Siblingship

Last month, Judy and I got the idea in our heads to test our DNA, via a sibling test, to see what it would show with regard to our relationship.  There was no ulterior motive, beyond curiosity on our part.  We deliberated the ramifications before hand, and determined that ultimately it didn’t actually matter what the test revealed, that things are as they are, and no test was really going to alter any perception on either of our parts.

We got the data back this week, and while most of it was extraordinarily technical unless you are a geneticist, I compiled the following “simplistic” breakdown in as plain English of what it revealed:

The theoretical ideal “sibship” index between two unrelated random individuals is 1.  A sibship index greater than 1 indicates a higher likelihood of sibship.  A sibship index lower than 1 indicates a higher likelihood of non-sibship.

Based on the genetic testing analysis by PCR of STR loci (specifically examined DNA location matches), sibship matches for “Judy” and “Ray” calculated as full siblings as follows:

DNA Location     Judy                Ray                 Match

D8S1179               13,15               13,15               exact

D21S11                 29,30               27,30               exact

D7S820                 11,12               12,14               very close

CSF1PO                12                    12                    exact

D3S1358               16,17               16,17               exact

TH01                     9.3                   9.3                   exact

D13S317               11                    11,13               very close

D16S539               12                    12,13               very close

D2S1338               17                    17                    exact

D19S433               13,14              13,14                exact

vWA                      16,19              16,19                exact

TPOX                    8                      8                      exact

D18S51                 13,17              13,18                very close

D5S818                 12,13               12,13               exact

FGA                       21,24               22,24              very close

As you can see, ten of the fifteen location samples in the table above matched exactly, while the remaining five were very close.  This indicates that our Combined full-sibship index is 125,219.00 (in other words, the probability of us being full siblings is 99.999%).  Additionally, our half-sibship index is 828.32 (in other words, the probability of us being half siblings is 99.8%).

So, are we true siblings?  Well, the greater the sibling index value over 1.0, the higher the likelihood that the two individuals are related as siblings. Our sibling index value came in at 125,219.

If a sibling pair has a sibling index value that is 10, that suggests that they are 10 times more likely to be true biological siblings than not siblings. If a sibling pair has a sibling index value of 100, that suggests that they are 100 times more likely to be true biological siblings than not siblings. Our numbers suggest that we are 125,219 times more likely to be full biological siblings than not.

Are we full or half siblings?  If the full sibling index value is greater than the half sibling index value, then the individuals are more likely to be full siblings. If the half sibling index value is greater than the full sibling index value, then the individuals are more likely to be half siblings.  Our full sibling index value came in at 125,219.  Our half sibling value came in at 828.32.

This means that if a sibling pair has a full sibling index value of 100 and a half sibling index value of 2, then they are more likely to be full siblings than half siblings. A comparison of our full versus the half sibling index values (125,219 / 828.32= 151.17), indicating that our full sibling index value is 151.17 times greater than our half sibling index value, indicating that our sibling pair is 151.17 times more likely to be full siblings than half siblings.

So what does all of this mean?  To be honest, absolutely nothing, and yet... absolutely everything.  It means that Judy and I test as full-blood brother and sister, validating what we knew in our hearts all along.