The Role of Gender in DNA Testing for Genealogy Research

DNA testing can connect you with living cousins, steer your research away from dead ends and lead your ancestral investigations in promising new directions. But trying to figure out which test (Y-DNA? mtDNA?) will provide the answers you seek can get confusing.

So first we should mention what DNA testing can’t tell you—regardless of gender. It won’t reveal ancestors’ names or their birth and death dates, or the country they were born in; you’ll have to turn to traditional resources to learn these details. It also won’t prove that two people are related, though it can provide strong evidence of an ancestral connection and even suggest when—and where—the most recent common ancestor (MRCA) might’ve lived. Bottom line: Genetic testing can’t replace traditional genealogy research, but it can complement it.

Gender plays a big role in determining which DNA test could solve your research problem—and in what answers a test can give you. Here’s how to use your genetic assets to your genealogical advantage.

Y-DNA vs. mtDNA

Nearly every cell in the human body contains 23 pairs of chromosomes, threadlike strands of DNA that make up our genetic code. Twenty-two of these pairs, called autosomes, look the same in men and women. The 23rd pair, the sex chromosomes known as X and Y, decide a person’s gender. Women have two X chromosomes, one inherited from each parent. Men have one X and one Y chromosome, the X from the mother and the Y from the father.

The Y chromosome is passed virtually unchanged from father to son, just like a surname. That’s why genetic genealogists use portions of the Y chromosome (also called Y-DNA) to trace paternal lineage—what’s listed on the top line of a pedigree chart (see the illustration on page 53). A man today should have the same Y-DNA (and theoretically, the same surname) as his father, his father’s father, his father’s father’s father, and so on. (Exceptions to the DNA-surname link occur in cases of adoption, infidelity or the occasional genetic mutation—more on this later.) Geneticists can determine whether two men, particularly those with the same surname, are related by comparing their Y-DNA. If they share an ancestor, their Y-DNA test results will be identical or nearly identical.

Y-DNA also can come in handy if a surname has changed through adoption or the assimilation process (for instance, my husband’s Italian ancestors changed their last name from Colucci to Colucy). Say your great-grandfather was adopted. If you found someone whose Y-DNA matched the Y-DNA of your great-grandfather’s male descendants, then you might make some headway in identifying your great-grandfather’s biological parents.

Since a woman doesn’t have a Y chromosome, she must turn to someone with the same Y-DNA as her biological father to trace her paternal lineage. That means her father, brother, uncle (on her father’s side) or male cousin (the son of the father’s brother) would need to take the test. My maiden name is Eisenstodt. If I wanted to participate in an Eisenstodt/Eisenstadt surname study, I’d have to ask my father or brother to take a Y-DNA test. I’d then compare the results to those of men in the study.

Geneticists use another source of DNA, mitochondrial DNA (mtDNA), to trace maternal lineage, represented by the bottom line of a pedigree chart. Mothers pass mtDNA (not to be confused with the X chromosome) to both their sons and their daughters, but only daughters carry the mtDNA to the next generation. Just as Y-DNA testing can tell you about your father’s father’s father, mtDNA can tell you about your mother’s mother’s mother.

MtDNA isn’t as useful as Y-DNA to genealogists because it doesn’t correspond to surnames. “Being able to utilize the last name is far more valuable than anything else,” says Bennett Greenspan, president and CEO of Family Tree DNA. “It allows us to be genealogists by virtue that we’re following the surname. The genetics portion provides scientific evidence.”

MtDNA can help determine whether two people (male or female) are related through a maternal line. An exact match would reveal whether they’re related, but because mt DNA changes (or mutates) so slowly, it’s hard to predict when the MRCA might’ve lived. “It could be within three generations. It could be a thousand years ago,” says Darlene Odenwalder, DNA project coordinator at Ancestry DNA. “A lot of people use mitochondrial tests for weeding people out.”

More often, MtDNA is used for tracking population movements on and between continents, and for revealing “deep ancestry”—where in the world your genetic origins lie. You may have heard of Brian Sykes’ book The Seven Daughters of Eve: The Science That Reveals Our Genetic Ancestry (W.W. Norton), which suggests 95 percent of people of European descent can trace their roots to seven “daughters of Eve” who lived 10,000 to 45,000 years ago. Sykes, an Oxford University genetics professor, founded the UK-based Oxford Ancestors (see the box at left), whose mtDNA test can identify which daughter of Eve you descend from. (If you’re not of European descent, scientists have identified 29 other “clan mothers” from whom you may descend.) Identifying your maternal clan can be interesting, but it’s not particularly useful in adding branches to your family tree.

You can use both Y-DNA and mtDNA to help prove or disprove family stories about ethnic ancestry. A lab can analyze your Y-DNA and mtDNA to see how similar they are to those typical of certain ethnicities. The gender rules apply: If you’re trying to prove you have an American Indian ancestor on your paternal line, you have to use a Y-DNA test. You’d use an mtDNA test to look for American Indian heritage on your maternal line.

