DNA Testing siblings and half-siblings
The first thing I did on my journey to learn who my biological father is was to get my maternal half-siblings autosomal DNA tested. We tested with 23andme which was the only DNA company doing autosomal tests at that time (2011). Since then we have transferred our DNA to FTDNA, Gedmatch, and MyHeritage, and I have tested at AncestryDNA. Testing a supposed full-sibling is often the first step taken when a person suspects that the man they have been told is their father is perhaps not actually the father biologically. If it turns out that the siblings are half-siblings, it is usually pretty easy to figure out which one (or neither) is the biological child of the alleged father. If they are half-siblings and the shared parent is not willing or available to DNA test then the matches in common with the half-sibling will help sort the matches into maternal/paternal.
What is a half-sibling? Why is it important to DNA test all my siblings and half-siblings?
A half-sibling is a sibling who shares only one parent with you and a full-sibling is a sibling who shares both parents with you. Maternal half-siblings share only the same mother, and paternal half-siblings share only the same father. Half-siblings can be very useful in genetic genealogy. For the most part, any shared segments between half-siblings came from the parent they share. If that parent is not available to be DNA tested, then segments of their DNA can be inferred from the shared segments of their children.
How much DNA do full-Siblings share? How much DNA do half-siblings share?
Full siblings share on average about 2550 cM and half siblings share on average about 1700. There can be quite a bit of variance in the amount of sharing between siblings. You can see an interesting chart showing the range of sharing in known relationships here.
There are other differences, so if you find yourself looking at 2100 cM shared, and wondering if they are a full-sibling or a half-sibling there are ways to be able to tell. If you have tested at Ancestry or a place without a chromosome browser you will need to upload to Gedmatch or another site with a chromosome browser. Then you can look to see if there are fully identical segments as well as half-identical segments.
This is because, for most matches, you have cM shared that are half-identical. This is because you are related through only one of your parents, and you have 2 copies of each chromosome. If you get a segment in common from your mother in one part of a chromosome and in the same part you get a segment in common from your father on the other copy of the same chromosome then that segment is fully identical. Clear as mud? Hopefully, the images in the next section will help explain the difference in the DNA comparisons between full-siblings and half-siblings!
What do full-siblings and half-siblings look like at 23andme?
At 23andme you can see the chromosome segments. This is me against 3 of my maternal half-siblings:
All the segments are half-identical. You can see the variance in the amount of DNA shared as well, in this case from 1624 cM to 1984 cM
If you look at full-siblings at 23and me in the Advanced DNA Comparison it will look something like this:
They call the fully-identical segments “Completely Identical”
Hopefully, you now have a better understanding of the difference between a full-sibling and a half-sibling and how they appear when both are DNA tested and compared to one another.
