# Welcome to the Relatedness Calculator

## Enter relative:

Result for ** identical twin's children**

Relatedness coefficient: **50%**

Degree of relation: **3**

The relatedness between two people is expressed with two measures:

**Relatedness coefficient:**what percentage of your genes you share.**Degree of relation:**how far you are from that person in your family tree.

So, your **identical twin's child**
is **50%** related to you
and **3** steps removed from you in your family tree.

Did you know...

- The curious case of double cousins occurs when two siblings from one family each marry two siblings from another family. Double cousins share 25% of their genes — the same as grandparents/grandchildren and half-siblings. No incest involved, but it's still kind of weird!
- Although it is illegal in many places, over 10% of marriages worldwide are between cousins. Famous cousin-marriers include Charles Darwin, Jerry Lee Lewis, and Shelbyville Manhattan.
- In terms of relatedness, you are just as likely to "take after" your uncle (25%) as your grandfather (25%).
- "Once removed," "twice removed," etc. refers to the distance up or down your family tree relative to a cousin. For instance, a "second cousin, once removed" could be either your second cousin's child or your parent's second cousin.
- Fraternal twins have the same relatedness coefficient as regular siblings (50%), but identical twins share 100% of their genes. So you're as closely related to your identical twin's children (50%) as to your own children (50%)!
- If there has been incest in your ancestry, you can calculate your relatedness coefficient to other family members by adding in any extra coefficients due to the incest. For instance, if your parents are cousins, then your relatedness to your father is 56.25%, because he's your father (50%), as well as your mother's cousin (6.25%).
- Despite what you may have seen on Futurama, it is not possible for Fry to be his own grandfather. He would need to share 125% of his genes with himself.

Richard Dawkins gives a great explanation of how to calculate
relatedness coefficients in *The
Selfish Gene*:

First identify all the common ancestors of A and B. For instance, the common ancestors of a pair of first cousins are their shared grandfather and grandmother. Once you have found a common ancestor, it is of course logically true that all his ancestors are common to A and B as well. However, we ignore all but the most recent common ancestors. In this sense, first cousins have only two common ancestors. If B is a lineal descendant of A, for instance his great grandson, then A himself is the ‘common ancestor’ we are looking for.

Having located the common ancestor(s) of A and B, count the generation distance as follows. Starting at A, climb up the family tree until you hit a common ancestor, and then climb down again to B. The total number of steps up the tree and then down again is the generation distance. For instance, if A is B’s uncle, the generation distance is 3. The common ancestor is A’s father (say) and B’s grandfather. Starting at A you have to climb up one generation in order to hit the common ancestor. Then to get down to B you have to descend two generations on the other side. Therefore the generation distance is 1 + 2 = 3.

Having found the generation distance between A and B via a particular common ancestor, calculate that part of their relatedness for which that ancestor is responsible. To do this, multiply 1/2 by itself once for each step of the generation distance. If the generation distance is 3, this means calculate 1/2 x 1/2 x 1/2 or (1/2)^3. If the generation distance via a particular ancestor is equal to g steps, the portion of relatedness due to that ancestor is (1/2)^g.

But this is only part of the relatedness between A and B. If they have more than one common ancestor we have to add on the equivalent figure for each ancestor. It is usually the case that the generation distance is the same for all common ancestors of a pair of individuals. Therefore, having worked out the relatedness between A and B due to any one of the ancestors, all you have to do in practice is to multiply by the number of ancestors. First cousins, for instance, have two common ancestors, and the generation distance via each one is 4. Therefore their relatedness is 2 x (1/2)^4 = 1/8. If A is B’s great-grandchild, the generation distance is 3 and the number of common ‘ancestors’ is 1 (B himself), so the relatedness is 1 x (1/2)^3 = 1/8.