Designating level of cousinship?

[royfarnol]

I don't see the logic in the numbers associated with these levels of cousinship. Is there something else taken into account that I don't know about? Thanks, Roy.

Shared cM Longest Block
4th Cousin 44.53 27.66
4th Cousin 34.31 22.47
3rd Cousin 50.16 21.89
3rd Cousin 47.81 21.80
4th Cousin 44.48 21.42
4th Cousin 39.82 20.65
4th Cousin 38.66 20.10
4th Cousin 34.35 20.05
4th Cousin 42.53 19.91
4th Cousin 42.54 19.21
3rd Cousin 46.43 18.56
3rd Cousin 45.52 18.13
4th Cousin 37.37 18.03
4th Cousin 40.91 17.68
3rd Cousin 50.96 17.38
4th Cousin 65.83 16.59

[josh w.]

I don't see the logic in the numbers associated with these levels of cousinship. Is there something else taken into account that I don't know about? Thanks, Roy.

Shared cM Longest Block
4th Cousin 44.53 27.66
4th Cousin 34.31 22.47
3rd Cousin 50.16 21.89
3rd Cousin 47.81 21.80
4th Cousin 44.48 21.42
4th Cousin 39.82 20.65
4th Cousin 38.66 20.10
4th Cousin 34.35 20.05
4th Cousin 42.53 19.91
4th Cousin 42.54 19.21
3rd Cousin 46.43 18.56
3rd Cousin 45.52 18.13
4th Cousin 37.37 18.03
4th Cousin 40.91 17.68
3rd Cousin 50.96 17.38
4th Cousin 65.83 16.59

Maybe they use a very arbitrary cutoff point for the combination of the two indices. All of your third cousins have a shared cM greater than 45.0 Only one fourth cousin does but with a lower longest block.

[josh w.]

Maybe they use a very arbitrary cutoff point for the combination of the two indices. All of your third cousins have a shared cM greater than 45.0 Only one fourth cousin does but with a lower longest block.

Because of adjustment for Ashkenazi endogamy, I have a fifth cousin with a shared cM of 150 and third cousins with shared cM of 80.

[josh w.]

Because of adjustment for Ashkenazi endogamy, I have a fifth cousin with a shared cM of 150 and third cousins with shared cM of 80.

and a third cousin with 50 shared cM.

[prairielad]

I believe the total number of segments, their sizes(cM),and SNP Total all come into play.
ie)3rd cousin
CHROMOSOME CENTIMORGANS MATCHING SNPS
2 1.38 500
3 1.85 500
3 2.81 700
5 1.93 500
6 1.5 4000
6 1.57 500
6 1.52 600
7 2.04 500
8 2.52 500
8 2.2 500
9 19.52 2795
12 2.46 500
17 2.61 500
20 1.38 500
45.29 13095SNPs

4th cousin2 1.38 500
3 2.61 600
6 1.16 1200
6 1.48 600
7 2.48 500
11 1.2 500
12 2.46 500
14 1.48 500
15 2.56 700
17 2.61 500
18 21.92 2700
19 2.3 600
43.64 9400SNPs

[royfarnol]

@Josh W & Prairielad

Thanks. I think you are both right.

In my experience I get the impression that all my cousin forecasts are over-optimistic but since they seem arbitrary I suppose there are a lot of cases where the forecasts are under-optimistic?

[josh w.]

I believe the total number of segments, their sizes(cM),and SNP Total all come into play.
ie)3rd cousin
CHROMOSOME CENTIMORGANS MATCHING SNPS
2 1.38 500
3 1.85 500
3 2.81 700
5 1.93 500
6 1.5 4000
6 1.57 500
6 1.52 600
7 2.04 500
8 2.52 500
8 2.2 500
9 19.52 2795
12 2.46 500
17 2.61 500
20 1.38 500
45.29 13095SNPs

4th cousin2 1.38 500
3 2.61 600
6 1.16 1200
6 1.48 600
7 2.48 500
11 1.2 500
12 2.46 500
14 1.48 500
15 2.56 700
17 2.61 500
18 21.92 2700
19 2.3 600
43.64 9400SNPs

Thanks. Now it is less mysterious. Is there a source of this information.

[prairielad]

Thanks. Now it is less mysterious. Is there a source of this information.

there are a few simple explanations in FAQ's
https://www.familytreedna.com/learn/...tions-cousins/

https://www.familytreedna.com/learn/...ionship-range/

https://www.familytreedna.com/learn/...sidered-match/

I don't think there is a in depth explanation of how FTDNA's algorithm works....

Data in my previous post was just taken from a family members chromosome browser data for a suggested 3rd and 4th cousin.