Well people I went ahead and got it done. Actually I got my brother to do it! And it has proved to be worth while.
Andrea, I think that is the only sensible way to go, as Andrew pointed out, it is not very effective to try to find ancestors via the female line, more's the pity.
Liz
It's perfectly effective to find ancestors by the female line, as goes for the male line - Going straight up the female line is what the mtDNA test does. The problem comes when you want to find relations to your mother's father's mother's mother's father's mother or your father's mother's father's father's father. All mtDNA passes down the female line. All Y chromosomes pass down the male line. However nothing will always pass down that line.
We have 2 copies of each chromosome (one from our father, one from our mother) which each have 2 identical bits. When forming egg or sperm, the 2 copies of each chromsomes swap DNA and then move apart. At this point you have 2 chromosomes - the mother's chromosome will have part of the father's and vice versa. Because these chromosomes change every generation, they're very difficult to track. (It's much easier to understand with an animation than words)
X and Y chromosomes however are not copies of eachother and can only cross over at the very tips, that is if they do at all. So X and Y chromosomes stay almost identical (Also, when doing the Y test what they look for are repeated sequences and their lengths, which vary. These will be in the section that does not cross over). While X chromosomes stay intact, they can pass down different routes.
I have 1 X chromosome, which came from my mother. She had 2 - One from her mother and one from her father. I've no idea which one I have, it could be either. Then my grandfather got his from his mother, and grandmother from both her parents. I could have any of those 3. Once you go back further, the number increases.
If you're female, you got 1 X from your mother, one from your father. That one came from your paternal grandmother. The other one could have come from either of your maternal grandparents. Your paternal grandmother's X chromosome, which you received, could have come from either of her parents. If add on the possibilities for your maternal grandparent's parents then there are 5 possible ones in total.
Perhaps there's a reason why X chromosome tests are not done that I don't know about, but I don't know. I think they'd be useful even if they are more complicated to work out where the relationship lies on the tree.
Hi
Well people I went ahead and got it done. Actually I got my brother to do it! And it has proved to be worth while. We have a very close match with some people in the US and we are trying to find out where from in our ancestry line at the moment. But I it was costly and we were lucky - I can see that many people would prob get nowhere with this.
Cheers!
Andrea
Just thought I'd link you to this page:
http://en.wikipedia.org/wiki/Genealogical_DNA_tests (It will also be useful for those who don't quite understand what the actual test is for, what it does, and what the results mean).
Go to the Haplotype subsection of Y Chromosome Tests and it says "If the two tests match on 37 markers, there is a 50% probability that the MRCA was fewer than 5 generations ago and a 90% probability that the MRCA was fewer than 17 generations ago". So as you can imagine, the link could be quite far back.
Andrew