Help me understand DNA testing

[ForresterModern]

I defer to you as the doc, but AFAIK, those women who do have a Y chromosome are not able to pass it on, as they are not fertile. That is a complicated subject in it's own right.

You are in general correct, that the presence of a Y chromosome, regardless of how many x chromosomes my also be present (but only to a degree) would make the person genetically a male, who should (theoretically) have the outward expression of a male. Developmentally we all start out as very tiny one celled females and those with the Y chromosome develop testosterone which typically causes them to develop male physical characteristics during the early and middle phases of fetal development.

But there are rare cases of exceptionally high numbers of x chromosomes (like 8 or 10) in cells with a single Y where the sheer preponderonce of x's outweighs the presence of the Y. You may also have situations where the Y may be present but not complete and therefore not carry ALL the genetic material necessary to express the full Male phenotype (appearence), which may look female. Lastly, there are spontaneous mutations of specific genes for metabolising certain sexual hormones which may make a genetic male (XY) not manufacture testosterone or not have appropriate cell membrane receptors and related mechanisms for getting cellular testosterone effects to occur, which would also leave you with the female appearence (although no normal internal female anatomy)---a phenomenon known as testicular feminization. The first and second groups COULD theoretically pass on Y chromosomal material if they were "female" enough anatomically and fertile, although the likelihood would make lottery odds look small. The last scenario is not compatible with fertility or reproduction so is not an issue.

Hence the reason I hedged a bit on my initial discussion.

[gilmore]

DNA testing has been commercially available only for seven years or so, but is proving to be a useful tool.

As was pointed out, men inherit Y DNA from our fathers, just as we usually do our surnames, hence that is tested for more often than the other kinds.

The larger the database of results, the more likely one is to match one's DNA with with some one else's. Family Tree DNA at www.familytreedna.com has the largest in the world, and is therefore the most useful. You can test for 12, 25, 37 or 63 markers. 12 is too few to tell you very much, and 63 is more than you need. You can upgrade and test more markers later, since FTDNA keeps the samples for 25 years.

Your DNA may match exactly with that of a man who has meticulous and reliable records going back over a thousand years. Or you may find no matches at all. Or you may find that your matches all hve surnames different from yours and come from the other side of the world than you thought your family did. It's a crapshoot.

Probably the most helpful thing that Y DNA tells you is who you are NOT related to. That is, by showing you who your close matches are, it gives hints as to where research is likely to be beneficial, and where it is likely to be useless.

Another thing to remember is that the results are in terms of probability, and rarely exactitude. That is, you will discover men with whom you have a common ancestor within a range of time, a range of generations.

DNA testing isn't really a substitute for old fashioned paper documentation, but can be a helpful adjunct.