I need more data from full and three quarter siblings that are either tested at 23andme or have uploaded to GEDmatch. Ever since I published my article on why full siblings don’t share more DNA, I have been getting inquiries from people who are concerned that they are only ¾ siblings, so I would like more cases in order to help them figure that out.
My article (click here for it) included the insight that if you sum the FIRs (fully identical regions) and HIRs (half identical regions) of full siblings you get a result of about 3600 cM. This is roughly the same amount shared with a parent, thus a true half of the inherited DNA. My article also discussed the expected numbers for ¾ siblings.
Here is the form to help me collect this information, Please do not include the X if you are using 23andme. Below are some more instructions if you need them.
If you are using GEDmatch, the instructions for collecting this data are in my previous article, click here. For the 23andme instructions click the read more below.
At 23andme when you use the chromosome browser to compare two people you get a display like the following. The number for Half identical is what I call HIRs and the number for Completely identical is what I call FIRs in the form.
Underneath the image of all those chromosomes there are many lines of numbers with the title Detailed segment data. At the bottom of those lines the segments shared on the X are listed. Subtract the X cM amounts that have half in the final column from the HIRs. Subtract the X cM amounts that have complete in the final column from the FIRs (only two sisters will have these). These new numbers are what are needed for the form. Below is an example of what those lines segment data look like. In this case I would subtract both 74.20 and 65.61 from the 2811 HIRs.