Friday, February 15, 2013

Discussing the X....

No, not that crazy person you used to date in college, I'm talking about the X chromosome.

It is simple, and complicated, at the same time.

The simple part: Males have XY for the 23rd chromosomes.  Females have XX.

The a little bit complicated part:  Males get their X from their mother, and a Y from their father, and females get one X from each parent.

This means that the possible X contributors for a persons X are limited, compared to the full set of ancestors for autosomal DNA on chromosome 1-22.

This chart, from Blaine Bettinger, at www.thegeneticgenealogist.com shows the *average* contribution of each ancestor for a male.



You can see all the ancestors who DO NOT contribute to the X at all.  This chart is for a male, but for a female just start in the pink block of the second ring.

For a male, the X comes from up to 2 out of his 4 grandparents, but for a female, the 2 Xs come from  up to 3 out of her 4 grandparents.  Only her paternal grandfather is eliminated.  For males, only 3 of their 8 grandparents can be contributors to their X, for female, 5 out of their 8 great grandparents can be contributors to their two Xs.  As you go further back in the generations, the number of possible ancestors who contributed to the X is much smaller than the actual number of ancestors.  

Because of the lesser number of possible contributors, relationships may appear/be predicted closer than they actually are based on the number of cM shared on the X.

At some date in the future, when I have tested my 6 children I will post some more charts showing how much actual contribution we have in our results.