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 line 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.

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.

As a result of these last two points, an alternating male-female lineage does the most to pass the X chromosome undisturbed (with the least recombining). Long X matching segments, when autosomal matching segments are short, may indicate an MRCA along a mostly alternating male-female lineage.

Also, 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.

When returning autosomal test results, the female test subjects result will have populated the values coming from both the X chromosomes; just as it does from the autosomes. The male test subjects 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 PAR or 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. Similarly, for yDNA we mean the Y.