Wednesday, June 5, 2013

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

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