DNA basics

Overview of DNA and chromosomes

We each have 23 pairs of chromosomes containing DNA plus we have some more DNA that can be tested in our mitochondria (mtDNA), the little organisms that make a cell’s energy. In human DNA, chromosomes 1 through 22 recombine for reproduction.
Brynne Ancestor MapA child gets one copy of each chromosome from each parent, thus a pair. However all of these first 22 chromosomes will be a mix of the genes from that parent’s parents. The DNA from the parent’s two copies of each chromosome is randomly recombined to make a single new version of that chromosome which is then passed on. This way siblings can inherit quite different DNA from their parents although normally they share about 50% with each other (see the diagram in my blog post for the comparison between me and my brother, we share 47%).

A really lovely example of four generations of DNA inheritance was done by Angie Bush for a presentation and discussed in one of my blog posts. The image here,  which shows the eight known sources of her daughter’s DNA, was made by Angie.


The X and the Y: the 23rd chromosome pair

The 23rd chromosome pair is either an XX for a girl or an XY for a boy. One X chromosome will be a recombined mixture from the mother’s parents but the second X for a girl is an unrecombined X from her father (recombined by her father’s mother) since he has only one X chromosome to pass along to her. To his sons he passes a Y which is what makes them boys not girls.

The fact that the X does not get recombined by fathers means that matches on the X chromosome can reach much further back in time than the other chromosomes, since they may not have been recombined as often. Another thing to note about the X is that a number of conditions like hemophilia, color blindness, and baldness are governed by recessive genes on the X so are infrequent in women who need two copies of that gene as compared to men who need only one copy since the Y is a blank for those genes. This type of gene is called sex-linked.

The Y chromosome has no second Y to recombine with, so it is passed from father to son with almost no change other than mutations. This makes the DNA on the Y chromosome very useful to test for genealogy purposes. The Short Tandem Repeat (STR) count on the Y changes frequently enough, every few hundred years or so, for paternal line (paternity) testing whereas the Single-nucleotide polymorphism (SNP) mutations on the Y are more indicative of deeper ancestry in the thousands of years.  Since the mitochondria only come from the mother, mtDNA testing gives information about the maternal line deep ancestry. Currently less useful for genealogy other than to disprove a maternal line of descent when the haplogroup does not match.


The various mutations of Y chromosomes from the original common ancestor of us all have been cataloged and assigned to groups known as haplogroups. These tell you much about the locations of your thousands of years back ancestors. ISOGG maintains the most current list of Y haplogroups and has a good write up of genetic genealogy basics. There are also haplogroups for mtDNA. So the completely male or female line can be traced back into prehistory this way. Eupedia.com has a nice set of maps and origin explanations for all these haplogroups.

DNA Testing Basics for Genealogy

Because the Y chromosome and mtDNA get passed down along only one line of your ancestry, Y: fathers to sons, mtDNA: mothers to daughters, testing them is a much clearer and easy to understand process. Meg Smolenyak’s book, Trace Your Roots with DNA: Using Genetic Tests to Explore Your Family Tree is the best introduction to the use of these tests for genealogy and tracing your roots that I have found.

However autosomal DNA testing, that is testing chromosomes 1-22 for the most important markers, is the most frequent and comprehensive tool being used. This is the type of test offered by 23andme, Ancestry.com, MyHeritage.com and the family finder test at Family Tree DNA. The problem is that it is so amorphous that it is hard to understand. Autosomal DNA inheritance is quite random and becomes more so the more generations the DNA passes through which many find frustrating.

Click here for my slides from my presentation “You DNA Tested,  Now What? which discusses ethnicity results and haplogroups, but mainly focuses on how to use your DNA results for cousin matching and genealogy. Click here for a slightly more advanced presentation “New Tools for DNA Cousin Matching which discusses the latest tools from the testing companies to help figure out your DNA cousin relationships.

