Asked by: Donna Broster, USA
It’s often said you’re equally genetically related to parents as (full) siblings: your ‘relatedness’ is a half. That means the chance that a bit of your own DNA is shared with your mother (by inheriting it from her) is 1/2. Similarly, the chance of sharing it with your sister (because you both inherited it) is also 1/2.
However, unlike for parents, the amount of shared inherited DNA between siblings varies, and it’s only 1/2 on average. In fact, sex chromosomes and mitochondrial DNA complicate the issue.
For example, considering only the X chromosome, the relatedness of a female to her parents stays unchanged, but to her sisters it’s 0.75 (because they both inherit an identical X chromosome from their father), and to her brothers it’s 0.25.