A: So let's get started...
Your first cousin is the child of your mother or father's full brother or sister. So a half first cousin would be the child of your mother's or father's half brother or half sister.
As an example, if your father John has a half-brother George, then George has a son Mike, Mike would be your “half cousin”. Now if Mike had a son or daughter, that person would be your “half cousin once removed.”
That being said, you get half of your DNA (excluding mitochondrial DNA), from your mother and half from your father. This is the same for your full biological sister or brother. So you statistically share 100% DNA with your sibling 50% from your mom’s side and 50% from your dad’s. Therefore anyone is 50% related to half brothers and sisters because they only share one parent’s DNA. Furthermore you share 50% of your full first cousins DNA if you share 2 out of 4 grandparents DNA. An exception to this would be if two brothers from one family married two sisters from another or a brother and a sister of one family married a brother and a sister from another, then I guess the cousins would share 100% DNA like siblings do and would be very closely related to themselves.
Anyways...let’s put this all together…you share 50% DNA with one parent who shares 50% DNA with their half brother or half sister who shares 50% DNA with their son or daughter (otherwise known as your “half cousin once removed”). So according to my calculations you statistically share 12.5% DNA with your “half cousin once removed” (50% x 50% x50% =12.5%). In reality, it is not so clear cut...but this the closest you will get to an accurate percentage...I hope this makes sense.
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