Digging up your roots: How DNA is used to trace your ancestors

Have you ever heard of genetic testing? Ever had a family member who sent a sample to companies like ancestry.com? With the rise in popularity of sites like ancestry.com, it’s becoming increasingly tempting to trace your roots through genetic testing. But, how do these genetic tests work? In this post, I will explain the science these companies use to break down your ancestry.

Genetic testing uses our DNA to identify our ancestral connections. DNA is made up of 4 different molecules: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C).  A gene, the functional unit of DNA, contains stretches of A’s, T’s, G’s, and C’s. Genes are clustered together on chromosomes, and we have 23 pairs of chromosomes in each cell of our body. Because we inherit one-half of our chromosomes from each of our parents, DNA serves as a molecular tool to trace our ancestors.

Instead of sequencing all of the genes in a person’s genome, genetic testing is carried out by sequencing small stretches of DNA called markers. There are different markers in our DNA that can help us find different things about our genetic heritage. One pair of chromosomes that we inherit from our parents is called a sex chromosome. As the name suggests, this chromosome determines our sex. Females have two X sex chromosomes while males have an X and a Y sex chromosome from their mother and father, respectively. This means a male will share the same Y chromosome as all of the male ancestors in his paternal line. So, if you are looking for a male ancestor on your paternal side, all you need to do is to get a “Y-DNA test”. This test will look for small sequences on your Y- chromosome and try to match them to other DNA markers available and group you into a specific category called ‘haplotype’, or your genetic roots.  

Another genetic test uses mitochondrial DNA instead of genomic DNA. Mitochondria are present in each of our cells and contain their own DNA. We inherit mitochondrial DNA from our mothers. Just like with Y- chromosomes in men, we can look at our maternal ancestral genes by looking at mitochondrial DNA. This test is particularly useful because it can trace maternal lineages of both men and women.

Lastly, marker DNA on non-sex chromosomes can also be sequenced. While everyone has the same genes,  certain regions in genes contain natural variants associated with different ethnicities. These variations are called Single Nucleotide Polymorphisms (SNPs). For example, there is an SNP that is responsible for eye color. Variations of this SNP can change a person’s eye color from brown (when T is present) to blue (when C is present). (Eiberg et al., Human Genetics, 2008) The T variation is present in 97% of Caucasians. (Eiberg et al., 2008) So when you hear someone say that their ancestry is 55% African, 30% European, 5% Asian and 10% unknown, these percentages are calculated by looking for known SNP variations that are present in different ethnicities.

Most of the genetic testing agencies have expansive databases collected from tons of people’s DNA sequences. With the boom in technological advances and bioinformatics analysis, it is becoming easier to compare genomes and hence, available for people taking tests to use.

So, my question to you is – When are you getting your ancestry traced?

Edited by Clara Boothby and Taylor Nicholas