There are a number of books on the basics of using DNA testing, but the field changes rapidly. The newest and perhaps best book these days is Blaine Bettingers’s The Family Tree Guide to DNA Testing and Genetic Genealogy. Also see my book recommendations on the DNA-NEWBIE FAQ.

I also recommend Kelly Wheaton’s series of free online lessons on Genetic Genealogy highly for learning more about DNA testing:


Can you find new relatives with autosomal DNA testing?

To find relatives you contact the people with whom you have the most matching DNA shown at your testing company or GEDmatch. More than one matching segment of at least 7-10 cM is a good starting criteria for finding closer relatives. Single segment matches of that size can be anything from 4th cousins out to 14th cousins. Overlapping segments are the key. If you overlap with more then one person and they match each other at the same spot (called triangulation) then all three of you share an ancestor, so exchange family trees and see if you can find him or her!

Click here for my blog post on finding DNA relatives which explains how to find the matching segment data. I also did a post that shows the step by step usage of the tools at 23andme.

Personally I have had great success finding new Norwegian cousins with DNA: 3rd, 4th, 5th, 6th, and further; but very little success on my German or half Ashkenazi side. One thing I have discovered is that DNA can persist through many more generations than expected. So I googled and found a good article which I link to from my blog post on single segment matches.

The ISOGG wiki has a chart of the expected amount of DNA shared with your various relatives which is very useful in determining how close a relationship might be. The problem is that if a population is very endogamous, that relationships will appear closer than they are. This is very apparent with Ashkenazi DNA and I even sometimes find that cousins who look close in Norway are not, rather we share more than one common ancestor.

This online calculator lets you enter the centimorgans or percentage shared with a match and will give you the probability for the different possible relationships: https://dnapainter.com/tools/sharedcmv4


What is your objective in doing this testing? If it is solving a genealogical brick wall let me know how you did. I solved the one that started me on this testing path (see the post on Lars Monsen) and another one along the way. If it is curiosity and finding more relatives then enjoy and read on.

I have summarized the testing companies and my thoughts on them on my DNA testing page.

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68 thoughts on “DNA basics

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  1. Kitty
    You ask What is your objective in doing this testing? If it is solving a genealogical brick wall let me know how you did. Yes I was hoping it would help but it was an awakening when my close male relatives (YNDA) tested 100% for a different surname. The brick wall got bigger and now not sure who we are!

    • Cheri –
      That reminds me of a story I heard about two women who were showing as 2nd-3rd cousins via DNA but could not figure out how they could possibly be related … until they noticed in an old census that their great-grandparents were next door neighbors …
      So develop a theory or two and get more people to test that would prove or disprove that theory 🙂
      There could be many reasons for the surname difference. Some countries, like Norway, did not use surnames until a little over 100 years ago. Some men on moving west in America changed their names to escape their past. Or it could be a non paternal event (NPE). So get some 2nd, 3rd, and 4th cousins tested on that paternal side. Do autosomal in addition to a few Ys.
      – Kitty

  2. Kitty, On FTDNA Chromosome Browser I sometime have a match on X Chromosome. What is the significance as far as trying to find in common relationships?? Thanks Sylvia

  3. Hi there. I am brand new to this and really grateful for your resources. I uploaded my data to GEDMatch last week and it says that it’s clear for one-to-many matches but none are turning up for myself or my husband even though we have tons of matches on 23andme.com. I think I’m doing something wrong. Can you possibly offer guidance on this process? Does it take time for data to be processed after the double asterisks clear from the “Your DNA Resources” box on the home page? Do I have to upload multiple kits from different providers to find matches? Thanks!

      • Those two stars did clear, but I still see no matches and my husband has no matches as well, so that’s my confusion. Is there still a lag in processing after that point? Thanks again!

  4. Hi, I’m interested in finding information of DNA comparison in the same family of a 60, 30, 20 and teenage sample group. Do you have any leads as to where or whom I can contact? I’m interested in understanding the genetic differences based in age.


