Should I test siblings’ DNA?

Do siblings have the same DNA? Is it really meaningful, genealogically speaking, to test their DNA if you have already tested yours?

We all know it – we inherit 50% of our autosomal DNA from our father and another 50% from our mother. However, we cannot fully predict the composition of our paternal and maternal halves. Will they have more of our grandfather’s genes or grandmother’s? While we expect the proportions to be roughly equal (25:25:25:25), this is rarely the case. For example, I am genetically more closely related to my paternal grandmother than my paternal grandfather and more closely related to my maternal grandfather than my maternal grandmother.

Full siblings (with the exception of identical twins) share on average 50% DNA – because their parental halves usually differ by 50%. Unlike the 50% we share with our parents, the siblings’ 50% is not set in stone. This means some siblings may share only 45% while others will share more than 50%. This is why siblings sometimes share more DNA with each other than with their parents. It really is not that uncommon – this happens in almost every second pair of siblings – including my sister and me.

Unfortunately (or in some cases fortunately!), we do not share DNA with all of our ancestors. While it is extremely rare to share no DNA with your 3rd great-grandparent, it is quite common to genetically ‘loose’ some of your 4th great-grandparents. This is why genetic genealogists continue to repeat the old mantra: start with testing your oldest relatives!

Let’s assume that our oldest living generation is the generation of our grandparents. Many beginners in genetic genealogy assume that once they have tested their grandmother’s brother there is no need to test grandmother’s other siblings. Why is this assumption wrong?

Testing your great-uncle gives you access to 50% of his father’s DNA (your great-grandfather) and 50% of his mother’s DNA (your great-grandmother). Accessing 50% of your ancestor’s DNA is not a bad score at all but just imagine all the genealogical treasures that were lost in the uninherited 50% (some of which may be crucial for smashing your brickwalls!).

Let’s do some simple calculations. Since we know that siblings share on average 50% of their DNA we can easily calculate how much more of your ancestors’ DNA you can access by testing multiple siblings.

Testing two siblings gives you access to an impressive 75% of their parents’ DNA (an extra 25%!). Testing three siblings increases the number to 87.5%, while testing four siblings results in the access to as much as 93.75% of their parents’ DNA.

You have probably noticed that the law of diminishing returns applies to testing siblings. The second sibling adds on average 25%, the third sibling adds 12.5%, the fourth siblings adds 6.25%, the fifth adds 3.125% and so on*. Therefore testing four siblings definitely makes sense and is worth the effort. However, if your family has been blessed with high fertility, there probably is not going to be much more information that can be gleaned from testing your mother’s seventh sibling.

As the siblings’ 50% is just an average these are only predictions. You might gain access to 100% of the ancestors’ DNA by testing four siblings even if the expected number is 93.75%. You may test four siblings and still be in the 80s. This is why the testing order and strategy should be determined on a case by case basis.

If by testing additional siblings you have already accessed >90% of their parents’ DNA it might be the right time to think about your next genealogical goal. Are you more interested in accessing an additional few percent of the parents’ DNA or would you gain more information from testing the siblings’ first and second cousins? This would give you access to new segments of the siblings’ grandparents and great-grandparents and would allow you to map their chromosomes (assign genomic regions to the specific ancestors) – extremely important for solving your DNA matches.

Apart from accessing more of the parental DNA, testing multiple siblings allows you to perform visual phasing and track recombinations – more on that next time!

Whatever your genealogical goals might be, testing siblings is definitely not a waste of time or resources and it can provide you with genealogical clues that you would not be able to discover otherwise. However, the law of diminishing returns applies and after testing 4-5 siblings, testing other family members (e.g. first cousins) may prove to be more helpful.

* This is a very simplified calculation that assumes independent segregation of genes but it illustrates very well the genealogical benefits of testing siblings’ DNA.

One Comments

  • Ruth Welch

    December 31, 2017

    IN 2014, I tested my 91 year old mother and 89 year old paternal aunt so I can tell which side of my family someone matches. Recently I tested my brother and not surprisingly he matches several people who I suspected were distant cousins and are now confirmed by his test. In fact, I traveled to Poland in 2016 with one of these cousins who I suspected was my cousin, and now I have confirmation! It pays to test siblings when working with 4-5th cousins!

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