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 Y 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.
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% |
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 |
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% |