Recombination- Observations, questions and answers

I love using DNA to help with my genealogy. It isn't a replacement for genealogical research, it is a tool, but it is a fascinating tool at that. Thus far I have tested my parents, my first cousin, my mom's half first cousin, my grandfather's first cousin, my dad's second cousin once removed, his half first cousin once removed and a third cousin. What I am finding as far as how much DNA we all share is fascinating.

As I identify more segments to family lines, the pattern of inheritance appears so random. It certainly appears that some siblings are more genetically related than others. What we inherit from our parents is sometime a small amount of DNA from one ancestor, or the entire DNA on some segments.It is a lottery.

We have 4 grandparents, 8 great grandparents, 16 great great grandparents, 32 great great great grandparents, 64 great great great great grandparents, 128 5th great grandparents and so on. While knowing your tree out to those 5th and 6th great grandparents definitely seems imperative in identifying DNA, so does having a full tree, in other words, you need to identify each generations siblings and do their DNA. Even then, a lot of the time, if you have a match that has a tree, you will look at it and be like, I don't see a connection.

Why? Well, human error I think it partly responsible for sure. I had to change names to the "wrong" names in my tree so that PAL would show up in her circle at ancestry DNA. Not every tree out there is right. From non parental events, to just plain bad research (or those lovely hints on ancestry. Why would you attach a record to a tree that is obviously wrong.)

So back to my trends. Let's look at the total cM's and how widely they vary among my testers. Using myself, each parent and a few of their largest matches (only one uses data from 23andme). Relationships in the table are the relationship to my parent.

	Mom	Me	Her nephew	Her ½ cousin	1st cousin 1R	2nd cousin 1 R
Mom	NA	3549 cM	1929.5 cM	590.6 cM	580.4 cM	152 cM
Me	3549 cM	NA	1039.7 cM	214.1 cM	233.5 cM	126 cM
Her nephew	1929.5 cM	1039.7 cM	NA	230.3 cM	415.4 cM	81 cM
Her ½ cousin	590.6 cM	214.1 cM	230.3cM	NA	NA	NA
1st 1R	580.4 cM	233.5 cM	415.4 cM	NA	NA	NA
2nd 1 R	152cM	126 cM	81 cM	NA	NA	NA

From this table you can see that though my cousin and I are the same generation, we inherited differently. I share more with Mom’s 2^nd cousin once removed from the maternal line (our 3^rd cousin) and he shares almost as much as mom with her 1^st cousin once removed (our 1^st cousin twice removed).

Similarly, her half first cousin shares 83 cM with a 2^nd cousin once removed and mom shares only 19 cM with the same person, though they are identically related, and her nephew shares 16 cM with the same person in a segment not shared with either. (Data not in table).

	Dad	ME	½ 1st 1 r	2nd 1R	Dbl 2nd 1R	4th cousin
Dad	NA	3552 cM	209.6 cM	106.8 cM	370 cM	81 cM
Me	3552 cM	NA	61 cM	63.2 cM	220.3 cM	0
½ 1st 1 R	209.6 cM	61 cM	NA	47.7 cM	NA	NA
2nd 1 R	106.8 cM	63.2 cM	47.7 cM	NA	NA	NA
Dbl 2nd 1R	370 cM	220.3 cM	NA	NA	NA	NA
4th cousin	81 cM	0	NA	NA	NA	NA

Dad and his ½ 1^st cousin once removed are the same relationship to his 2^nd cousin once removed, yet he shares twice the DNA. He shares a large amount with his 4^th cousin, yet I share none with the same person, and I only have about ¼ the DNA in common with his ½ 1^st cousin once removed but ½ with his second cousin once removed and about 2/3 of the DNA with the double 2^nd cousin once removed. 

What does this mean, it’s possible that we inherit less of one ancestor’s DNA than others. Until we can map an entire genome for one person we can’t be certain, but it certainly looks like DNA isn’t share and share alike. What we get is a mix match of DNA that often includes greater portion from one ancestor than another.