What is genetic ancestry testing?

Genetic ancestry testing is one way people can learn about their family history. Ancestry testing uses common DNA variations called single nucleotide polymorphisms (SNPs) to provide clues about relationships between families and where a person’s ancestors might have lived. Established geographical groups and individuals who are related to one another tend to share more of these common DNA variations.

Three types of genetic testing are commonly used to provide ancestry information:

• Y chromosome testing
• Mitochondrial DNA testing
• Autosomal DNA testing

Y chromosome testing

This type of testing analyzes genetic variations on the Y chromosome, one of the sex chromosomes. The Y chromosome is present in males, who have one X chromosome and one Y chromosome, while females have two X chromosomes. As a result, Y chromosome testing can only be performed on males. Since the Y chromosome is passed from father to son, variations in the Y chromosome can be used to explore ancestry in the direct male line.

Mitochondrial DNA testing

Mitochondria are structures within cells that convert the energy from food into a form of energy that cells can use. Although most DNA is packaged into chromosomes within the nucleus of the cell, mitochondria have a small amount of their own DNA, known as mitochondrial DNA. Mitochondrial DNA testing analyzes genetic variations in this type of DNA. Both males and females have mitochondrial DNA, so mitochondrial DNA testing can be performed on anyone. However, only egg cells provide mitochondria to the developing embryo. So only females pass mitochondrial DNA to their children. As a result, mitochondrial DNA testing only provides information about the female ancestral line.

Autosomal DNA testing

In humans, each cell typically contains 23 pairs of chromosomes. Twenty-two of these pairs (chromosomes 1-22) are called autosomes, and they look the same in males and females. The X and Y chromosomes make up the 23rd pair. (Males have one x and one Y chromosome. Females have two X chromosomes.) The autosomes contain the DNA that are needed for most of the body's functions. The X and Y chromosomes are the sex chromosomes are involved in sex determination and development. Autosomal DNA testing evaluates large numbers of SNPs in the autosomes. Autosomal DNA testing provides a more comprehensive analysis of an individual’s genetic ancestral information than either Y chromosome testing or mitochondrial DNA testing, which only represent single ancestral lines.

During autosomal DNA testing, an individual’s pattern of SNPs is compared to testing (reference) databases that contain SNP information from people whose ancestry is already known. After SNP patterns are analyzed, a report is generated with predictions about the most likely locations of a person’s ancestors.

Limitations of genetic ancestry testing:

• Different reference databases are used across genetic testing companies 
• Populations are not represented equally in all reference databases 
• Tests generally do not account for historical population migration 

Several reference databases are available for use in genetic ancestry testing. Because the specific reference databases and the methods used for analyzing individual samples can vary across genetic testing companies, ancestry estimates may not be consistent from one company to another. Additionally, different populations may not be equally represented within a given reference database, so the results for some populations may be less informative than for others. Even within a given reference database, information about a population will change over time as more genetic information is added to the database.

Human migration patterns can also complicate the interpretation of genetic ancestry test results. Most human populations have migrated many times throughout their history. Because of this movement, genetic ancestry test results may reflect where a person’s family originated and not where they lived. The fact that populations may have mixed with nearby groups as they moved around may also cause ancestry estimates to differ from an individual’s expectations.

Many genetic ancestry testing companies provide online forums and other services that allow people to share and discuss their test results with others. Sharing results with others can lead to unexpected findings about existing relationships and reveal relationships that were previously unknown.

It is important to remember that the information provided by genetic ancestry testing is just one part of a person’s story. These tests typically analyze only a portion of a person’s genetic information, and the tests do not consider the environmental factors, such as social relationships and cultural experiences, that are also important in shaping someone’s sense of identity.