  5. You are not going to find differences based on age but multi generation inheritance is fascinating. See the article about professional genetic genealogist Angie Bush’s family here on this site blog.kittycooper.com/2014/09/using-the-chromosome-mapper-to-make-a-four-generation-inheritance-picture/

  6. I have a question about DNA testing. I went to your class at Rootstech and realized that I should have an autosomal test done for my Father and Mother. I purchased the test from Ancestry. My parents are 86 and 84 and my time is short, so I traveled to visit them but after many attempts my Mother could not give any saliva – absolutely none!!!! It was very frustrating. She was diagnosed with Pancreatic Cancer in November 2014 and we understand only has weeks left. We tried all the tricks we found on the internet, but still no saliva. I’m not sure what to do now.

    • Brent, so sorry to hear this.

      I recommend getting family tree DNA’s test for the elderly because it is a cheek scraping, no saliva required. Plus they keep the test material so you can run other tests on it for 25 years. I have also heard that it can be collected by an undertaker as well.

      You can then upload the ftDNA result along with your family’s other tests at ancestry to GEDmatch or just upload the ancestry tests to ftDNA for comparisons.

  7. Thanks Kitty. Great information. I will order the Farmily Tree DNA test for my Mother. I knew you would have the answer. By the way, your class was exceptional. Thanks for all you do to teach DNA concepts.

  8. Hello,

    I had a DNA test run at Family Finder and I am still googling info so that I can understand how to check relationships..I have many matches and one I pretty much knew was a cousin and we share a great-great grandfather, though I have never met this cousin. So, I am trying to get more knowledgeable so that I can use these tests in a more useful way for my genealogy projects.

    Thank you for your blogs!

  9. Wow! In the past year, I’ve tested myself, my daughter, my mom, my dad, and my spouse using ancestry. (I wish I had known about the cheek kits at ftdna when doing my parents’ kits.)

    Just recently, I uploaded the results to gedmatch and have been semi-lost with the differences from ancestry.com and gedmatch. Your site has been so helpful.

    THANK YOU SO MUCH for your work on this blog. I appreciate how much info you share with others like me who are lucky enough to benefit from your knowledge, dedication, and hard work. I can’t thank you enough!

  10. Several months ago, I uploaded my Ancestry DNA to GedMatch. Do I need to do an update on that information or is it automatically updated as more matches are found on Ancestry?

  11. I am so confused with all the terms about DNA. I am trying to figure out who I am related to on my fathers side. All i know about my father is that he was a Greek Merchant Mariner named Nicholas. I have matches that I have found on my mothers side, using ancestry. How can I determine on GED which matches are those of my mother vs. those of my father?

    • Is your mother tested? Or any relatives on her side? That is how you separate paternal and maternal is have maternal family tested.

      Check out DNA detectives on facebook and/or DNAadoption.com for the methodolgy for finding an unknown parent with DNA

  12. Hi
    I am curious what you think about these results. Who is May to the father and son. Thank you so much for taking a look at these!!

    Father & Son
    Total cM 3572
    largest cM 214
    Gen 1.0
    X-DNA 0

    Son to May
    Total cM 1095
    largest cM 63
    Gen 1.9
    X-DNA 0

    Father to May
    Total cM 1885
    largest cM 92.0
    Gen 1.5
    X-DNA 167
    largest cM 64

    • Soryy!! with GEDmatch when searching matches it show the largest segment being 92. but when comparing one to one, there are actually 4 chromosomes that are over 100. unless i am reading it wrong. this is what it shows for one to one.

      Ch 1 – 101.8
      Ch 2 – 132.8
      Ch 6 – 106.5
      Ch 11- 100.

      Also for X. there are 3 shared X Chs.

  13. Thank You so much! She is the father’s Aunt. His Mother’s full sister. The total seems soo high though, esp the X. the Mother and Aunt did have the same father, but wouldn’t it dilute more going down from Mother to son. and the 1.5 and 1.9 Gen is weird but i guess thats only based on the numbers.

