How Much DNA Do Second Cousins Share?

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Do you want to know how much DNA second cousins should share?  What about half-second cousins?  In this post, I’ll tell you everything you need to know about shared DNA between second cousins, including whether it is possible for second cousins to share no DNA.

If you have second cousins on your DNA match list, consider yourself very lucky.  True second cousins share at least one great-grandparent with us, and so they can be a great source of information about our family tree.

Even though most people don’t grow up knowing their second cousins, they aren’t really distant at all.  In some families, second cousins have great familial relationships.

To make sure that we are all on the same page, I want to make sure to define what I mean by a second cousin in this post:

• Second cousin:  A person who shares two great-grandparents with you
• Half-second cousin:  A person who shares one great-grandparent with you
• Second cousin DNA match:  A person whose relationship with you is estimated to be second cousin, because their DNA falls within the typical range of shared DNA for second cousins

How much DNA do second cousins share?

Generally speaking, second cousins will share between 75-360 centimorgans (cMs) of DNA.  A centimorgan (not to be confused with a centimeter) is a term used to measure the genetic distance between two positions on a chromosome – in other words, the length of a shared DNA segment.

I always prefer to use the number of shared centimorgans to describe shared DNA instead of the percentage that is reported by certain DNA testing companies.  To me, the number of centimorgans gives more nuance.

Anyhow, without further ado:

• Two people who are confirmed full second cousins who share both great-grandparents should share between 75-360 centimorgans (cMs).

There have been a few reported cases of outliers, or exceptions to this range, ranging from as low as about 50 cMs to as high as 500 cMs, but I am not sure how well-documented these cases are, so I would recommend to stick to the 75-360 cM range.

Do half-second cousins share less DNA?

Half-second cousins will share, on average, about 50% less DNA than full second cousins.  It’s really common to have half-second cousins due to death and divorce, so it’s good to know how much DNA a half-second might share with you:

• Half-second cousins will share between 30 cMs-215 cMs

Do you share DNA with all second cousins?

You will share DNA with every person who is your second cousin, whether or not they are full or half-second cousins to you.  A team of mathematicians calculated the probability that second cousins would share no DNA and they found that it is statistically impossible.

Additionally, there have been no cases of verified second cousins (either full or half) sharing no DNA.  One reason that this might be is because you would need DNA test results to verify the relationship of every person involved in order to truly “prove” the relationship.

How many DNA segments should second cousins share?

Second cousins will generally share several detectable DNA segments across a few different chromosomes.  Full second cousins will share more DNA segments than half-second cousins.  On all of the DNA test results that I manage, the number of DNA segments shared between half-second cousins tends to be about half.

There is no real guideline as to how long DNA segments shared between second cousins should be.

Important:  You can’t make a determination about whether you are full or half-second cousins based on the number of shared DNA segments.  It always varies, and there is no “normal” number.

How much DNA should you share with the child or grandchild of your second cousin?

Chances are very high that you will share DNA with the children and grandchildren of a second cousin, although there is a minuscule chance that you won’t share genetic material with them.

How minuscule?

It’s estimated that there is only a .10% chance that second cousins once-removed (the child of a second cousin) would share no DNA with you, and a 2.3% chance that the grandchildren of a second cousin would share no DNA.

Note:  If your parents were second cousins, then that means that you and your cousin are third cousins.  In this case, the amount of shared DNA decreases and the probability that you won’t share DNA increases dramatically.  Read more about third cousins and DNA, if you are interested.

What’s the difference between a second cousin and a second cousin DNA match?

A true second cousin is a person with whom you have a verified relationship.  Ideally, you’ve had a peek at their tree and you have spotted the great-grandparent, or great-grandparents, that you share in common.  You might have even known your second cousin your whole life!

A second cousin DNA match is a little different.  DNA testing companies use the amount of DNA that you share with your DNA match in order to estimate a relationship.  As you can tell with the range of shared DNA between half and full second cousins, there is an overlap.

There is actually an overlap of shared centimorgan ranges for a variety of relationships, and so the DNA testing company has to make an educated guess, typically using the amount of shared DNA.

This means that some of your second cousin DNA matches may very well be actual second cousins, but it also means that the person might be slightly more distantly related – or even more closely related – than a second cousin.

Conclusion

I hope that you found the information about shared DNA between second cousins that you were looking for in this post.

If you have any questions about something you have read here, have a question about a second cousin match, or would just like to add your experience relating to second cousins on your DNA match list, I would love to hear from you in the discussion below.

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