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Genealogy 101: DNA Resources

Contributors: Joan Parks, Amy Anderson, Megan Firestone

What is DNA?

Autosomal DNA

DNA which is inherited from the chromosomes in the nucleus of a cell, excluding the 2 sex chromosomes.  Humans have 22 pairs of this type of chromosome and one pair of sex chromosomes (the X and chromosomes).  This is what most DNA tests are analyzing.

Mitochondrial DNA (mtDNA)

DNA located in the mitochondria, which are organelles located within the body of a cell.  Because it is not in the nucleus, it is only passed down from mothers, not fathers.  Both sons and daughters receive the same mtDNA, but sons cannot pass this along to their children--this is because mtDNA is transmitted through the female egg.  You inherited your mtDNA exclusively from your mother.  You can have this tested by 23andMe and Family Tree DNA if you are interested, but this is really more useful at a population level, not at an individual level.

Sex Chromosomes

The X and Y chromosomes determine a person's sex.  Most of the time, men have an X and a Y, while women have two Xs. The X chromosome can be inherited from either father or mother, but the Y can only be inherited from the father.  Analysis of the Y chromosome therefore follows the patrilineal line only.   23andMe and Family Tree DNA can both analyze the DNA on the Y chromosome.  The X chromosome is often included in the Autosomal DNA tests, but it is not analyzed separately.

 

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Privacy Issues

Why did my___________ancestry not show up?

 

 

The table below also speaks to this problem--by the time you have gone back 6 generations, you are likely to have little if any DNA from a specific person.  At each generation, the DNA that is inherited is a random group of characteristics (except for the parents--you always get 50% from each of them).  It is very possible to have no DNA from a direct relative.

 

Generation   Number of Ancestors Relationship Average Inherited DNA
    You 100%
1 2 Parents 50%
2 4 Grandparents 25%
3 8 Great-grandparents 12.5%
4 16 Great-great-grandparents 6.25%
5 32 Great-great-great-grandparents 3.12%
6 64 Great-great-great-great-grandparents 1.56% 

Sample DNA Profile of Siblings

This is a comparison of the DNA from my 3 siblings in Ancestry.com.  We all match as siblings, and our Mother matches us as a parent, but there are significant differences in our DNA.

Region Amy Sister 1 Sister 2 Sister 3
England, Wales, & Northern Europe 75 80 65 78
Ireland & Scotland 15 15 16 16
Germanic Europe 9 0 11 0
European Jewish 1 1 0 0
Sweden 0 3 6 6
Portugal 0 1 2 0


Sample DNA Profile of Identical Siblings

This is a comparison of identical siblings.

Region Joan Sister 1 Sister 2
England, Wales, & Northwestern Europe 67% 67% 67%
Ireland & Scotland 33% 33% 33%