Recently I was asked whether there is a way to tell a full sibling from a half sibling. Sometimes the total centimorgans are low for one but high for the other. You expect to share about 2550 cM with a full sibling and 1700 with a half sibling; so which is it if you share 2100 cM?
The answer is that full siblings will share many fully identical regions (FIRs), over about 25% of their chromosomes. Half siblings will have no FIRs of any significant size. The exception being some small ones if their parents are from the same population group but still far far fewer than a full sibling.
Here is a comparison of my first eight chromosomes with my brother made at GEDmatch. The green bars are where we are fully indentical.
You can click on the image to see the entire display at GEDmatch that this was made from.
The old 23andme used to show these FIRs and HIRs in the “Family Traits” display when you selected Genome-Wide Comparison. You can see my brother and mine on this page: http://blog.kittycooper.com/about-me/our-dna-test-results/autosomal-dna-picture/
I have not been able to find this tool in the new 23andme experience, so I guess it is gone. If anyone finds it let me know. There is a new way it shows traits under Share and Compare and many new traits are listed. For example, most of my family has a widow’s peak.
So how to get the FIRs from GEDmatch? In the one-to-one comparison be sure to you click “Yes” next to the question “Show graphic bar for each Chromosome?” You will get a display of every chromosome (not in scale to each other) and the numbers that go with the matches as well as the blue bars for each half matching region (HIR) and the green bars for the FIRs. [UPDATE: you can also click next to “Graphic Only” to get just the picture]
A word of warning, double first cousins, where two brothers married two sisters, will also have many FIRs (about 12.5%) but their centimorgan count will look like half siblings.
Use the article at ISOGG on autosomal statistics as your reference guide for the expected centimorgans for various relatives: http://www.isogg.org/wiki/Autosomal_DNA_statistics
UPDATE 3 jan 2017: Now that you can look at just the image at GEDmatch I thought it might be useful to look at a pair of known half siblings (click for a larger image)… no large green bars.