L. David Roper
Often in genealogy two individuals are related in more than way. It is desirable to have a way to combine all relationships into a measure of the total kinship. Also, the names of kinship (e.g. 3rd cousin two generations removed) do not give a clear indication of the amount of kinship. It is desirable to have a clear numerical way to compare all kinds of kinships.
A numerical measure of kinship is described in this article. It is based on the simple fact that when parents have children, the genetic material of the siblings are onehalf the father's and onehalf the mother's. Siblings do not have identical characteristics (unless they are identical twins), because some mixing of the genes occurs (called recombination) between chromosome pairs when eggs and sperm are produced, which is different for different siblings, except for identical twins. However, we assign the number of 1 to the relationship of siblings, since they all have onehalf of each parent's genetic material (but a slightly different onehalf). Then, all other numerical relationship will be given relative to the relationships of siblings.
Assigning
a numerical kinship to two people will be based on the fraction of a progenitor
couple's genetic material that the two people share. I.e., if one person has
1/2 of the progenitor genetic material and the second person has 1/4, then the
numerical kinship is 1/4. I take the inverse of this number to quantify the
relationship; in this case it is 4. Therefore, the larger the number is the
less is the relationship.
When two
people are kin in more than one way, the total kinship is the sum of the
separate kinships.
First
Cousins: Since
cousins are siblings of siblings with one parent not an original sibling,
cousins each have 1/2 of the progenitor couple's genetic material. So, the
ratio of siblings to first cousins sharing of genetic material is 2:



Cousin1 (1/2) 
Father 

Sibling1 (1) 





Mother 

Sibling2 (1) 




Cousin2 (1/2) 




Double Cousins: Double cousins occur when brothers and sisters marry
sisters and brothers. Thus, double cousins have the same two sets of
grandparents. So, the ratio of siblings to double cousins is 1, the same as for
siblings. (Actually the double cousins sharing of genetic material will be
somewhat less, because recombination has occurred twice instead of once in the
case of siblings.):
Father1 

Sibling11 









Mother1 

Sibling12 






Double Cousin (1/2+1/2) 

Double Cousin (1/2+1/2) 
Father2


Sibling21 









Mother2 

Sibling22 



Second Cousins: Second cousins have one further dividing of genetic
material beyond cousins. So, the ratio of siblings to second cousins sharing of
genetic material is 4:




Second Cousin1 (1/4) 



Cousin1 (1/2) 

Father 

Sibling1 (1) 







Mother 

Sibling2 (1) 





Cousin2 (1/2) 





Second Cousin2 (1/4) 





First Cousins One Generation Removed: Here one person has ½ and the other has ¼ of the
progenitors’ genetic material. The smallest number is the shared genetic
material. So, the ratio of siblings to 1stC1GR sharing of genetic material is
4, the same as for second cousins. (Actually, it is somewhat more, because
recombination has occurred once for one person and twice for the other instead
of twice for each in the case of second cousins.)



Cousin1 (1/2) 

Father 

Sibling1 (1) 







Mother 

Sibling2 (1) 





Cousin2 (1/2) 





1st Cousin Once Removed (1/4) 





First Cousins Two Generation Removed: Here one person has ½ and the other has 1/8 of the
progenitors’ genetic material. The smallest number is the shared genetic
material. So, the ratio of siblings to 1stC1GR sharing of genetic material is
8.



Cousin1 (1/2) 


Father 

Sibling1 (1) 









Mother 

Sibling2 (1) 






Cousin2 (1/2) 






1st C. Once Removed (1/4) 






1st C. Twice Removed (1/8) 






First Cousins Three Generation Removed: Here one person has ½ and the other has 1/16 of the
progenitors’ genetic material. The smallest number is the shared genetic
material. So, the ratio of siblings to 1stC1GR sharing of genetic material is
16.
Second Cousins One Generation Removed: Here one person has ¼ and the other has 1/8 of the
progenitors’ genetic material. So, the ratio of siblings to second cousins
sharing of genetic material is 8, the same as third cousins or first cousins
two generations removed:




Second Cousin1 (1/4) 




Cousin1 (1/2) 


Father 

Sibling1 (1) 









Mother 

Sibling2 (1) 






Cousin2 (1/2) 






Second Cousin2 (1/4) 






2nd C. Once Removed (1/8) 






Half Siblings: Two people that have only one common parent share only ½
of their genetic material. So, the ratio of siblings to half siblings sharing
of genetic material is 2, the same as for cousins. (Actually it is somewhat
more, because there has been only one recombination, instead of two in the
Parent1 

 
Half Sibling1 (1/2) 
Common Parent 


Half Sibling2 (1/2) 
Parent2 

Aunt/UncleNiece/Nephew: Anuts/uncles (siblings) have entirely the progenitors’
genetic material and nieces/nephews have ½ of the progenitors’ genetic
material. So, the ratio of siblings to aunt/uncleniece/nephew sharing of
genetic material is 2, the same as for cousins:
Father 

Sibling1 (aunt/uncle) 

 



Mother 

Sibling2 




Niece/Nephew (1/2) 




Great Aunt/UncleGreat Niece/Nephew: Great aunts/uncles have entirely the progenitors’ genetic
material and great nieces/nephews have 1/4 of the progenitors’ genetic
material. So, the ratio of siblings to aunt/uncleniece/nephew sharing of
genetic material is 4, the same as for first cousins one generation removed or
second cousins:
Father 

Sibling1 (aunt/uncle) 


 




Mother 

Sibling2 





Niece/Nephew (1/2) 





Great Niece/Nephew (1/4) 





Cousins Kinship Chart: We can draw a chart of the numerical kinship for cousins.
The number in the chart is the ration of the sibling kinship to the kinship
given. I.e., a larger number indicates a smaller kinship. The shared genetic
material of the progenitor couple is the inverse of the number given. A formula
that fits all the cousin kinships is K=2^{C+G}, where C=cousins level
and G=generations removed.
Cousin order/Gen. Rem. 
0 
1 
2 
3 
4 
5 
1 
2 
4 
8 
16 
32 
64 
2 
4 
8 
16 
32 
64 
128 
3 
8 
16 
32 
64 
128 
256 
4 
16 
32 
64 
128 
256 
512 
5 
32 
64 
158 
256 
512 
1024 
6 
64 
128 
256 
512 
1024 
2048 
Aunt/UncleNiece/Nephew Chart: The following is a chart of the numerical kinship
for aunts/uncles and nieces/nephews:
Aunt/Uncle Great Level 
Kinship 
0 
2 
1 
4 
2 
8 
3 
16 
4 
32 
5 
64 
6 
128 
Multiple Kinships: One can use these charts to calculate the net kinship of
two people who are kin in more than one way. Just add the inverses of the
numbers for the different kinships, and then take the inverse of the result.
E.g., if two people are both first and second cousins, the total kinship is
½+¼=3/4 for a kinship number of 4/3=1.33.