Genetic Genealogy and Consanguinity
Quora queries and answers about shared DNA
I’ve been seeing a lot of questions on “how much DNA do I share with x” questions on Quora lately. People seem to be curious about how closely related they actually are to their, for example, third cousin’s kids. A handy chart I’ve found and continue to share is this one:
It’s available on Wikimedia Commons/Wikipedia for anyone interested in more details (see the copyright notice at the bottom of the image).
The percentage of DNA shared is actually the percentage of DNA markers shared. All humans share about 99% of their DNA code. The slight difference includes markers that we track between generations (Y-haplogroups, mitochondrial markers, and ethnicity/population markers).
The image above gives percentage estimates - the average expected amount of shared DNA halves each step away from the proband (the point person, you). But due to the cellular processes involved in reproduction, there is actually a range of potentially shared DNA.
How big of a chunk is shared helps determine the relationship between you and the other person. These chunks are measured in centimorgans (cM) - which are actually units of relative likelihood to crossover. And they parse out as in the image below (another Wiki find):
Note the ranges. On Ancestry, the smallest possible match is 6 cM (or 6 million basepairs). 23andMe doesn’t state cM, but I’ve been told the percentage they give can be converted to cM by multiplying the percentage by 68 or using their chromosome browser tool.
So, how much DNA do you share with your third cousin? Almost as much as you do with a stranger.
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See more on mitochondrial haplogroups from ISOGG.
Have questions? Stop by Lineage Research on Quora.