DNA Markers
Sixty years after the discovery of the double helix, we have entered into the realm of personalized genetics. Companies such as 23andMe sequence customers’ full genomes, and thus provide them with information about their genetic make-up: For about $100 you can learn how likely it is that you will get breast cancer or Alzheimer's, whether you have a reduced sensitivity to bitter taste, and from which part of the world your genetic ancestors hail.
To figure out a person’s ancestry, researchers often use the help of Y chromosome haplogroups. In fact, the study of Y-chromosome haplogroups has helped researchers attain much of the knowledge about human migration patterns that is now readily available on the Web.
Haplogroups
A haplogroup describes a population of individuals that share certain genetic features. In fact, a haplogroup describes a set of haplotypes, which are combinations of alleles (variants of genes) that are located near each other on a single chromosome and are inherited together. This sounds very technical, but is actually simple to understand. Alleles, which are different versions of one gene (yellow and green are alleles of the color gene in a pea plant), are not always inherited together: In meiosis a sperm or egg cell is created. In order for this to happen, the cell first duplicates its 23 human chromosomes for a total of 46 chromosomes. When this cell then divides, duplicate chromosomes line up next to each other and exchange genetic information before they are separated into two daughter cells.
Recombination
This exchanging of genetic material is called recombination. For alleles to be inherited together, as in a haplotype, there must be no recombination between the alleles. Because each male only has one Y chromosome, the Y chromosome does not line up with any other chromosome during meiosis, and does not undergo recombination. Therefore, if by chance there is a change (a mutation) in an allele on the Y-chromosome, it will get passed on to the next male generation without change. A set of such mutations is termed a haplotype, while a group of males that show several such changes in common is referred to as a Y-chromosome haplogroup.
How Haplogroups Help Us Trace Our Lineage
These Y-chromosome haplogroups can be used to trace the genetic lineage of humans. Imagine an ancestral human named “Adam," whose Y-chromosome underwent a mutation at one distinct site. Let us call this mutation A. His male progeny then all displayed this A. Several generation down the line, another mutation B arose on the Y-chromosome of one of Adam’s descendents (let’s call him Bob). All of Bob’s male progeny will then carry both the A and B mutations, but, in the entire population, more men will carry the A mutation than the B mutation.
Even if we didn’t know that Adam was Bob’s ancestor, we could till surmise this fact by assessing the prevalence of each mutation A and B in the population. We would know that every individual with both A and B mutations was descended from Bob, and every individual with only an A mutation was descended from Adam. In this way, a genetic tree of Y-Chromosome Haplogroups can be constructed. If this genetic tree is then mapped to the places of the world where people with certain mutations are found, a history of human ancestry and migration can be established!
Additional Resources:
DNA Ancestry Project
Trace your own genetic ancestry! Or if you don’t want to spend the money (or don’t really care), just learn about how it is done.
SNP Fact Sheet from the Human Genome Project
Learn about the type of mutation that is responsible for the creation of Y-chromosome haplogroups, other haplogroups, and many diseases.
Distribution of European Y-chromosome haplogroups by Region
Interested in the haplogroup distribution of modern-day Europe? Visit this site.
PBS Religion and Ethics on Personalized Genetic Testing
What are some of the ethical and public policy concerns of personalized genetic testing?
Mitochondrial DNA on Genetic Home Reference
Learn about the matriarchal lineage marker, mitochondrial DNA. MitoDNA is passed from woman to woman, and can thus be used in the same way as the Y-DNA to establish haplogroups.
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