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 identical, in other words we got the same segment of DNA from each of our parents there.
GEDmatch comparison of me and my brother (full siblings)
You can click on the image to see the entire display at GEDmatch that this was made from.
The more large segments you share, the more closely related you are to a DNA match. To learn more about segments, read the blog by one of my favorite DNA writers on the yahoo DNA-NEWBIE list, Jim Bartlett, called Segmentology.org
He explains why you share such large chunks of DNA with your closer relatives by showing how recombination works and how few cross-overs there usually are in a recent post at http://segmentology.org/2015/05/24/segments-bottom-up/
While the total amount of shared DNA in centimorgans (cMs) is a fine indicator for your closest relatives out to second cousins, farther relationships can be more and more random in the amount of shared DNA. You can consult the charts and numbers at ISOGG here – http://www.isogg.org/wiki/Autosomal_DNA_statistics – to determine those closer relatives.
The question I often see is are we really 2nd or 3rd or 4th cousins? The answer is usually “maybe.” A 2nd-4th cousin designation by your testing company is purposefully vague. Best to look at the amount of shared cMs in segments greater than 7cM, number of segments, and the sizes of those segments; plus, of course, who else this new DNA relative matches!
DNA inheritance gets more and more random the further away the relationship is. The amount of DNA you share with someone more distant than a 3rd cousin is impossible to predict and even those 2nd and 3rd cousins seem highly variable. So the statistical study conducted by genetic genealogist Blaine Bettinger is deeply appreciated by all of us hunting down the relationships with our DNA connections.
Shared DNA statistics from Blaine Bettinger, used by permission
Blaine has created this beautiful chart. His blog has several posts explaining the study which is the source of these new statistics. See http://www.thegeneticgenealogist.com/2015/05/29/the-shared-cm-project/ for all his posts on this study. I had previously discussed his project when recruiting people to add their statistics; apparently he is still taking in statistics so click here to add yours.
“My half-sister and I have been tested to confirm half-siblings. Is there any way to know which parent we share” – this is a rephrasing of a question posed by a woman on the DNA-ADOPTION mailing list at Yahoo. A discussion of the problems of mtDNA ensued. Because the mtDNA haplogroups go back thousands of years, a match is no guarantee of a recent ancestor like a mother. So that information can only rule out the maternal side if there is a mismatch. Deeper mtDNA can get closer but still this is not certain territory yet.
However since they are both female, the X chromosome can give a definitive answer because their father(s) would pass his only X chromosome unchanged to each of them. A man has one X and one Y chromosome and a woman has two X chromosomes. Whether the father passes an X or a Y to his child determines its sex, but since neither is recombined the child gets exactly the same X or Y chromosome as the dad has.
Since a girl has two X chromosomes, one is from her dad. Two sisters with the same dad will therefore ALWAYS match on the entire X chromosome since they have one complete X in common. If the mothers are related, then some of their X will be a full match as well. So let’s look at some X chromosome comparisons at GEDmatch:
This is what the two half sisters from the question look like. The blue indicates that there is a match, the colors above it whether on one chromosome (yellow) or both (green), while red is no match. A few small errors can creep in when processing DNA so an occasional little red line can be from an error. Click the read more once you have decided if they have the same mother or the same father.