Frequently Asked Questions About DNA Testing

By: Laura Ryan, PhD
Last Updated: July 24, 2018

If you find yourself getting confused by some of the many complex ideas behind DNA testing, you’re not alone! Our readers send in hundreds of questions each month, and here on this page we do our best to answer the most common ones.

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On this page:

How can a DNA test predict where my ancestors came from?

The process by which genetic information is copied to be passed down generations is not perfect, and sometimes errors can occur. These errors are called mutations, and while they may cause disease in some cases, they are often neutral, and are responsible for all the immense variation we see within the human race. New variants that arise are passed down through generations, and may become widespread enough in a certain group of people that they become markers that can be used to identify that particular group. Different groups of people often carry different gene variants in their DNA, and by comparison of this data geneticists can accurately predict where our ancestors are likely to have come from.

While we may enjoy a jet-setting lifestyle today, this was not true of our ancestors 500 years ago, or even 100 years ago, when travel was difficult and often dangerous. The fact that our ancestors tended not to travel the world, and so crucially did not mix their genes with populations based in far away lands, is what underpins our ability to predict where our ancestors came from. If a group of people has been largely isolated over the course of time, the prevalence of gene variants unique to that group in the population is likely to be high, and it is unlikely to be seen in individuals from other unrelated populations. It is interesting to note that particularly genetically isolated peoples include both those separated from mixing their genes by physical barriers, such as islanders or mountain dwellers, but also those peoples isolated by self-imposed, religious or cultural barriers, such as Jewish people. However, if there is a lot of intermixing between populations, such as has been the case in mainland Europe, the genetic differences between groups will, in time, become diluted, and it becomes difficult to distinguish between them, making ancestry prediction more difficult.

How accurate are ethnicity estimates?

In general, the larger and more geographically comprehensive a company’s database is, the more accurate its ethnicity estimates are. To be able to accurately analyze markers in your genome and connect them to a specific ethnic group, or region of the world, a company must know what markers are specific to people of that ethnicity or area, and this means collecting a huge number of samples from people all over the world. The tests are extremely reliable on a continental level, but the results may become less reliable as you move into more specific geographical areas, and results indicating a percentage of ethnicity of less than around 5% often have to be taken with a grain of salt. There is also always the potential that certain markers in your genome have not been picked up on by the testing company, and so are not included in your report - this is especially true for markers indicating ethnicities that are less common amongst test takers.

How can ethnicity test results vary from one provider to the next?

In essence, as different testing companies are matching your genomic data to reference points on their own unique company databases, and using their own methods of analysis, they will return slightly different results to you, depending on the composition of their specific database and their methodology. This accounts for why different companies can give you different levels of specificity. You may also find that certain ethnicities crop up at low levels in reports from some test providers but not others. There are a number of possible reasons for this. One potential explanation is that in the past, a particular tribe of people carrying a genetic variant migrated to both location A and location B, and while one test provider may assign that variant to location A, another provider may assign it to location B. It’s also possible that unexpected results reflect ancient common ancestry between the group you’re descended from and another, much more distantly related group. Unfortunately, where complex migration and population mixture is concerned, there isn’t necessarily a ‘right’ answer - it’s all a matter of perspective! The answer to this question builds on the question: “How accurate are ‘Ethnicity Estimates’, so please take a quick look for more information.

What is DNA recombination?

The human genome is packaged into 22 pairs of autosomal chromosomes, and one pair of sex chromosomes, giving a total of 46 chromosomes. During certain cellular activities, such as in the formation of human sex cells (eggs and sperm), a process called recombination occurs. During this process, the two chromosomes of each pair (one derived from our mother, one from our father) line up next to each other, and exchange small regions of their information with each other. This creates new combinations of characteristics that are different to those of either of the parents. This reshuffling process generates a huge amount of variation and genetic diversity within species that reproduce sexually.

What is an SNP?

DNA is a code, made up of a combination of four different molecules (nucleotides) that we can think of as four different letters (A, C, G, and T). When this code is copied from one generation to the next, occasionally one letter of the code is copied incorrectly, and replaced by another letter. This is called a point mutation, or a variation (there are other types of mutation, but those are not relevant here). Some point mutations can cause disease, or even reduce your likelihood of developing a certain disease, but many are neutral, and contribute to natural characteristic differences between people. When a particular variation in a single nucleotide is passed down through generations and becomes prevalent to an appreciable degree in a population, it is called a Single Nucleotide Polymorphism, or SNP (‘Single Nucleotide’ because it occurs in one specific letter of the code, or nucleotide, and ‘Polymorphism’ from the Greek for ‘existing in several different forms’).

