Half or Full Sibling?

It is becoming increasingly common for people to ask how to tell a full sibling from a half sibling with autosomal DNA testing at 23andme.

Firstly, full siblings will usually match around 50% to each other.  Half siblings will match around 25% to each other.  These are averages, and although I have yet to see anyone reporting a full sibling at closer to the 25% than 50%, and have yet to see anyone reporting a half sibling at closer to the 50% than the 25%,  in theory its possible.

Secondly, full siblings will have fully identical segments, half siblings will not.  To check for this you look in Family Inheritance, not Family Inheritance Advaned.

Half Sibling *

All the segments are light blue, which means half-identical.  Where the light blue segments are doesn't matter.

Full Sibling

Some segments are light blue (half-identical) but many are dark blue (fully indentical).  

So, that is the quick and easy way to check if someone is a full or half sibling!

Generally speaking, if the two half-siblings have different maternal haplogroups then they are paternal (same father) half siblings.  If they have the same maternal haplogroups, and are males with differing paternal haplogroups, then they are maternal (same mother) half- sibling.

*A full aunt or uncle would appear the same as a half-sibling.