What's hidden in your chromosomes?

DNA & Genealogy

Should I test my siblings too?

This is a question often asked and heard in the genetic genealogy community.  Should I test my siblings too?  The answer depends on the type of DNA test and the makeup of your family.


If you have already tested your own mitochondrial DNA to the highest level (FMS = Full Mitochondrial Sequence) to identify the haplogroup and mutations from your direct maternal line, then there is generally no need to test any full siblings, as their mtDNA should be the same as yours (barring heteroplasmy or current generation mutations).  If you have half-siblings by a different mother, then their mtDNA will differ from yours.



Sis Bro CuzIf you are male and have already tested your own Y-DNA to identify the haplogroup and STR marker values of your direct paternal line, then there is generally no need to also test any full brothers, as their Y-DNA should be the same as yours (barring any current generation mutations).  If you are female and have not been able to test your father’s Y-DNA, then testing a brother is ideal, as females do not inherit Y-DNA.  If you have half-brothers by a different father, then their Y-DNA will differ from your father’s.




Definitely do autosomal DNA tests (eg. Family Finder, AncestryDNA, or 23andMe) on as many of your siblings as you can, as each child inherits a different mix of DNA from their parents at conception, and will therefore have a different selection of matches in their results.

Each parent can only pass down half of their autosomal DNA to each child, so each child will inherit a random mix of segments from their parents, and consequently a random mix from each grandparent.  The diagram below demonstrates how the DNA can be recombined at each successive generation.  Three children of the same parents can get quite different mixes of DNA from their grandparents, which means they may match relatives and cousins differently.

DNA inheritance mixing through the generations



The following chromosome browser view shows two siblings (brothers) and the DNA they inherited from their grandparents through their mother.

The orange coloured segments were inherited from their mother's father, and the blue coloured segments were inherited from their mother's mother.

Note the different mixes on most chromosomes, and that some chromosomes were passed down intact from just one grandparent (eg. chromosomes 11, 21 & 22 for Grandchild 1, and chromosomes 14 & 22 for Grandchild 2).


DNA siblings inherited from their maternal grandparents

Also note the percentage mix from each grandparent - one grandchild inherited 60% from his grandfather and 40% from his grandmother, and the other child inherited the opposite (coincidentally).



Some of your cousins will be detected as matches to some siblings, but will not be detected as matches to other siblings.

The chart below shows the likelihood of detecting cousins:


Probability of Detecting Cousins

Relationship Match Probability
1st cousin or closer  ~ 100%
2nd cousin  >  99%
3rd cousin  ~  90%
4th cousin  ~ 45 - 50%
5th cousin  ~ 10 - 15%
6th cousin or further  <  2 - 5%

Sources:  Family Tree DNA and 23andMe



AncestryDNA Probability of detecting cousins who have also tested

Source: AncestryDNA Account - Matching Help & Tips

(Note: AncestryDNA match probabilities are higher than FTDNA's and
23andMe's due to AncestryDNA's lower thresholds for including
matches, and due to
filtering of matches)


It is very rare for 2nd cousins not to be detected, but for 3rd and more distant cousins, it is worthwhile having additional family members tested to give you the best chance of detecting all your closest relatives.  Even in my own family, some siblings match 3rd cousins, and others don't.



The following examples use Family Finder’s Chromosome Browser tool to demonstrate how siblings can inherit quite different mixes of DNA and will match some relatives more than others, or not match others at all.  The thresholds for all examples are set at the default 5cM.


Example 1

DNA segments shared by one kit owner and two 1st cousins (full siblings).

At the 1st cousin distance, there is plenty of shared DNA to detect the matches, but you can see how the segments differ between the two siblings, and there are large segments that may match one sibling but not the other.  These two 1st cousins share 593cM and 668cM respectively.


Kit owner compared with 2 x 1st cousins (siblings)


Example 2

DNA segments shared by two kit owners (full siblings) and their four 1st cousins once removed (4 full siblings).

Notice the chromosome and segment differences between the two siblings' matches to their same cousins. 

At this distance there is still plenty of DNA in common with each cousin to easily detect their relationships.

Total shared segments for these 1st cousins once removed range from 247cM to 381cM.


Two siblings, compared with their 4 cousins, all siblings


Example 3

DNA segments shared by two kit owners (siblings )and their four 2nd cousins (4 siblings):

Notice the differences between the two siblings' matches to their same 2nd cousins. 

