Genealogical Ponderings

the Professional Family History Blog

Professional Family History Blog
  1. Demystifying DNA 1: Introduction


    DNA testing is becoming a more and more important part of genealogical research. It is estimated that 12 million people have now been tested, 7 million of those with AncestryDNA. Should you test and why? With so many tests available and so many companies to choose from it can be difficult to know where to start.

    This is the first in a series of articles that starts at the beginning, demystifying all that is DNA testing in genealogy.


    The Tests

    There is more than one type of DNA test and the test you choose will depend on what you are using DNA to look for:

    • Y-DNA testing
    • mtDNA (mitochondrial DNA) testing
    • autosomal DNA testing
    • X-DNA testing
    • deep ancestry or ethnicity tests


    The Testing Companies

    Some companies offer individual tests and some offer more than one type of data from a single test. The major testing companies are:

    • Ancestry DNA
    • Family Tree DNA
    • 23andme
    • Living DNA
    • My Heritage


    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:

    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.

    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.

    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.
    • mtDNA is passed from mother to her children (but is not passed on by her sons) and can be used to determine maternal ancestry, again unchanged for many generations.
    • 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.
    • Autosomal DNA is passed from parent to child in varying combinations and can be used to assess close cousin relationships.

    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

    Each type of DNA test will be discussed in detail in subsequent blog posts, looking at when to use each type of test, who can each test and what the results look like. Next up, Y-DNA.



    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.

    2 thoughts on “Demystifying DNA 1: Introduction

    1. Kevin Rankin

      Found the information very interesting and comprehensive. I look forward to the subsequent blogs in the series.


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