Monday, October 24, 2005

Who's Been Wearing Your Genes?

From the Bangor (Me.) Daily News:

Have a fourth cousin? You share about 58 genes

Monday, October 24, 2005

[by Roxanne Moore Saucier]

Fifty-eight genes.

That's the scientific estimate of how many genes I share through one of my connections with Vice President Hannibal Hamlin, who served during Abraham Lincoln's first term.

I didn't count up these genes by myself, you understand. Geneticist Tom Roderick, a former president of the Maine Genealogical Society, did the calculation, which works for anyone who can find the same relationship to another person.

Let's let Tom explain it:

"Your relationship with the vice president, second cousin four times removed, is one of ninth degree - that is, one-half to the ninth. It happens to be the same relationship as fourth cousins. You share 0.00195 of your genes with him, but if you consider that we as humans have about 30,000 genes, then you share about 58 genes with the former vice president. I wonder which 58 they are?"


[Read the whole story]
Degrees of relation between cousins can be figured out by using a table such as this, or (for the mathematically inclined) a simple formula.

Let's have two variables named C ("cousinhood") and R ("generations removed"). The degree of relation equals R + (2*C + 1). L.L. Bean is my fourth cousin (C=4) twice removed (R=2). So, the degree of relation between us is 11. Raise 0.5 to the 11th power, and multiply the result by 30,000 (the approximate number of our genes). The answer is about 14. L.L. Bean and I share 14 genes (which explains my penchant for wearing waterproof boots).1

Here's a cousinhood calculator I whipped up:

1It should be noted that probability is involved in calculating the number of genes shared by siblings or cousins. A parent and child will share 50% of their genes, but two siblings need not share this number. Nevertheless, the law of probability dictates that they will (approximately).

To see why, imagine a card trick in which each of my parents has 30,000 cards to choose from. I am asked to draw 15,000 cards from each deck, mark them, and return them to the deck. Now my brother is asked to do the same. If the cards were well shuffled (randomly distributed), there should be about 15,000 cards that my brother and I drew in common. On the other hand, there is a tiny chance that we selected completely different sets of cards, or sets exactly the same.

The results are similar if two first cousins are asked to each draw 7,500 cards from the 15,000 cards that both of their respective fathers drew from their own father's deck. At best, they will draw the same 7,500 cards; at worse, they will draw none in common. Chances are, they will share about 3,750 cards in common.

It should also be noted that this calculation does not account for the small number of mitochondrial genes passed from mother to child.


How about direct descendents. 5th G. Grandparents?


As you've noticed, the calculator works only for cousins. See this chart for siblings and direct ancestors.

Genetically speaking, a 5th great-grandparent is the equivalent of a third cousin. It's amazing how quickly the genetic contributions of our ancestors are diluted.

As I mentioned above, the number of genes you share with direct ancestors is definite, while the number shared with siblings and cousins is only probable.

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