Demystifying DNA 1: Introduction to DNA Testing

The "Sciencey" Bit

Before we get into the detail of what each type of DNA testing looks at and why you should choose a particular test, let’s start with the “sciencey bit”, keeping things simple.

DNA exists within almost every cell of the human body and contains long strings of nucleotides containing bases: adenine, cytosine, guanine and thymine, more commonly referred to as A, C, G and T. I am sure you are familiar with the double helix structure of DNA illustrated above. This is actually two DNA strands woven together, the interaction of the bases of one strand (blue circles) with the other (purple circles) forming the ladder like construction of base pairs, A always pairs with T and C always pairs with G.

Most of the tests we are interested in are concerned with the DNA within the nucleus of each cell. This DNA is packaged into chromosomes and the chromosomes are grouped in chromosome pairs of two similar, but not identical chromosomes. This relationship is illustrated, thus:



We have 22 pairs of chromosomes, known as autosomes, and a 23rd pair, the sex chromosomes. Females have two X chromosomes in this pair and males have one X chromosome and one Y chromosome. Chromosome pairs are shown below in an arrangement known as the human karyotype:

So already you can start to see where the Y-DNA, X-DNA and autosomal DNA tests come in.

The last type of DNA is a special case. Mitochondrial DNA is found within a different component of human cells: the mitochondria. It is not packaged into chromosomes but into a single ring of DNA.

Inheritance of DNA

In each case our DNA makes us unique. It is the ways in which the DNA varies from one individual to another and the ways in which it is passed down the generations that enable us to use DNA for genealogy research.

However, let’s not forget: human DNA is made up of around 3 billion base pairs and around 99.5% of those are the same in all of us – they are what makes us human. It’s only that 0.5% we are looking at when we talk about DNA testing in genealogy.

Which type of DNA test do you need?

We will talk about each type of testing in more detail in upcoming blog posts. In very general terms:

• Y-DNA is passed from father to son and is unchanged for many generations. It can be useful for surname projects and looking at ancient origins.
• mtDNA is passed from mother to all of her children (but is not passed on by her sons) and can be used to determine maternal ancestry, again unchanged for many generations. It is useful for identifying remains and looking at ancient origins.
• X-DNA is passed from parent to child but a daughter will receive X-DNA from father and mother, a son will only receive X-DNA from his mother. It can be useful to place autosomal matches in a particular part of your family tree.
• Autosomal DNA is passed from parent to child in varying combinations and can be used to assess close cousin relationships. It has mostly replaced Y-DNA and mtDNA for looking at living parent-child relationships now.

It’s easy to see why there is so much confusion about which test to take. Before deciding which test to take it is important to be clear in your mind what you are looking for, what is the question you want answered? 

Next up

Subsequent blog posts cover each type of DNA test in detail. For ease of reference the links are here:

Demystifying DNA 2: Y-DNA tests

Demystifying DNA 3: mtDNA testing

Demystifying DNA 4: Autosomal DNA, Ancestry DNA and Family Finder tests

Remember

With all types of DNA testing, the DNA test is one source of information that can be used to aid genealogical research. You would not base a family tree on a single census record and in much the same way, DNA needs to be used in the context of known information and documentary research.