DNA Studies: Children of Cousins

In the previous post, I compared the amount of shared DNA between siblings.  Here, we compare the widening differences in the amount of shared DNA between cousins.

With each generation, half the DNA of the previous generation is lost.  Each parent contributes half their DNA to create a new human being.  Beginning with your grandparents, you will not inherit your DNA in precise halves.  In other words, you have 50% of your mother's DNA and 50% of your father's DNA, but not necessarily 25% from each grandparent.  The amount skews with each new child and subsequent generation.

Amount of shared autosomal DNA expected between close relatives.  The actual amounts will vary.

I am fortunate to have been able to test cousins of my grandparents.  My paternal grandfather, Clifford Lutter, only had one first cousin through his mother.  My paternal grandfather, Beulah Cook, had two first cousins through her mother.  If my grandparents were alive to test their DNA, we would expect them to share about 12.5% of their DNA with their first cousins.  Instead, the children of my grandparents (my father and his siblings) have tested their DNA.  We would expect to lose half the DNA with the next generation, placing us around 6.25% shared DNA between first cousins, once removed.  My sister and I are the next generation, so we would expect to share about 3.125% with our first cousins, twice removed.

The comparisons at 23andMe illustrate that even close relatives will share varying amounts of DNA.  Long segments of DNA are broken down in recombination, so that little DNA is identical by the time we reach the third cousin relation.

My grandfather's first cousin versus the next two generations.
The expected amount of identical DNA between first cousins once removed is 6.25%
The numbers for this group range from 6.71% to 9.25%
The expected amount for the next generation is 3.125%.  My sister matches only 2.7% while I match 4.44%

Grandmother's maternal first cousins versus the next generation.
In this set, the actual shared amount varies more widely.
The first cousins once removed share between 3.85% and 6.3%
In the next generation, my sister shares a greater percentage than our aunt and uncles,
while I share far less at 2.14%

Grandmother's maternal first cousin versus the next generation.
The amount of shared DNA among the first cousins once removed ranges from 5.64% to 8.6%
Again, my sister shares more DNA with this cousin than three members of the prior generation,
while I share less than expected at 2.95%

  

You see in the above comparisons that your ancestor's DNA quickly breaks up into smaller, non-identifiable segments randomly allocated down different descendant lines.  At this point, I have tested most of the known close relatives of my paternal grandparents (except for the descendants of Ruben Charles Cook and Eleanor Lovelace- hint hint).  It is not surprising that most relatives inthe DNA databases share only one tiny segment with me.

1920 Federal Census
Ruben Charles Cook and Eleanor Lovelace
Newark, Essex County, New Jersey

DNA Studies: Siblings

Full siblings (same mother and father) will share about 50% of their autosomal DNA.

My sister and I thought that we look and act so much alike that we would test within the higher range for siblings.

We were surprised to discover that we share less than 50% of our DNA.

23andMe DNA Relatives function
You inherit half your DNA from each parent.  Full siblings will match within the 50% range.
The blue P indicates a relation through my father.
The purple M indicates a relation through my mother.
Hence, my sister has both designations.
(You can only use this function if you have a parent in the database.)

My father's siblings also submitted their DNA.  They match one another from a low of 43.6% to a high of 55.9%


And my paternal grandmother's cousins submitted their DNA.  They are brothers.  Their match is exactly 50%.

23andMe DNA Relatives function
This sibling duo shares exactly the predicted amount of identical DNA.

Remember that you have two sides to your chromosomes: one from your father, the other from your mother.  Current DNA testing does not tell us which side of the chromosome holds the match to a DNA relative.  (One of the reasons why you need to test cousins from different branches of your tree.)  When you compare full siblings, you can see the areas where they inherited the same DNA from both parents.

23andMe
The sky blue areas represent where my sister and I inherited identical DNA from one parent, but not the other.
The dark blue areas represent points where both of us inherited the same DNA from both parents.

Half siblings will match on only one side of the chromosome- the one from the parent in common.

23andMe
Half sibling comparison
The sky blue areas represent shared DNA from one parent.
No dark blue/completely identical areas because they are related on one side only.

Next up:  I will show you the cousin to cousin comparisons and demonstrate how quickly and randomly our ancestors' DNA breaks into smaller, indistinguishable segments.

Grandparents were Cousins?

John Hickman shares a larger than usual segment of identical DNA with my father, David.  The comparison at 23andMe looks like this:

23andMe autosomal DNA comparison

John caught my interest because the segment is three times larger than most of the segments that are shared with the hundreds of other DNA cousins.  In addition, he resembles my father and he is a lawyer who teaches politics- some of my (other) interests.

John Hickman is exploring whether his Hale line is related to Nathan Hale (1755-1776).  Nathan Hale is believed to not have had issue before dying at age 21, so if John's Hale is related to Nathan, it is through one of Nathan's ancestors.

My paternal grandmother, Beulah Cook, was descended from Solomon Brewer (1746-1824).  Nathan Hale was the second cousin, twice removed of Solomon Brewer.  Their common ancestors were John Strong (1610-1699) and Abigail Ford (1619-1688):  Great grandparents of Solomon Brewer and 3X great grandparents of Nathan Hale.

We need a paper trail connecting John Hickman's Hale line to Nathan Hale's line.

In the meantime, I tested my some more relatives.  Surprise!!!  A first cousin of my paternal grandfather shares a segment of DNA with a first cousin of my paternal grandmother.

23andMe autosomal DNA comparison
between my father's paternal cousin and his maternal cousin.
They match.  The question is HOW.

This is the same segment of DNA where John Hickman matches my father, so we compare John against my father and my father's maternal and paternal cousins.  John matches all of them.  The comparison at 23andMe looks like this:

23andMe autosomal DNA comparison.
John shares an ancestral line with David, David's paternal cousin, and David's maternal cousin.
The question:  which line?

This gives us some options: 

---The connection is still the Strong/Ford/Hale/Brewer ancestors- and unknown to us at this time, both of my father's parents are descended from these lines; OR

---The connection is through a different line- but still indicates that my grandparents were their own cousins; OR

---The connection is through lines of the maternal lines of the first cousins, which are not related to my father.

The common ancestor is someone I have not uncovered- someone waiting behind a brick wall.  Many of these ancestral lines lived in the same geographical area, so it is entirely possible and perhaps likely that the lines linked up in the past, split, and then rejoined when my grandparents married.

This is why I say that DNA testing gives us the answer first; we have to figure out the equation.

And it is John Hickman's DNA- a cousin of unknown relation- that provides us with the link between the families.

John is also a published author with his recent book, Selling Guantanamo: Exploding the Propaganda Surrounding America's Most Notorious Military Prison.  I find it fascinating how so many relatives are also researchers and authors.  John is especially fascinating because our potential common ancestors were deeply involved in the politics of their day.  Their actions created the records we use today to document our genealogies.