DNA analysis can confirm or disprove pedigrees based on paper records. Particularly useful for verifying paternal lines is Y‑chromosome DNA ("Y-DNA"), which is passed down (almost) unchanged from father to son. Mitochondrial DNA, which is passed down along from mother to daughter, is a separate topic not discussed here.

Why Y-DNA?

Humans have 23 pairs of chromosomes in each cell, for a total of 46. One chromosome in each pair comes from our father, the other from our mother. In 22 of the pairs, the two chromosomes look alike, and have matching sets of genetic information. In males, however, the two chromosomes of the 23rd pair are different: males have an 'X-chromosome' and a 'Y-chromosome', whereas females have two matching X-chromosomes. Having a Y-chromosome is what makes you a male.

It's pretty obvious that your Y-chromosome had to come from your father, and he got it from his father, who got it from his father, etc, etc. Because family names (surnames) are inherited the same way, we can use the Y-chromosome to follow the direct male line. In the Merrill project and similar 'surname projects', we're looking at only the DNA on the Y-chromosome.

What can be learned from Y-DNA?

Were it not for randomly occurring mutations, all human males would have the same Y- DNA. Mutations occur only rarely, which allows one to determine ancestry. All males having identical or nearly identical Y-DNA must have had a common male ancestor within the last few thousand years or so. However, males having the same surname may not have similar Y-DNA for two reasons:

1. DNA does not reflect non-paternity events such as name changes, adoptions and illegitimate births.
2. Surnames did not come into general use until about 500 years ago, long after the common ancestor.

Furthermore, even males having a common ancestor within the last few thousand years will have slightly different DNA due to random mutations.

What do all those numbers mean?

Imagine DNA being a long string of letters that spell out the genetic 'code'. The DNA 'alphabet' has only four 'letters' (C, G, A, and T). The test looks at short sections of DNA at specific locations (or 'markers') along the length of the Y-chromosome. These markers are places where a short sequence of letters is repeated, over and over again (almost like stuttering), before moving on.

The report you receive from the testing lab has a number associated with each marker, which is the number of repeats you have at that marker. The number of repeats stays fairly constant over many generations, but every so often, the number of repeats at a marker can change. By comparing the number of repeats with someone else, you can get an idea of how closely related you are to that person. But instead of looking at just one marker, the test looks 37 or 43 (depending on where the test was done), because the more differences there are between us, the less likely it is that we're related.

Summary:

• Similar Y-DNA proves common ancestry in the paternal line.
• Malesof the same surname do not necessarily have similar Y-DNA.
• Minor Y-DNA variations can confirm or disprove pedigrees based on paper records.

How can I get started?

For a living male, a Y-chromosome DNA test for genealogy purposes can be done for under $200 in less than two weeks. The markers that are tested have no biological or medical significance. Results are confidential. Anonymous Y-DNA results for tens of thousands of males are presently available on line, identified by surname. You can optionally choose to make your results (your DNA results, your identity and your pedigree) public. Internet discussion groups, organized by surname, permit related persons to share results and confirm their pedigrees.

Interpretation

There are about 15 human Y-DNA haplogroups. These are subpopulations having specific genetic mutations that occurred up to 60,000 years ago. Broad prehistoric migrations have been traced by determining the prevalence of individual haplogroups is specific geographic regions. Within each haplogroup, teams of researchers are studying when and where more recent mutations occurred. The most recent mutations, occurring during the last 500 years or so, can be associated with particular family surnames by using traditional genealogical methods.