We get half of our DNA from our mothers, and half from our fathers.
Matching DNA segments are measured in Centimorgans (cM). We inherit 3400 cM from our mothers, and 3400 cM from our fathers.
You have 23 chromosomes (22 autosomal, and one sex chromosome). On each chromosome, you have one strand that came from your mother, and one that came from your father.
Our parents each get half of their DNA from their mothers (our grandmothers), and half from their fathers (our grandfathers). That DNA gets mixed up before it gets passed down to us so we can get more DNA from some grandparents and less from others – so while we get an average of 25% of our DNA from each grandparent, the actual amount we inherit could vary.
On each of our chromosomes, we have one strand that is a mixture of our paternal grandparents, and on strand that is a mixture of our maternal grandparents.
Because each of our Grandparents inherited their DNA from their parents, our DNA consists of approximately 12.5% DNA from each of our Great-Grandparents.
On each of our 23 chromosomes, we have two strands: one strand consists of DNA that came from our paternal ancestors, and one strand from our maternal ancestors.
Our DNA consists of an average of about 6.25% of each of our Great-Great-Grandparents, and about 3.125% of each of our Great-Great-Great-Grandparents.
4th Great-Grandparents = about 1.56%
5th Great-Grandparents = about 0.78 %
6th Great-Grandparents = about 0.39%
And so on.
With each generation, there is the chance you’ll have more DNA from some ancestors and less from others. This is because the DNA each of us gets from each of our parents is always a random mixture from each of their parents.
Here is an example of a tree, showing the average amount of DNA you share with each of your ancestors:
When it comes to cousins and the randomness of DNA inheritance, you’ll find that you share various amounts of DNA with each of your cousins, aunts, uncles, etc; the more distant the relative, the greater the variance in the amount of DNA.
The following image demonstrates the percentages of shared DNA we expect to share with our relatives. Notice that the amount of shared DNA is cut in half with each step (for example, your 1st cousin shares half the amount of DNA with you as your Aunt/Uncle).
Again, please remember, these percentages are an average. The actual amount you share with your relatives will vary. That variance will broaden the further away a relative is.
The variance is because of the randomness of DNA inheritance. For example, I have a 2nd cousin with whom I share the expected amount of DNA with. My daughter, however, shares much less DNA than expected with this cousin. When we investigate by looking at our matches on a chromosome browser, we can see that there is a very large strand of DNA that I share with this cousin that I did not pass down to my daughter.
Because of this variance, and because the variance widens with each generation removed, we might find there are times when 3rd and 4th cousins don’t match one another, or share very little DNA with one another. We might also find that there are times when large segments of DNA pass down from generation to generation, undisturbed, and may make it appear that very distant cousins are related more closely than they are.
When it comes to immediate and close family, there is little variance, so it is easier to determine the relationship based upon the amount of shared DNA.
For a better visual, there is a short video explaining how DNA is inherited here: http://learn.genetics.utah.edu/content/chromosomes/typesauto/
You'll find more charts and information on shared DNA here: http://isogg.org/wiki/Autosomal_DNA_statistics