Glossary – DNA – Deoxyribonucleic Acid

What is DNA and why do I care?

Good questions. Let’s take a look at the answer in general, then why we use DNA for genealogy.

The Recipe for You

DNA, deoxyribonucleic acid, is the book of life for all organisms. In essence, it’s the recipe for you – and what makes you unique.

DNA is formed of strands that twist to form the familiar double helix pattern.

The two strands are joined together by one of 4 different nucleotides, one extending from each side to connect in the middle. The nucleotides are:

  • Cytosine – C
  • Guanine – G
  • Thymine – T
  • Adenine – A

The nucleotide names don’t really matter for genetic genealogy, but what does matter is that the sequence of these nucleotides when chained together is what encodes information on long structures called chromosomes. Each person carries 22 chromosomes, plus the 23rd chromosome pair which is gender specific.

Using DNA for Genetic Genealogy

There are four different kinds of DNA that genealogists use in different ways for obtaining ancestors’ information relevant to genetic genealogy. Thankfully, we have 4 different kinds of DNA available to us because of unique inheritance patterns for each kind of DNA – meaning we inherited different kinds of DNA from different ancestral paths. If one kind of DNA doesn’t work in a particular situation, chances are good that another type will.

Genetic genealogy makes use of 4 different types of DNA.

  • Y DNA – passed from males to male children, only (your father’s paternal line)
  • Mitochondrial DNA – passed from females to both genders of children, but only females pass it on (your mother’s matrilineal line)

Y and mitochondrial DNA inheritance paths are shown on a pedigree chart in the graphic below, with the blue boxes representing Y DNA and the red circles representing mitochondrial DNA inheritance.

In addition to Y and mitochondrial DNA, genetic genealogists also use two kinds of DNA that reflect inheritance from additional ancestral lines, in addition to the red and blue lines shown above – meaning the ancestral lines with no color.

  • Autosomal DNA – the 22 chromosomes that recombine during reproduction.
  • X Chromosome – always contributed by the mother, but only contributed by the father to female children – this is the 23rd chromosome pair which recombines with a unique inheritance pattern.  You can read more about that in the article, X Marks the Spot.

Receiving What Kind of DNA from Whom

While the Y and mitochondrial DNA have unique and very prescribed inheritance patterns as shown by the red arrows pointing to the blue Y chromosome below at far left, and the red mitochondrial circles at far right, the 22 autosomal chromosomes are contributed equally by each parent. In other words, for each chromosome, a child inherits half of each parent’s DNA. How the selection of which DNA is contributed to each child is unknown.

A child’s gender is determined by the parent’s contributions to the 23rd chromosome, not shown above. The following chart explains gender determination by the X and Y combinations of the 23rd chromosome.

Received from Mother Received from Father
Male child X Y
Female child X X

The Y chromosome is what makes males male.

No Y chromosome?  You’re a female.

However, this X chromosome inheritance pattern provides us with the ability to look at X matches for males and know immediately that they had to have come from his mother’s lineage – because males don’t inherit an X chromosome from their father.

Autosomal DNA and Genetic Genealogy

The 22 non-gender chromosomes recombine in each generation, with half of each chromosome being contributed by each parent, as shown in the illustrations above.

You can see that in the first generation, the child received one blue and one yellow, or one pink and one green, chromosome. In giving each child exactly half of their DNA, each parent contributes some amount of ancestral DNA from generations upstream, as you can see in the mother/father and son/daughter generations.

For example, each child receives, on average, 25% of each of their grandparent’s DNA – although they can receive somewhat more or less than 25%, depending on the random nature of recombination.

Therefore, genetic genealogy testing companies compare tester’s autosomal DNA with other testers and look for common segments contributed by common ancestors, resulting in autosomal matching.

When relatively large segments match between three or more relatives who are not immediate family, we can attribute that DNA to a common ancestor. Of course, the challenge, and the thrill, is to determine which common ancestor contributed that common DNA to our triangulated match group. It’s a great way to verify our research and to break down brick walls.

Let’s face it, you received ALL of your DNA from SOME combination of ancestors, and if you carry large enough pieces from any specific ancestor, we can, hopefully, identify the source of that DNA segment by looking at the genealogy of those we match on that segment.

It’s a great puzzle to unravel, and best of all, it’s the puzzle of you.

More Info

The great news is that you can utilize your Y DNA, mitochondrial DNA and autosomal DNA differently, to provide you with different kinds of information about different ancestors and genealogy lines.

If you’d like to read more about how the 4 Kinds of DNA can be used, please read the short article, 4 Kinds of DNA for Genetic Genealogy.

You can also enter any word or phrase into the search box in the upper right hand corner of this blog to find additional useful information about any topic.

If You Want to Test

If you’d like to learn more about the various kinds of DNA tests available, and which one or ones would be the best for you, please read the article, Which DNA Test is Best?



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9 thoughts on “Glossary – DNA – Deoxyribonucleic Acid

  1. Roberta, When I downloaded my raw dna results from 23 and me, I saw the nucleotides you list. Is there information on how these determine and combine to show the expression of ethnic and continental history available for the lay genealogist?

  2. I never imagine when i must took a several kind of the DNA Testing JUST to confirmed my Ethnicity which my DNA Test Results confirmed when i’m just an ordinary Southeast Asian or Southern Han* Chinese based from my Uniparental DNA Haplogroups:
    – Y DNA Haplogroup O-CTS11856 (Big Y), O-CTS5492 (Geno 2.0) and a Y111 STR’s.
    – mtDNA Haplogroup B4c (FTDNA) or B4c2 (Geno 2.0).
    and My Ethnic World Region (Autosomal DNA) to the people around me, in Surabaya – East Java – Indonesian.
    I’m about 73% Southeast Asian + 26% Northeast Asian + <=1 Siberian from FTDNA FF – My Origines 2.0.

  3. Thank you, Roberta, for this excellent, concise primer. Have been receiving your articles for about two years now, and cannot think of enough superlatives to offer you for all you have shared so willingly with us. I will be posting a link to this page in my surname project (my cousins’ DNA) as soon as I can, for there are so many now who are purchasing commercial DNA tests, but without any prior knowledge of genetics at all and as a result are confounded by the results. And while there are many informative pages out there if one can wade through the techno-jargon, your articles are more than instructive, informative–they are simply pure delight to read. God bless you!

  4. Excellent summary.
    I too love the symmetry of the helix you have used in illustration above and often use it myself for presentations. Why, oh why, is is so hard to get a really good-looking representation of the real DNA helix? With strands seen along one side of the cylinder alternately close together and then further apart, not evenly spaced.
    Christopher Schuetz

  5. Roberta, I don’t mean to be picky but in the 1st paragraph it is 22 PAIRS, plus the 23rd chromosome pair which is gender specific. The gametes have 22 chromosomes.

    By the way, as a biologist, I am putting together a short course in sex and genetics for members of the Davis Genealogy Club on the request of a number of members. It takes a different approach than I generally find in Genealogy blogs. If you are interested, I would be happy to send you the text.
    Your comments would be greatly appreciated.

  6. What type of privacy do you provide for test DNA results? Do you surrender all access and records of DNA results after testing/evaluation?

    • I don’t know who you’re asking this question of. I don’t do any testing of anyone. I write about genetic genealogy Various companies do testing through their labs. You will need to read the terms of conditions of each company individually.

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