DNA tests can be very confusing. What can they tell you that you don't already know? Which company should you go with? What type of test should you choose? I'm going to walk you through your options and conclude with my personal recommendation.
Let's do a five-second remedial course on DNA
Humans have 23 chromosome pairs. Each pair inherits 50% of your parents' DNA. The 23rd chromosome pair determines your gender. The 23rd chromosome pair is either made up of an X and an X, or an X and a Y. XX is female. XY is male. With me so far? I hope so :)
There are three major companies who offer DNA tests for genealogy purposes:
Next, there are three common TYPES of DNA tests people do for genealogy:
1. Autosomal DNA
Let's talk about those three types.
1. Autosomal DNA
Autosomal tests are far and away the most common, analyzing all 23 of your chromosome pairs. They can predict your ethnicity breakdown (e.g. 25% Great Britain, 24% Ireland, 17% Italy/Mediterranean...) and your likely relatives within 4-5 generations.
When a site doesn't tell you what type of DNA test it does, you can assume it's Autosomal. AncestryDNA, FTDNA, and 23andme all offer Autosomal testing.
A commercial Autosomal test cannot tell you which parent your DNA comes from. However, if one or both your parents take the test, and so do you, you can compare which relatives you share and make deductions about which side they belong to.
Y-DNA tests analyze only your Y-chromosome. I believe they focus exclusively on finding common relatives (no ethnicity breakdowns), potentially tracing you back much further than 5 generations.
You may recall from biology (and my quick remedial blurb from before) that a female is XX and a male is XY, so you may have already deduced that only a male can take a Y-DNA test. That is correct.
The Y-chromosome is passed down from male-to-male. As long as each generation has a son, the Y carries on. Let's use the Duke of Cambridge's family as an example. Little Prince George carries the Y chromosome of his father, William. William carries the Y chromosome of his father, Prince Charles. Therefore little Prince George is carrying Prince Charles's Y chromosome. George, in turn, will pass down that Y to his son someday, if he has one. Charlotte, on the other hand, will not be able to pass down the Y, because she has two Xes.
If your family is anything like mine, pretty much everyone had a son or 10 prior to 1900, so that Y could have been passed down as far back as history goes. How cool is that?!
FYI there are different levels of depth to Y-DNA tests. The more markers you test, the further back you may be able to go.
FTDNA is the only site I've mentioned here that currently offers full Y-DNA testing. 23andme will analyze your Y-chromosome as part of their autosomal test, but not in the level of detail that FTDNA offers. So on 23andme, you may see relatives identified as matching your Y-chromosome, but only FTDNA will have a thorough report and the best matches.
MtDNA, or Mitchondrial DNA, is carried by males and females, but only passed down from a female.
I'll use the famous Duke of Cambridge's family as an example again. Prince George and Princess Charlotte both have their mother, Kate's, MtDNA. Charlotte will be able to pass that on to her future daughters. George will not. Princess Diana's MtDNA ends with William and Harry. Since they are both male, they are not able to pass it down.
MtDNA tests are not as widely recommended for genealogy as Y-chromosome tests because MtDNA mutates less often. The less often DNA mutates, the harder it is to pinpoint common ancestors. Ideally, you would want to be able to trace DNA back to where it mutated and see who has the same mutation, and be able to say "Ah, see they all got that mutation from So-And-So". The Y-chromosome mutates often enough to make this relatively reliable, but MtDNA... not as much.
FTDNA is the only major site to offer MtDNA testing at the moment. 23andme will analyze your MtDNA as part of their autosomal test, and tell you what "haplogroup" you belong to (meaning what major mutation branch you can be traced to, which is relatively broad), but much like Y-chromosomes, FTDNA will analyze it in-depth.
Okay, totally confused now? :) Basically all you have to remember is:
- Autosomal DNA tests all of your chromosomes from both sides of your family
- Y-DNA is for men only and traces your paternal line further back than autosomal can
- MtDNA is for males and females, traces your maternal line further back than autosomal can, but isn't great for "recent" (e.g. 100-200 years) relative identification
So which do you pick?
If you're just starting out, autosomal all the way. For genealogy, I prefer AncestryDNA. It's easy to use, has the most genealogists on it, and is backed by excellent publicity through "Who Do You Think You Are?" that is attracting more users every day.
I wouldn't recommend a Y-DNA test as a first step. I wouldn't really recommend it at all unless you're a very savvy genealogist and web user. Quite frankly, FTDNA's interface for Y-DNA stinks. Their support is confusing. The resources for understanding your results aren't sufficient. You blow a lot of money for something you may potentially be unable to understand how to use (and that's not your fault-- FTDNA needs to step up their game and I wish more influential genealogists would complain about it).
If you ARE a savvy, resourceful genealogist who can self-teach through confusing interfaces, Y-DNA can be a decent resource for breaking past a brick wall in your pre-1800s paternal line.
I wouldn't really recommend MtDNA at all. I haven't done it yet, but after doing a couple of Y-DNA tests for my family (and Y-DNA is more straightforward), I was so unimpressed that I think MtDNA would be a waste of your money for the time being. It might- MIGHT- get you some hits that make sense, it MIGHT be presented to you in a legible enough way to act on, and if you're lucky, your matches might have traced their family far enough back to share helpful info. Proceed with caution!
I hope this helps you on your genealogy quest!