If you are new to DNA testing for genealogy, you probably wonder why only males can be tested for this project. The chart and explanation below show how the sex of a child is determined by the presence or absence of a Y chromosome. We are fortunate that the sex chromosomes are different from the other 22 pairs of chromosomes. The Y and X chromosomes are so different that they do not exchange genetic material (DNA). The Y chromosome is passed from father to son only and contains no DNA from the mother. These properties allow us to use the Y chromosome to track the male line only, making it a very powerful tool for genealogy.

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    The chart below shows two lines that descend from a common ancestor, Thomas WEST.  Each son, John 1767 and Thomas 1769, received a copy of the Y-chromosome of their father Thomas. We expect brothers John and Thomas to match exactly on their Y-DNA. The same condition holds for each succeeding generation. The last two men in each line, Robert 1917 and William 1924, are 4th cousins, each 5 generations from their ancestor Thomas. The names have been changed, but we have actual data for a genealogy like this in which Robert 1917 and William 1924 have an exact match on their Y-DNA. The matching DNA of Robert and William tells us that they both descend from the same ancestor, traditional genealogy research identified that ancestor as Thomas WEST, born 1734 in Virginia.

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