WEST Surname DNA Project
Why only males have a Y chromosome

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.

    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.