their family lines coming from a specific lineage and you also share only that specific lineage, it is usually quite accurate.

J Man
07-13-2013, 11:30 PM
The examples are accurate, but if you come from a population that has quite a bit of pedigree collapse, as I do, this doesn't work as well.

To explain, many of my cousins have large amounts of DNA in common with me, but we are much more distantly related in real life. The reason is because we share so many common ancestors and sometimes have connections on my mother's side as well as my father's side that it throws this off.

IF you are matching with someone that ONLY has one of their family lines coming from a specific lineage and you also share only that specific lineage, it is usually quite accurate.

I have noticed this with my Finnish matches as well even though I am only 25% Finnish by known ancestry. I tend to share multiple small segments with Finns in the 5-7 cM range. What I mean by this is that with many of my Finnish matches I share more than one segment with them whereas with my Irish/British and Italian matches I usually only tend to share one segment.