  14. ok Added! Is there a sample of what you’ve collected specifically “aunt to sister’s child” would love to see it if its available.

  15. Soryy!! with GEDmatch when searching matches it show the largest segment being 92. but when comparing one to one, there are actually 4 chromosomes that are over 100. unless i am reading it wrong. this is what it shows for one to one.

    Ch 1 – 101.8
    Ch 2 – 132.8
    Ch 6 – 106.5
    Ch 11- 100.

    Also for X. there are 3 shared X Chs.

  16. This is the info when I do one to one. i dont know why it slightly differs than its noted on the one to many matches.

    Father & Son
    Largest segment = 281.5 cM
    Total of segments > 7 cM = 3,585.2 cM
    24 matching segments
    Estimated number of generations to MRCA = 1.0
    No shared X-DNA segments found

    Son to May
    Largest segment = 100.0 cM
    Total of segments > 7 cM = 1,099.4 cM
    37 matching segments
    Estimated number of generations to MRCA = 1.9
    No shared X-DNA segments found

    Father to May
    Largest segment = 132.8 cM
    Total of segments > 7 cM = 1,878.8 cM
    56 matching segments
    Estimated number of generations to MRCA = 1.5
    Chr Start Location End Location Centimorgans (cM) SNPs
    X 2,321 29,921,522 51.4 4,214
    X 40,686,138 122,152,592 65.3 7,027
    X 125,094,846 154,886,292 52.7 4,281
    Largest segment = 65.3 cM
    Total of segments > 7 cM = 169.3 cM Actual.

  17. So based on the actual breakdown, do you think the numbers are too high? She is definitely the aunt, but what else could cause them to be so high?

    • The numbers are fine. An aunt is normally about 1700 cM but it can vary in either direction as the statistics at ISOGG show. Big chunks are expected. The son’s numbers are fine too. He got a little more of the DNA shared with her and that is random.

  18. I just ran my sister’s 23 and me test through ged match in comparison with my ancestry test. It shows that we share 1905cM. Out of all the things I’ve read I’m led to believe that we are half siblings. Does that seem to be the case?

    • Sister and Myself
      Largest segment = 167.2 cM
      Total of segments > 7 cM = 1,905.1 cM
      54 matching segments
      Estimated number of generations to MRCA = 1.5

      Largest segment = 26.5 cM
      Total of segments > 7 cM = 26.5 cM Actual.

      Figured this may help. Thank you for your time!

        • In other words, full siblings will have lots of blue bars as well as green bars and half siblings will only have the blue bars. That is also about the amount of DNA an uncle/niece will share, so double check if there was an in family adoption that you were unaware of.

          • Thank you! I had looked at your example previously and mine doesn’t look like yours. We have a lot of green yellow and red. Only on chromosome 16 do we share a whole smaller section. I tried to copy the image but I was unsuccessful. My sister and I are 6 years apart so I’m not sure how the adoption would apply to us. You would have better insights into that area. I’m not the best at this stuff. Again thank you so much!

          • By an in family adoption, the typical case is where a different older sister has a child out of wedlock that the mother then raises as one of her own. So the other children, presumed sibs, are actually aunts and uncles to this younger sibling.

  19. We do share a lot of blue bars but not the solid green portions like you and your brother. Considering our age difference of only 6 years, it would be fairly safe to rule out adoption? I did notice that we are on the upper end of the ranges for half siblings but not enough to be full siblings, which made me wonder if there was something else to look into.

    You have been so unbelievably helpful!

    • I just took another look at the GEDmatch. We have a lot of blue but it appears to be about half of what you have compared to your brother.

      • I kept mixing up blue and green (just back from a trip and tired). If you do not have lots of green bars you are not full siblings.

  20. Hello Kitty

    Thank you so much for your blogs. They are so helpful, especially if you are new to this.

    My son has just received his DNA matches result from Ancestry and he is matching to my brother as a first cousin. Is this possible?

