Did you know that there are specific ways to tell the difference between a full sibling and a half sibling? In simple terms, half siblings share either the same mother or father, while full siblings share both, but it can actually be broken down even further. It all has to do with the number of centiMorgans (cM) shared.

For example, full siblings are expected to share about 2550 cM, while half siblings only share around 1700 cM. So what if you share something in between that… what does that make you?

The answer is simple — full siblings share many fully identical regions (FIRs); fully identical means a segment with a match on both the paternal and maternal chromosomes, so full siblings share about or around 25% of their chromosomes. Half siblings, on the other hand, don’t really have any FIRs of significant size. Exceptions can be made, but regardless, they will always share significantly fewer than full siblings.