The Y chromosome is a sex chromosome. Sex chromosomes carry the genetic code that makes each of us male or female. All people inherit two sex chromosomes. One comes from their mother and the other from their father. Men receive a Y chromosome from their father and an X chromosome from their mother. Men and only men inherit their father’s Y chromosome. It is passed down from father to son in an unbroken line all the way back to our earliest ancestors in Africa.
About 60,000 years ago, some groups of Homo sapiens migrated out of Africa, while others remained. Our direct paternal lineages trace these migrations.
This unique pattern of inheritance means genealogists can use Y-DNA testing to see how closely related two males are on this paternal tree of mankind. There are two main types of Y-DNA testing for genealogists: Y-STRs and Y-SNPs
Y-DNA SNPs
DNA is made up of four chemicals that we abbreviate as A, C, T, and G. DNA has two strands, with one chemical on one strand pairing with a complementary chemical on the other strand. We call these pairs nucleotides. Sometimes a small copy error occurs where one chemical is substituted for another on the DNA strand. For example, an A can be copied as a T or a C copied as a G. When this occurs on a single nucleotide, we call it a Single Nucleotide Polymorphism or a SNP (pronounced snip).
These changes are usually rare, and many generations can pass without a SNP occurring. This means that SNPs that occur in a specific lineage are unique and seldom change back. Most SNPs are ancient. They occurred thousands or tens of thousands of years ago. Some are more recent, and as science evolves we are discovering more that occurred in just the past few generations.
Y-DNA STRs
There are some areas in your DNA code where a chemical sequence is repeated many times. For example, AGTAAGTAAGTA is three repeats of the sequence AGTA. It is thought these sequences repeat themselves so that even if a section is lost, the genetic code is preserved. We call these areas Short Tandem Repeats (STRs), because they are short segments of DNA, that occur one after another (in tandem), and repeat themselves many times. The number of repeats that a specific person has in an STR typically gets passed down to their sons unchanged.
However, sometimes a copy error occurs and a repeat is gained or lost. We call this unique set of Y-STRs a Y-STR signature. Because there are many more places within an STR than an SNP for a copy error to occur, STRs have a faster mutation rate than SNPs. Unlike SNPs, STRs rarely go more than a few hundred years without a change.
Ancient Migrations
The desire to explore has been with us since the dawn of humanity. The yearn to see what lies over the horizon, or the promise of a better life has carried groups of people to all corners of the globe. As groups of people drifted apart, each one developed its own unique set of mutations that it passed down to the future generations. Over time, these small changes grew and grew to give us the rich diversity of ethnicities and races that we see today.
Modern science allows us to see what mutations different groups have in common and where they differ. This helps us reconstruct a timeline of when different groups went their separate ways. This is useful for anthropologists to study our ancient migration routes out of Africa. We call these migratory routes haplogroups. Each haplogroup is defined by a specific SNP, but to some degree, all members of a given haplogroup will share similar STRs. This means a general ancient haplogroup can be predicted from an STR signature while a more specific haplogroup can be confirmed with a SNP.
Applications to Genealogy
More recent mutations are often exclusive to clans or families within historic times. This is especially true for STR signatures, because of their fast mutation rate. By comparing these “genetic signatures”, genealogists and family historians can better understand how members of their own extended family fit together. They can use it to prove or disprove genealogical theories and discover new relatives they never knew they had.
FamilyTreeDNA offers a variety of Y-DNA testing options for anthropological and genealogical studies and interests. You can learn more about them in the Y-STR Testing and Introduction to Y-SNP Testing articles.
What will a Y-DNA Test tell me?
- Identify your family’s detailed placement on the Great Family Tree of Mankind
- Discover your surname history and how it relates to other families
- Validate your known paternal line genealogy and get additional clues for research beyond the historical records
- Help expand the tree - your results may enable the creation of a new branch representing a patrilineal ancestor’s genetic signature that you might be able to identify
- In a future update to the FamilyTreeDNA website, you will be able to see if your paternal line is related to ancient DNA from archaeological remains, and when your common ancestor lived.