Autosomal DNA

The problem with Y-DNA and mtDNA is that they can tell you about only a small section of your family tree: the top and bottom lines of your pedigree chart, not the branches in between. To fill in the middle branches, geneticists use autosomal DNA—from the other 22 pairs of chromosomes called autosomes, which make up 99 percent of our genetic composition.

If you want to know generally whether your ancestors belong to a certain anthropological group, order a biogeographical test, which evaluates autosomal DNA and will estimate what percentage of your ancestors were Native American, East Asian, Sub-Saharan African and Indo-European. But since autosomal DNA is a random mix of genetic information inherited from both our mothers and our fathers, geneticists can’t tell which parent a particular gene comes from unless both parents also get tested. So your results might say you have East Asian ancestry, but there’s no telling who on your family tree contributed that heritage, or when the person lived.

These tests come with additional challenges: DNA companies analyze your genetic makeup by comparing your test results to a reference database of DNA samples from modern individuals of various regions. Our ancestors moved around a lot, and they intermarried, so today’s inhabitants of a given region are genetically different from people who lived there before migration occurred. Scientists can make inferences about your ancestry based on trends among specific populations, but can’t say with absolute certainty, for example, that you’re 70 percent European, 20 percent American Indian and 10 percent Sub-Saharan African—or that your ancestors belonged to a certain tribe in Africa. Scientists haven’t yet agreed upon definitions for genetic ethnicity, so if you test with different companies, your results could be different.

Avoiding “false positive” family relationships

The Y chromosome and mtDNA remain largely unchanged as they’re passed from generation to generation, but mutations do occasionally occur. Just as your fourth-great-grandfather might’ve changed the spelling of his last name, his Y-DNA might have changed, as well. Genetic mutations are passed down from generation to generation. Like surnames, they distinguish one family group from another.

Two men can take Y-DNA tests to see how closely their DNA matches and, therefore, how closely they’re related. They can choose to have between 12 and 67 genetic markers tested, and then compare the numeric values that the lab assigns to each of those markers. The more markers tested, the greater the chance of finding genetic mutations (or differences) and, therefore, the lower the chance of having an exact genetic match. In other words, a 67-marker test is more precise than a 12-marker test and less likely to imply a “false positive” relationship.

If the results show an exact match, the two men probably are related within a genealogically significant time period—their MRCA lived recently enough that he may appear in written records. Depending on the number of markers tested, one or two differing values still could indicate a connection. The testing company would provide guidelines for interpreting the results and estimating when an MRCA might’ve lived. In general, the more marker values that differ, the longer ago the MRCA lived—if the men are, in fact, related.

Keep in mind, though, that DNA can mutate at any time. “I happen to be one step off from my father,” says Greenspan. “My son is identical to me, but my brother is identical to my dad. My dad passed a single mutation to me but not to my brother.” Greenspan and his brother were a match on 66 out of 67 markers.

“When you build up enough markers, what you can see is that there really is a near-exact match between the two of us,” he says. So don’t jump to conclusions if your DNA tests seem to suggest an infidelity or adoption. “There’s some wiggle room,” says Greenspan. “I am closer to my dad, my brother and my son than I am to anyone else.” When in doubt, look for evidence in records.

Some markers mutate faster than others, too. “On the Y chromosome, we may look at 43 different markers. Each of those markers has a different rate of mutation,” says Woodward. The rate of mutation associated with a particular marker affects a testing company’s prediction of when the MRCA might’ve lived. “It depends on which marker might have changed.”

Taking a DNA test

Ready to apply what you’ve learned to real-world research problems? You’ll get the most bang for your buck if you start by identifying the specific question you want answered. That’ll determine what type of test you should order and who should take it—you or a relative (see the sidebar on pages 50 and 51 for sample scenarios). Pricing depends on the number of markers tested—the more markers, the higher the price—as well as on the analysis you’d like done. For instance, you can ask for a haplogroup analysis that tells you what genetic group your ancient ancestors belonged to.

Once you’ve decided on a test, you’ll need to order a cheek-swab kit or mouthwash kit that’ll arrive in the mail. For the former, use the toothbrush-like device to painlessly scrape the inside of your cheek; for the latter, vigorously swish the supplied mouthwash for the recommended time and spit it back into the container. Mail your sample along with the signed consent form to the testing company.

You’ll have to wait two to eight weeks for results, which come in the mail or are posted on a password-protected Web site. You’ll get a report with a series of numbers—your DNA markers and their values or your ancestry breakdown by percentage, depending on the test you ordered. The company also will send background information explaining how your DNA was analyzed and what your results could mean.

Most testing companies securely store samples in case customers want to order additional tests later on. But they will destroy them upon request. If you’re concerned about privacy, ask the company about its policy.

Once you have your test results, you can look for genetic matches in your surname study or in online databases. Comparing your roots research to someone else’s could help one or both of you break through a brick wall. Odenwalder worked with someone whose DNA test linked her kin to folks in Ireland. A family Bible confirmed the ancestral connection. “As more and more people are tested and these databases grow,” she says, “people are truly making connections.”