Calculating Kinship

L. David Roper

(www.roperld.com)

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 one-half the father's and one-half 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 one-half of each parent's genetic material (but a slightly different one-half). 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/Uncle-Niece/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/uncle-niece/nephew sharing of genetic material is 2, the same as for cousins:

 

Father

 

Sibling1 (aunt/uncle)

 

 

-

 

 

 

Mother

 

Sibling2

 

 

 

 

Niece/Nephew (1/2)

 

 

 

 

 

Great Aunt/Uncle-Great 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/uncle-niece/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=2C+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/Uncle-Niece/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.

 

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