How Much DNA Do You Share With Your Blood Relatives?

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How much genetic material do you share with your biological relatives?  Take a look at the family tree below and the figure in the red box is the percentage of genes your body has in common with your blood relatives.  For example, your first cousin has 12.5% of the genes you do (implying, inversely, that 87.5% of their genes are different).  Your third cousin twice removed, on the other hand, would have only 0.195% of the same genes, meaning 99.805% of their genes would be different.  This assumes, of course, that you have no double relations in your family tree (e.g. sharing a great-great grandmother from two sides of your family tree).

Not sure I understand. I would expect to share well over 99% of DNA with my immediate relatives. How do you calculate that I share only 50% DNA with them?

Joshua Kennon

Your body was created by taking roughly 50% of the genes from your father and 50% of your genes from your mother.  If you have siblings, different genes in different combinations were passed onto then due to sexual reproduction creating a roulette effect, resulting in siblings sharing roughly 50% of the genetic material, as well.

The exception is identical twins, who share 100% of DNA due to coming from the same

egg and the same sperm, which split and began reproducing independently from the same gene pool combination.

On a related note, a common misconception is that identical DNA must result in identical outcomes.  Identical twins, for example, have different fingerprints, which you can read about here: http://www.nytimes.com/2009/10/06/science/06qna.html.  If one twin suffers from autism, there is only a 90% chance the other will, too (not 100%).  If one twin is gay or lesbian, there is only a 50% chance the other is too (not 100%), etc. etc.  This is because of, in simplified terms, the fact that our genes have ‘on/off’ switches attached to them that can be flipped through conditions in the womb, environment throughout life, exposure to certain chemicals, and a host of other factors.  For a very basic explanation of how genes get flipped on or off, read http://www.thetech.org/genetics/ask.php?id=63

And, of course, it’s fascinating how our ancestral DNA plays a role in our day-to-day lives.  Most of the humans alive on planet Earth cannot drink milk; the figure is around 70% for adults.  If you live in the United States and are white, though, you wouldn’t realize this because your ancestors developed a mutation that allows us to process cow’s milk.  This explains more: http://www.thetech.org/genetics/ask.php?id=250

If you prefer a hierarchy written out, Genealogy.com has that at http://freepages.genealogy.rootsweb.ancestry.com/

Source: www.joshuakennon.com

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