More Than You Ever Wanted to Know about DNA

At our last Genies meeting we scratched the surface on DNA Inheritance.  When there is not much time, it is easy to explain it all away by saying “it’s random!”  But there is much more to it.  You have to go back to the basics.  The question we are answering today is:  Why do siblings have different DNA?  

If you find this to be intimidating, just come to our Genies meeting next Monday the 15^th and we’ll go over it all.

Let me describe what happens.  Today we are only talking about autosomal DNA. The autosomal DNA test is the one provided by Ancestry, Family Tree DNA, 23andme, My Heritage and Living DNA.    Autosomal actually means not a sex chromosome.  There are 22 autosomes and 23^rd is the sex chromosome.  They are the genetic material that is found in the nucleus of every cell.  Here's a Wikipedia picture of all 23 of the wigglies.

 

Each chromosome is actually a pair of chromosomes.  Each parent sends one of their pair for each of their chromosomes.  Since the child receives one of each pair, he receives 50% of each parent’s chromosomes.  There is also a process of recombination in which some segments are optionally swapped between the pair in the child’s chromosome.

I clipped this picture from Wikipedia.  It shows how for a specific chromosome children numbered 1 and 4 may not match at all.

 

In general siblings match about 40% of their DNA.

Chromosomes are measured in centiMorgans (cM).  For each person, the total of their chromosomes’ lengths is somewhere between 6700 and 7040 cM.  A child inherits about 3500 from each parent.  Relationships can be determined from the total cM of matching DNA segments.

In each generation the child only gets half of the DNA of their parents.  Each person receives 50% from each parent, approximately 25% from a grandparent, 12.5% from a great-grandparent, 6.25% from a second great and so on. 

So that is the scientific theory.  A few years ago Blaine Bettinger and others thought they should find out what the actual amounts of shared DNA between known relatives are.  They started a study and by 2017 over 25,000 relationships had been submitted and they are still accepting data. 

Results are published on his website https://thegeneticgenealogist.com/2017/08/26/august-2017-update-to-the-shared-cm-project/.  There are several charts there and I have copied one below.

Some expected amounts in the far-right column are siblings 2550, 1^st cousin 850, 2^nd cousin 213, 3^rd cousin 53.  There is a wide range of relationships for a given cM match. 

 

One more fact from this chart:  All known second cousins share DNA.  For more distant cousins you start to see the size of a shared segment become too small to be significant or disappear altogether. 

For more information and discussion:

Check out the Ferndale Genies blog at https://ferndalegenies.blogspot.com/.  In the left column are fixed pages.  Half way down you will see the pages entitled DNA.  Many of these are handouts from previous talks.  They will give you basic info about DNA and ideas of questions to ask. 

Also come to the Genies Meeting next Monday and this will be one of the topics.

See you there!