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Somal (Sex chromosomes X & Y)

The Somal (Sex) chromosomes are the 23rd pair of nuclear DNA that exist in cells. This as compared to the autosomes, that are known as chromosomes 1 to 22. Or the mitochondria which is the only DNA that exists outside the nucleus in the general cell body. Somal chromosomes are unique for a number of reasons.

First, as is now well known, the X and Y chromosomes determine the biological sex of the person. Biological females have two X chromosomes while biological males have an X and a Y. There are some anomalies that exist in nature but this characterizes most in the human species.

Second, for the most part, the Y chromosome does not recombine with any other chromosome. As a result, it is very stable for many generations. And hence the usefulness in a surname study to tag the Patrilineal family branch back for hundreds of years.

Thirdly, a father does not pass down his X chromosome to his son, Only either the fathers' X or Y chromosome will exist in the sperm that fertilizes an egg and thus determine the sex of the resultant embryo. A father always passes his Y to his son.

Fourthly, the X chromosome, in the male or father, does not recombine when passed down to a daughter (for the most part). Note that a daughter will always get this "mostly intact" X chromosome from their father. Whether it can be recreated in test results (by phasing) is another issue. But this leads to unique characteristics of the X. Segments will survive longer down alternating male-female ancestral lines. Statistically twice as long in such instances as recombination with the other X will only happen every other generation. Long X matching segments, when autosomal matching segments are short, may indicate an MRCA along a mostly alternating male-female lineage.

Finally, the two X chromosomes in females behave identically to the autosomes. So in a Matrilineal line, the X chromosome and autosomes behave the same and equally share in recombination each generation. A Matrilineal line loses the longer matching segments on the X chromosome the quickest and in-step (statistically) with the autosomes. (But the mtDNA helps make up for that down these lines as it does not recombine being an isolated DNA strand.)

When returning autosomal test results in a RAW Data File (that typically include the xDNA also), the female testers result will include values coming from both of their X chromosomes; just as it does for the autosomes. The male testers result will either simply replicate the single X value in the tuple returned for each SNP or may include the values of the recombining tips (aka ((Pseudo-Autosomal Regions|PAR), PARs) of the Y chromosome in the corresponding X chromosome result. The non-recombining part of the Y chromosome is always reported separately and often as a 24th chromosome if not marked directly as the Y value.

In the text, when you see xDNA, we mean the X chromosome and X testing. Similarly, for yDNA we mean the Y.