DNA Studies: Siblings

Full siblings (same mother and father) will share about 50% of their autosomal DNA.

My sister and I thought that we look and act so much alike that we would test within the higher range for siblings.

We were surprised to discover that we share less than 50% of our DNA.

23andMe DNA Relatives function
You inherit half your DNA from each parent.  Full siblings will match within the 50% range.
The blue P indicates a relation through my father.
The purple M indicates a relation through my mother.
Hence, my sister has both designations.
(You can only use this function if you have a parent in the database.)

My father's siblings also submitted their DNA.  They match one another from a low of 43.6% to a high of 55.9%


And my paternal grandmother's cousins submitted their DNA.  They are brothers.  Their match is exactly 50%.

23andMe DNA Relatives function
This sibling duo shares exactly the predicted amount of identical DNA.

Remember that you have two sides to your chromosomes: one from your father, the other from your mother.  Current DNA testing does not tell us which side of the chromosome holds the match to a DNA relative.  (One of the reasons why you need to test cousins from different branches of your tree.)  When you compare full siblings, you can see the areas where they inherited the same DNA from both parents.

23andMe
The sky blue areas represent where my sister and I inherited identical DNA from one parent, but not the other.
The dark blue areas represent points where both of us inherited the same DNA from both parents.

Half siblings will match on only one side of the chromosome- the one from the parent in common.

23andMe
Half sibling comparison
The sky blue areas represent shared DNA from one parent.
No dark blue/completely identical areas because they are related on one side only.

Next up:  I will show you the cousin to cousin comparisons and demonstrate how quickly and randomly our ancestors' DNA breaks into smaller, indistinguishable segments.