My Dad and I
I'll start with the one that I've had the most comments on at 23 and me. When people compare me to my dad they see something like this:
given the small format, you may want to click these to get a closer view. Above is my comparison to my dad. The comments I normally get from people are along the lines of "how can it be that you are exactly like your dad".
The key is that my dad gave me half of his DNA. He built half of every chromosome I have (except the X). So I'm half identical on every chromosome. What it's easy to forget is that most chromosome browsers show you "half identical" segments...not fully identical.
When you have a match to another person (unless they are a full sibling) you will normally only have half identical segments in common.
What I'm showing here is that for me, there is the opportunity to have a match to my paternal family on every chromosome, in every position. Even though every chromosome is covered, it's only half my DNA.
My Dad, his Maternal Half Sister and their Maternal Uncle
Okay. Time to add some complexity. Here is the broad overview of my dad, his uncle "R" and his half sister "C". This is from my dad's perspective his half sister only has DNA from their shared mother's family. His maternal uncle is his mother's brother so he has the same family in common.
I'm going to close in on some of the chromosomes at the top to illustrate some differences. I apologize if you have trouble telling blue from green. I didn't have any choices for colors at 23 and me. My dad's uncle R is in green. Half sister C is in blue.
Things to note when looking at this. Notice that even though R, C and my dad all share the same very close family and all the same ancestors on my dad's maternal side, there are differences in how R and C match my dad. These matches represent DNA coming to my dad from his mother.
You can see that there are places, like on chromosome 1 where sister C matches my dad but Uncle R does not. That makes sense because uncle R and my grandmother were not twins, they got different DNA from their two parents.
You can see that there are places where uncle R matches but sister C does not like on chromosome 5 and a good chunk of chromosome 4. Again here uncle R and my grandmother have the same DNA and my grandmother handed some segments down to my dad, but not his sister. This also makes sense because my dad and his sister are not twins.
They both got half their mother's DNA, but the halves they got were not identical. The halves were randomly generated from all the DNA available to my grandmother.
Also notice these pictures compared to my match with my dad. Even my dad's uncle combined with his sister is not enough to recreate the DNA my grandmother gave my dad! There are maternal relatives in those white gaps that don't match his sister or his uncle.
From the Sister's Perspective
Now I want to look at these same matches from my dad's sister's perspective. The same three relatives all share the same family. The matches are similar in many ways which makes sense, but they are different in striking ways too. My dad here is in Blue and R again is in green.
What is important there is what you don't see in my dad's close up. Remember that his mother fills up the entire chromosome for both my dad and C. It's not that he doesn't have relatives of his mother there, it's that they will most likely be different relatives than C and R have in that same spot.
For example. C and R may carry a large segment of DNA from one maternal relative while my dad carries a large segment there from another maternal relative. My grandmother had both to give, my dad got one and my aunt got the other. In really simplistic terms, maybe my dad matches his maternal great grandmother there, while C and R carry DNA from the great grandfather...or something to that effect.
What's the Point of all this?
The point is that even closely related people who share the exact same set of close ancestors and get huge amounts of DNA from those ancestors, do not get the exact same DNA from those ancestors. Uncle R is not definitive for my dad's maternal matches. Neither is sister C. Together they come very close, but still not close enough to cover all the ground. In those cracks are the exact same ancestors shared by everyone in the group, just broken up in a different way.
Somewhere, another cousin may fill the gaps in. Perhaps one of my dad's first cousins through a different aunt or uncle will hold the keys to those white areas. They will have matches to the same families but in different spots on different chromosomes.
Here is my last example image, I promise. This is from the perspective of sister C again. My dad is blue, Uncle R is green and common cousin P is purple.
P is related to C and my dad's great grandmother Clara Beadle. Clara is uncle R's grandmother. P shares about 206 total cM with sister C, but I'm going to focus on chromosome 22 for a minute because it's a perfect illustration.
Uncle R in green overlaps my dad a little there in blue. Both do not overlap with P, all are related to Clara Beadle in some way but that doesn't mean they will always match each other where P does because they all got different DNA from the Beadle family. Each of these matches could represent Beadle DNA. P cannot be the definitive check for Beadle DNA because he doesn't cover enough ground.
Here is what I mean by P can't be definitive for Beadles in my family:
P shares 206cM on 8 segments with sister C.
P shares 156cM on 6 segments with my dad.
P shares 145cM on 10 segments with uncle R.
The Beadle family is about 1/4th of uncle R's DNA and yet P shares the lowest amount of DNA in total with R! My grandmother has more in common with P and handed more of those common segments to her children C and my dad. Uncle R is not less related to his Beadle grandmother than his sister was, he just got different DNA from her!
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