Kinship: Grandparents, Cousins and Other
Slekskap: Besteforeldre, søskenbarn og annen slektninger
This page is intended as a guide to the various family relationships.
Everyone has two parents, four grandparents, eight great grandparents, sixteen great-great grandparents (also known as 2nd great grandparents), 32 3rd great-grandparents, etc. The number doubles with each generation and quickly gets very large as one goes back in time. If you are mathematically inclined, the formula is x = 2n, where x is the number of grandparents and n is the number of generations from yourself. For example, Erlend paa Hassagher and Baard paa Lein are two of my 11th great grandfathers, so they are 13 generations back and are two of 213 (8,192) grandparents in that generation. Lars Brøskift is the only 12th great grandparent whose name I know, and he is one of 214 (16,384) grandparents in that generation. Go back 25 generations to Viking times, and there would be 225 (33,554,432 !) grandparents in that generation.
The numbers quickly
become staggering. While the math is right, the progression seems
counter-intuitive. One exceeds the number of people in Stadsbygd and Rissa
combined sometime between the 10th and 11th generations
and all the people in
Not only that,
mathematical modeling of common ancestry shows that this extends to all
Europeans and to a large number of non-Europeans as well. For a fascinating look at the mathematical
modeling of common ancestry, see the web site of Dr. Mark Humphrys of the
You share 50% of your genetic kinships with either of your parents, 25% with any of your grandparents, 12.5% with any of your great-grandparents, and 6.25% with any of your 2nd great-grandparents. Just as the number of grandparents doubles with each generation, the amount of ancestry you share with any one of them halves. Of course, since many are related to you by more than one line, you would have to add the contribution of each line to that person's total.
Your first cousins are the children of your parents' siblings, sharing two of your four grandparents with you. Second cousins are the grandchildren of your grandparent’s siblings, so they share two of your eight great-grandparents. Third cousins share two of your sixteen 2nd great grandparents. Fourth cousins share two of your 32 3rd great grandparents, and so on. Just as it is possible to have half brothers and sisters, you can also have half cousins. It is also possible to have double cousins. My grandfather and his two brothers married three sisters, so my second cousins are really double second cousins, sharing four of our eight great-grandparents -- the same degree of relationship as if we were first cousins.
Lastly, the child of your first cousin is your first cousin once (one generation) removed. Your child and that person would be second cousins (the children of first cousins are second cousins, the grandchildren of first cousins are third cousins, etc.). The grandchild of your first cousin is your first cousin twice removed, and so on. In reverse, the first cousin of your parent is your first cousin once removed, and the first cousin of your grandparent is your first cousin twice removed, and so on.
You share 100% of your genetic kinship with your identical twin, but 50% with other siblings, including fraternal twins. You share 25% with your grandchildren, nieces and nephews, uncles and aunts, and grandparents. You share 12.5% with grandnephews and grandnieces, first cousins, grand uncles and grand aunts, and great grandparents. You share 6.25% with your first cousins once removed, great grand uncles and great grand aunts, and with your great-great grandparents. To compute the genetic kinship with any degree of cousin, divide the genetic kinship of the one before by four. So, if 1st cousin = 12.5%, then 2nd cousin = 12.5%/4 = 3.125%; 3rd cousin = 3.125%/4 = 0.78125%; 4th cousin = 0.1953125%, and so on. Kinship gets “thin” rather quickly.