22
Feb

Do you share DNA with all of your relatives?

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If you have done a DNA test, you might have some questions after looking through your DNA match list.  Maybe someone that you thought should be on the list doesn’t show up?  Or maybe you haven’t done a DNA test yet and are wondering who might show up on your match list?  After all of this, you might be asking yourself whether or not you share DNA with all of your relatives.  In this post, I will explain to you who you definitely share DNA with, and which of your relatives might not share any genetic material with you.

Which relatives will I definitely share DNA with?

You will share relatives with anyone who is related to you at a second cousin relationship or closer.  These relationships include:

  • Parent
  • Child
  • Siblings (including half-siblings)
  • Aunts, uncles (including half-aunts and uncles)
  • Grandchildren, grandparents
  • Great-grandchild, great-grandparents
  • First cousins (including half-first cousins)
  • First cousins once-removed (including half-first cousins once-removed)
  • Second cousins (including half-second cousins)
  • Second cousins once-removed*

*There is a 0.1% – very, very small – chance that you would not share DNA with a second cousin once-removed.  In practice, most people will share at least some DNA with all of their second cousins once-removed, with few exceptions.

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Do I share DNA with my third cousins and beyond?

This is where things start to get a little confusing.  If people are related to us, shouldn’t we always share DNA with them?  The reality is that we can be related to someone in a genealogical sense, meaning we descend from the same common ancestor, and share absolutely no genetic material beyond the basic DNA that all humans have in common.   To illustrate how this happens, I have a graphic that I created to show how DNA segments are inherited.

Imagine that all of the different colors are DNA segments that come from a particular ancestor.  You can see in the graphic that as the generations pass from the grandparents, to the children, and finally, to the grandchildren, the segments get smaller and smaller and sometimes are not passed down at all.

The effect of the way that DNA segments are passed down is that eventually, after a few generations, two descendants of common ancestors will not have inherited DNA from the common ancestor that matches the DNA from the common ancestor that the other descendants have.

Using the above graphic as an example, we can see that Nicole and James are first cousins.  They share a substantial amount of DNA inherited from their common ancestors (the grandparents), which is show with the pink and red segments.  James also has some of the blue segment, which Nicole doesn’t have – but that’s not material here.  If Nicole and James both marry and have children, their children will have even less of those pink and red segments.  If Nicole’s offspring eventually has grandchildren, and James also has grandchildren, the possibility now exists that these third cousins will not share DNA with each other.

The reason that this happens is because a child inherits 50% of their DNA from each parent, but this also means that 50% of the DNA from each parent is not inherited by that child.  The smaller those pink and red segments are, still using our example, the smaller the chance that these segments will get passed down to future generations.  You can see an example of this exact phenomenon in the graphic I show.  Mike inherited a small light blue segment from his mother, but Nicole did not inherit this segment from him.

The cool thing about this is that this is also a perfect example of why it is important to ask more relatives to do a DNA test.  That light blue segment that Sam passed down to James (that Nicole did not inherit from Mike) is the exact DNA segment that will match third cousins and beyond that don’t match Nicole.  This light blue segment might be the key to finding one of Grandma’s mysterious ancestors, and James is the only person who has that light blue segment.

If you think you might like to order another test or two, please consider using either of the following two links to purchase your test, since I would get a small commission at no extra cost to you and it helps me support this website ­čÖé  Thank you for your support:

What is the probability that I will share DNA with a distant cousin?

The bottom line is that the more distant the relative, the less likely they will show up as a match for you on a DNA test.  The chance that you will match your cousin falls dramatically as the cousin relationship gets more distant.  The following percentages represent the range of cousins detected for the various cousin relationships:

  • Third cousin:  89-98% detected
  • Fourth cousin: 45-75% detected
  • Fifth cousin: 14-32% detected
  • Sixth cousin: 4-11% detected
  • Seventh cousin: 1-3% detected
  • Eight cousin:  less than 1% detected

The reason that there is a range the percentage of cousins detected is because some companies claim to be able to detect more cousins than the other companies.  Each company uses slightly different methods in their laboratory processing, and this might account for the difference.  The end result, however, is the same: the more distant the cousin, the less likely they will show up as a match on your list.

Before you worry about someone not showing up on your list…

If you searched for this article because you were expecting someone who falls in any of the above relationships to show up on your match list and they don’t, before you worry that you have stumbled upon a surprise, make sure that:

  • You wait a few days to give all of your DNA matches a chance to “populate”.  Basically, what this means is just to give your testing company’s software time to compare your DNA with all of the other kits in the system.
  • If you tested with Ancestry DNA, make sure that you have chosen to allow matching to other DNA customers (you can do this in your test settings)
  • Make sure that you and the person who you were expecting to see on your match list tested at the same company

Conclusion

I hope that this post helped you understand a little bit about why you don’t share DNA with all of your relatives, even though they are still related to you.  If you have any questions about something that you read here, or you just want to share your experience with DNA matches, I look forward to hearing from you in the comments.

Thanks for stopping by!

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Do you share DNA with all of your relatives?
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Do you share DNA with all of your relatives?
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If you have done a DNA test, you might have some questions after looking through your DNA match list. In this post, I will explain to you who you definitely share DNA with, and which of your relatives might not share any genetic material with you.
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Who Are You Made Of
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