Surprising Variation in Levels of DNA Match

[Cas (stallc)]

I share a match with siblings, the male shares 111 cM across 7 segments with me 2% DNA shared, rated 2-3rd cousin. The female shares 18 cM across 3 segments 1% shared DNA, rated 5-8th cousin.

The sister does not show to me on his shared matches as under 20 cM. The connection is my  3 X G grandmother and theirs were sisters.

DNA is passed very randomly assigned indeed down through generations.

Cas

[TreeDigger]

Everyone inherits half their DNA from thier mother, and half from their father.  Quality doesn't affect that.

Yes, they do. However, the older a person gets, the more mutations their DNA acquires. For example, it's known elderly parents pass on more mutations to their offspring. That means i.e. a specific segment passed on by a father aged 30 may theoretically look different when passed on at age 60.

[Alexander.]

I was fortunate to be able to test DNA of several of my grandparents, and was surprised how much more of my DNA came from each of my grandmothers compared to my grandfathers. It was nowhere near a 25% from each as you might expect:

paternal grandfather - 20%
paternal grandmother - 30%
maternal grandfather - 21%
maternal grandmother - 29%

If this effect is amplified over generations it might not take many generations before one may inherit no DNA from a particular ancestor.

[TreeDigger]

That, exactly. And very fortunate indeed.

The theoretical percentage of DNA inheritance as it goes down the generations is just that: theoretical. A 'hand hold' if you will to point in the most likely direction. Things like Identical By State (IBS; DNA inherited within ethnic groups due to shared geography/ancestry) bits tacked on to segments, mutations, etc can throw things off as well.

For example:
I share 471cM & 18 segments with Person A, a full 1st cousin of my father
I share 204cM & 10 segments with Person B, the cousin's brother & full sibling
I share 252cM & 11 segments with Person C, who is the daughter of Person A

And when we did a so-called 'DNA fingerprint' paternity test back in 1989, the results raised some eyebrows. Of the 40 bands of DNA, 20 were identical to my mother, 19 identical to my father, and one did not match either! Turns out that last one was chockablock full of mutations 

DNA usually follows theoretical lines of descent within small percentages of deviation. But there definitely are some serious outliers.