Have you ever wondered how two siblings can have the same parents but can look quite different from each other? What Implication does this have for interpreting DNA test results?
My mum had dark brown straight hair, dark eyebrows and hazel/green eyes and her sister had blonde/light brown curly hair and blue eyes. Both my mum and her sister had the same parents.
The very simple answer to the first question is obvious: DNA is variable which is why humans are genetically diverse. This variability has implications for interpreting DNA test results.
Our DNA is held within our chromosomes. Each person inherits 23 pairs of chromosomes – 1 copy of each chromosome is received from mum (from the egg) and the other from dad (from the sperm). The 23rd chromosome pair is the sex chromosome/X Y (this determines gender).
In a biological process called meiosis one chromosome is chosen at random and put into the egg and then the DNA within it is "shuffled up" in a process called recombination which means that the DNA passed down is random.
Autosomal DNA is the term used to describe the 22 other chromosome pairs and this is what most DNA testing companies test.
Two things to note about Autosomal DNA and DNA Testing:
1. A male or a female can take an Autosomal DNA test to show a DNA relationship because autosomal DNA is inherited equally from each parent.
2. Autosomal DNA tests (like Ancestry, 23 And Me etc) show matches on both mother’s line and father’s line but are unable to distinguish which side unless a parent has also tested.
In a biological process called meiosis one chromosome is chosen at random and put into the egg and then the DNA within it is "shuffled up" in a process called recombination which means that the DNA passed down is random.
We inherit about 50% of our parent’s DNA: 50% from mum and 50% from dad. Both mum and dad inherited 50% of their parent’s DNA. So we get about 25% of their parent's DNA ie 25% from each of our 4 grandparents. Our grandparents inherited half of their parent’s DNA. We get about 12.5% from each of our 8 great grandparents and so on.
Number Relationship % DNA
16 2nd Great Grandparents 6.25%
8 1st Great Grandparents 12.50%
4 Grandparents 25%
2 Parents 50%
% are approximate
For example – I share 50% of DNA with my mum and 26% with my aunt
It is important to note that although we get 50% of our DNA from each of our parents, the 50% that is passed down from our ancestors is random. Also that the percentages vary.
The amount of DNA inherited becomes smaller and smaller with each generation going back in time, as it reduces by half each time until no DNA is detectable. See above table.
This is why DNA testing companies can generally detect DNA to identify 100% of first and second cousins, 98% of third cousins. The percentage drops to about 70% for 4th cousins and to 50% for 5th cousins. This means that a person you know to be a fourth cousin on your family tree, say, may not have DNA in common with you but they are still a 4th cousin relative.
It is also the reason why it is a good idea to test not only yourself but your siblings also as they will have a proportion of more distant DNA matches that you won’t. This is because you haven’t inherited that piece of DNA. Likewise, you will have some distant DNA matches that your sibling won’t have for the same reason. Of course, you will both still be related to these matches so it is helpful to see both your own DNA matches and your siblings when building up or confirming your family tree.
The table above also shows why it is a good idea to get the oldest members of your family to do a DNA test so that you can go back a further generation and investigate these matches to add to your tree or further validate it.
Understanding the impact that variability has on DNA test results is important to avoid pitfalls of misinterpretation.
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