At this distance, the amount of shared DNA in common with each cousin is getting much less in total, smaller segments, and fewer matches in common with the siblings. 

Total shared segments for these 2nd cousins range from 165cM to 210cM.


2 siblings compared with their 4 sibling 2nd cousins


Example 4

DNA segments shared by one kit owner and two 2nd cousins once removed (siblings).

At this distance, the siblings only shared two overlapping segments with their cousin. 

They both match the cousin, but have each inherited different matching segments from their parent/grandparents.

Total shared segments for these 2nd cousins once removed was 50cM (blue) and 64cM (orange).



Example 5

DNA segments shared by one kit owner and four 3rd cousins (4 siblings). 

Alas!  Notice that there are only 3 colours showing, as the 4th sibling is not detected by Family Finder.

Total shared segments for these three 3rd cousins range from  20cM to 57cM. 

Family Finder predicted them as a 3rd cousin, 4th cousin, and a 5th cousin, and one was not detected at all.

By comparing the missing cousin on GEDmatch using a one-to-one comparison report, it was confirmed that there were genealogically-relevant matches on the same chromosomes as the other siblings, of 8cM and 9cM in length, but not totalling the minimum 20cM required to be included by FTDNA as a match.



Matches & Mismatches

The above examples demonstrate how easy it can be to miss cousin matches at the 3rd cousin distance and greater.

I have more examples similar to the above, from different relationship perspectives and distances.

In additon to the above, in my kits I also have:

  • a 2nd cousin 1R detected as a 5th+ cousin (total 23cM, largest 9.86cM).
  • a 3rd cousin who matched 2 siblings as 5th cousins, but doesn't match 2 other full siblings at all.
  • a 4th cousin 1R who was not detected by FTDNA or GEDmatch, but does match a 2nd cousin 1R in common on the same line, so is related but just inherited different segments.
  • many other 1st, 2nd and 3rd cousins and other relationships who DO MATCH as predicted or known.


If you only test yourself (or another relative) and no siblings, you may miss many cousin matches. 

If you only test one cousin, it may not be one that matches you.  Encourage your cousin to test their siblings if possible.

If you test multiple siblings and multiple cousin siblings, you have a much higher chance of detecting both your known cousins' families, and many more unknown families.  The same applies for testing your parents' siblings, if possible.


What next?

If you don't match a cousin as expected, don't panic and assume that you're not related after all, or that there was some hanky panky going on somewhere, or that a non-paternal event (NPE) occurred. 

One-to-one comparison

Simply upload the two kits to GEDmatch and run a one-to-one comparison report set at lower cM & SNP thresholds, and you may find that you do match each other after all, but just not enough to meet the total 20cM threshold required to be included in your Family Finder match list.

Matches in common

Also check for matches in common, as cousins may not inherit the same DNA segments, but may both match other cousins in common.

  • Using GEDmatch, run the report called People who match one or both of 2 kits to identify matches in common.
  • Using Family Finder, download both match lists to Excel (download link is at the bottom of any Family Finder match page), and sort by match name or email address, and then scroll through the combined list to identify matches in common.

I'm sure most people are aware of the importance of testing the oldest generations while you still can (parents, grandparents, great aunts, great uncles, etc).  Once you've done that, test your siblings.  If you don't have any siblings, try to find some willing cousins or other extended family members instead.


Ethnic Origin Differences

Because siblings inherit different segments of DNA through their parents, they may also show different results in their ethnic makeup.  The example below shows the FTDNA myOrigins predictions of two parents at the top, and their four children below.  Note the differences: only one child inherited the Norwegian (green) and only one child inherited the Southern European (purple).

Combinations and percentages are quite different, with one child only getting two clusters, and one getting as many as five. 


myOrigins predictions of parents (top) and their 4 children (bottom)myOrigins:  Top 2 larger maps are parents;  Bottom 4 smaller maps are their children.


This demonstrates that you shouldn't take ethnic predictions too seriously, as they can vary within the same family.  Individuals do not inherit DNA from all of their ancestors, nor do family members inherit it equally.  Results can also vary between testing companies, due to different reference populations, different algorithms and different timeframes for the estimates.





23 Pairs of Chromosomes. One Unique You. Get your DNA story at 23andMe.com.


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