Why Siblings Have Different Test Results: Autosomal DNA

Is it possible that siblings can have different test results for their ethnicity? Why of course it is! In fact, it is normal for multiple siblings NOT to share the exact same percentage of each representative ethnic part that makes up their genetic whole. It’s about varying part to whole ratios. It’s also the nature of autosomal DNA that makes it RANDOM – as random as getting dealt a hand of cards from multiple decks through a single shoe.

Understanding this card metaphor will help us understand why siblings have different test results with their DNA as well as why two related people might not show up as a genetic match. It’s all back to the random nature of autosomal DNA. We get dealt nature’s hand in all things, and DNA is no different; however, instead of having 4 decks of cards with 208 individual cards and 16 sets of suit/value combinations, genetics hands us more than 3 billion base pairs of DNA.

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 For a full breakdown of the basic concepts of genetics: read my full article.

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DEAL ME IN! CARDS & CHROMOSOMES

Metaphorically speaking: 1 person = 1 deck and each (genetic chromosome) gene = 1 card. It’s not an exact metaphor as there are 52 cards within a playing card deck and there are approximately 20,000 genes (the bits that encode hereditary function) within our genetic deck. Our genes only represent about 1.5% of the total human genome, by the way.

When we are born we get dealt a full deck – well, most of us anyway and is actually the genesis of that famous pejorative statement. That means we are all born with 52 cards. As to the composition of those cards, we get an unspecified random quantity from our mothers and an unspecified random quantity from our fathers; that is to say, we don’t necessarily get 26 from mommy and 26 from daddy (50/50).

An “unspecified random quantity” means that we don’t know which suit/value cards we are getting from each and how many cards we are getting from each, but we all get a total of 52 cards. Ceteris paribus.

That means were all kinda born with a messy deck! We’re not all suited and in numerical order as when you purchase a brand new deck of playing cards from the store. I may have 6 aces and only one 8 and my sibling might have only one ace and six 8s, it’s all back to the randomness of autosomal DNA. Our parents combine their decks during the miracle of life, yet mother nature decides how those 2 decks are recombined into 1; some cards are kept others are discarded. They are then shuffled and stuffed into a card shoe called you.

YOUR HAND! DECKS & DNA

Take a moment and think of the possible number of unique single decks that combining two 52-card decks can produce; while the initial numbers might not be mathematically earth-shattering, factor that along with the introduction of new decks over time and the permutations do become staggering!

The ratio of genetic material that you inherit from each parent ranges across siblings and can be 60/40, 70/30, 80/20 or almost any other permutation in between. If 4 siblings are sitting at a card table, the dealer doesn’t care if they are related, they all get randomly generated hands. Unless they are twins, of course.

How DNA is shared between family (the amounts they have in common) is measured in centiMorgans (cM). ISOGG defines a centiMorgan as: “a unit of recombinant frequency which is used to measure genetic distance” (ISOGG website) and is a great working definition here.

If we inherit an unknown percentage of cards from 2 decks to produce a single deck – also assuming that those initial 2 decks are not the same and are the result of the same process – how are we able to tell where all the cards came from? Simple, we count and match! “All right grandpa, whaddya got?”

If we laid out all of our cards and compared them to the decks that our parents (grandparents) are holding, then we can see exactly where we got that king of hearts. Metaphorically speaking, of course. CentiMorgans allow us to scientifically compare our cards to the hands which our ancestors, siblings and extended family are holding. ISOGG has a great article on autosomal DNA statistics on their site and is a go-to for many genetic genealogists.

ETHNICITIES! GENES & GAMES

Compound the ideas of cards, decks, and hands and things get even more wild when we consider that our genetic makeup is the product of different GAMES as well. Not all decks of cards have similar designs across all nations and if we have ancestors that came from different regions, even across Europe, then we may have some unique looking playing cards differentiating our hands. We’re all “decked” out differently, get it?

If each person has 52 cards inherited in different quantities from 2 other 52-card decks which were inherited from 4 different randomly recombined 52-card decks which came from 3 different games, then it’s no wonder that siblings will have different genetic test results, especially when it comes to ethnicity percentages.

I might be only 25% British Isles while my sister may be 50%. I might show 30% Lithuanian while my cousin might only show 15%. My deck (hand) would also have British/English-style cards in them along with Lithuanian-style cards. Out of my “British/English” cards I will match those in my family that have those same individual cards, say a 10 of hearts. I also have a smiling Mona Lisa in my hand.

This phenomenon also explains why some extended family do not always show up on genetic test results. While I may have that 10 of hearts, my 4th cousin might not. This is definitely true in my case where I have a confirmed 4th cousin relationship through genealogy but who does not show up on my Ancestry DNA match list.

DNA tests, along with this card metaphor, are not exhaustive and should never replace actual genealogical research, it should only supplement it.

When we get our DNA test results and view them on a site like Ancestry, FTDNA, 23andMe, GedMatch, or the like, very few of our matches are close matches – at least for me. Most of my 32 pages of matches on Ancestry are distant cousins with a ‘genetic distance’ of at least +1. But it’s that one strange, random card that has been passed down from generation to generation that we are all holding that creates that autosomal bond between us; metaphorically it may be a card that my 8x great-grandfather received and passed on all the way down to me!

LINEAGE! SCRATCHES & SNPs

The implications on lineages and population genetics gets even more profound when you consider that accidents happen. We’ve often wanted to play cards at home with our family only to find out that the 5 of diamonds is missing, or worse scratched up. What do we do? Well, we either substitute it or mark over it. In the study of genetic genealogy this is called an SNP, or Single Nucleotide Polymorphism.

While SNPs do not play a major role in determining your close relatives, they definitely play a determinant role in identifying your deep ancestry. SNPs are simply the switching of matched base pairs along a section of a double-helix DNA strand. 23andMe describes this phenomenon by stating that “Cells sometimes make mistakes during the copying process – kind of like typos. These typos lead to variations in the DNA sequence at particular locations, called single nucleotide polymorphisms, or SNPs” (23&Me website). They have a neat, non-scientific breakdown of the concept as far as measuring genetic relatedness through testing.

SNPs are basically useful in determining which HAPLOGROUPS we belong to. They also explain why we are related to Neanderthals and that 2.7% of Europeans still carry Neanderthal DNA!

In conclusion, and in reality, our genes choose us and we don’t get to choose them. Fortunately or unfortunately, the hype about ethnicity results is out there – this is why trendy articles make superfluous claims about one sibling being more Irish than the other. Genetically, maybe; in reality, not so much. The ultimate takeaway in any discussion of autosomal DNA testing is that it should never replace factual, methodical genealogical research. In my mind this is what separates pure genealogy – as dates and names – from family history, or the stories and cultures of our ancestors. This is why siblings can have different test results!

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