For example, at a specific position in the code of the human genome, the ‘G’ nucleotide may appear in most people from Iceland. In other European peoples, an ‘A’ nucleotide may be more common in that same position. ‘G’ and ‘A’ are two possible nucleotide variations for the SNP at this position, and by analysis of that SNP, we may be able to predict whether a person has any Icelandic ancestry.

What are haplogroups?

The concept of the haplogroup is complex and difficult to wrap your head around, but it can be roughly summarized like this: a haplogroup denotes an ancestral clan, or a particular line of descent, that often dates back many thousands of years, and is defined by a very specific mutation. If a new mutation occurs in an individual, and over the course of hundreds of generations becomes prevalent to a certain degree in a population, it may become a new haplogroup, although this is likely to take millennia! Incredibly, for a handful of haplogroups (those that arose more recently) we can pinpoint the exact person who first developed the mutation and gave rise to the group. Originally, we all belonged to the same Africa-based haplogroup. As groups of people branched off and started to migrate out of Africa to colonize the rest of the globe, new haplogroups were gradually created. Haplogroups are named alphabetically and in order of discovery, but there are many websites that list them in the order that they developed if you want to delve a little deeper into your pre-historic origins! Because neither yDNA nor mtDNA undergo recombination, we can learn about both our maternal and paternal haplogroups by sequencing these types of DNA, meaning that we can discover the ancestral origins of both sides of our family.

Why doesn't yDNA undergo recombination?

yDNA is the DNA found on the Y chromosome - one of the two sex chromosomes. For recombination to occur, two chromosomes that are extremely similar in structure and content must line up, and swap genetic material (see “What is DNA Recombination”). The ‘pair’ of the Y chromosome is the X chromosome, but these chromosomes do not even loosely map on to each other, and are not at all similar (the X chromosome is in fact much, much bigger than the Y), so they cannot align and swap genetic material in the way that autosomal chromosome pairs can. For this reason, they undergo no recombination, and are passed on with virtually no change from one generation to the next.

Why doesn't mtDNA undergo recombination?

mtDNA is unique in many ways. It exists in the mitochondria - which are the energy producing centers of the cell - rather than in the nucleus. mtDNA only makes up a tiny proportion of a person’s genome. Your mtDNA is inherited solely from your mother (see ‘Why does mtDNA only pass through maternal lineages’ below), and so all of the copies of this DNA in the mitochondria are identical. mtDNA does technically undergo a kind of recombination, but as it is recombining with identical copies of itself, the overall genetic code remains unchanged before and after the recombination event. In this way it is very different to chromosomal recombination, which results in new combinations of genes between each generation (see What is DNA Recombination above).

Why does yDNA only pass on from father to son?

The answer to this question is simpler than you might expect. Human sex chromosomes can come in two forms; X or Y. The sex chromosomes of females are both X, while males have one X, and one Y chromosome. Females can only pass on X chromosomes (as this is all they have), while males can pass on either an X, or a Y. It follows then, that males inherit one Y chromosome from their father, and one X from their mother, while females inherit an X chromosome from each parent. As yDNA is DNA found on the Y chromosome, and these are only passed on from fathers to their sons, yDNA can only be inherited in this way.

Why does mtDNA only pass through maternal lineages?

This is a very interesting question, and one to which scientists do not have a detailed answer. What we do know is that the mitochondria carried in sperm cells are destroyed inside the egg cell after a fertilization event has occurred. In addition to this, the mitochondria are often carried at the base of the sperm cell’s tail, which is often lost during fertilization. Both of these factors explain why there is generally no male mtDNA in a fertilized embryo, and so we inherit mtDNA exclusively from our mothers.

How can genes that don't affect me affect my children?

If you are what’s called a ‘carrier’ of a disease-causing variant, then although you carry a copy of this variant in your genome, you do not suffer the effects of the illness it can cause. However, this does not mean that your children will not be affected. So how does this work? We each inherit two copies of each autosomal (non sex-determining) gene; one from our mother, and one from our father. If the two variants of a gene that we inherit are not the same as each other, often we only see the effects of one of these variants in our body, while the other appears to be silenced. Two different variants of the same gene are called ‘alleles’. If an allele is ‘dominant’, it will be the allele that makes a contribution to what goes on inside our bodies, while if an allele is called ‘recessive’, it is the allele that appears to be overridden.

It’s confusing, I know! Let’s take eye color as an example; although both parents may have brown eyes, it is still possible for their child to have blue eyes. This is because if both parents carry one allele for brown eyes, and one for blue, their eyes will be brown, because the allele for brown eyes is dominant. However, if they both pass on the recessive allele for blue eyes to their child, that child’s eyes will be blue. It works the same way for disease causing variants; if a child inherits the recessive disease-causing allele from both parents, that child will show signs of the disease. If both parents are carriers of a certain disease-causing gene variant, the likelihood that each of their children will be affected by the illness is 25%.