    Many thanks again

    • I was given to understand that an uncle/aunt and niece/nephew shared 25% of their DNA, and first cousins shared 12.5% which was why I queried it. Thank you for clearing that up.

  21. Thank you Kitty. I had looked at that link. It all gets quite confusing and I know it isn’t an exact science. I have not been able to check the percentages on Gedmatch yet, as it is still uploading the information. As the website stated great-uncle/aunt and first cousin as approx 12.5% I was getting confused.

    Thanks again.

  22. Hello again Kitty
    My son’s DNA results are now on Gedmatch. Can I just check with you; when I go on to my brother’s kit number my son’s results are:

    Total CM: 1982.8
    Largest CM: 147.7
    Gen: 1.4
    X Largest: 54.1
    X Total: 28.1

    When I go on to my son’s kit number my brother’s readings are exactly the same. Is this correct? I apologise if I am missing the obvious here.

  23. Thank you so much for your blog!

    1/2 siblings both femaile should share 196cM’s on the X chromosome. Is that correct? They share a total of 1801 cM’s and 63.4 on the X. Wouldn’t they be half aunt/niece rather than 1/2 siblings?

  24. Thank you Kitty! I forgot to add they are would have shared a father not mother….so this would prove that they are aunt & niece rather than siblings, which also makes sense as there is a 20+ year gap in age, although that’s not necessarily a factor.

  25. Hello MsKitty,
    Both my brother and I did DNA tests through Ancestry. He is designated as a “close family – 1st cousin” to me. Our shared DNA is 1729 centimorgans across 63 segments. We have a first cousin on my father’s side who also did a test. He is a first cousin match to my brother, but no match to me. My brother and I also have DNA matches that are different “levels” of cousins for the two of us, for example 2nd cousin to me and 3rd, 4th or 5th cousins to him. Rather than a full sibling, am I assuming my brother is a half-sibling. Is it reasonable to assume that we do not share a father? Or am I oversimplifying? Thank you.

  26. I just learned that my father is not my biological father, and that I have a 1st cousin/niece that is an adoptee. She and I have been working together to try to find anything we can. She is pretty much dependent on me because her records are closed and all she knows is her mother’s age. I am 76 years old so I know it is going to be very difficult to find my real father because there are no living relatives. Can you give me any suggestions as to where to start? I have written a few 3rd cousins, but no luck so far. Should I contact all of them (there are a lot)? Any help or pointers will be appreciated.

  27. I’m not sure if this is the appropriate place to ask for advise. My sister & I are 55+ and 35 yrs ago our Mother eluded to my sister that she doubted her paternity and never giving her a straight story on the fathers information. Our Mother now has dementia with failing health and my biological father passed away 20 yrs ago. I so want to help her find the answers she so desperately needs. We both submitted our DNA to MyHeritage (herd isn’t back yet), I’m confused about what we do with the information to extract the answers we are looking for. I’ve been trying to find a blog or group with similar circumstances to bounce ideas off of and have had no luck. Thank you in advance for any insight 😉

  28. I found a surprise when I got my Ancestry DNA results back, a close relative – 1524 cm over 58 segments. I have an extremely small family so this was strange to not know who this was. When I figured out the numbers and it could be grandparent, aunt/uncle, half sibling or double first cousins. I was confused to start with, debated to send a message to her but I did, I was wondering if she was adopted and when she was looking she was wondering if I was. We are related on my Dad’s side. I found out she was estranged from her Mom so she wouldn’t ask her, my father has passed. I have looked at it every which way and can only see that it is half sibling, really not a total surprise if you knew the whole situation. Surprisingly her father had done a DNA test and he hadn’t shown up for me. We become friends on facebook and I find out she has a twin brother that looks just like my Dad. They are 10 years younger then me. Then she talked to her husband because I think she was seeing that maybe her father wasn’t her father and he said to leave it alone and she took herself off of Ancestry. I have screen shots everything, I have come to a point where I also just have to accept unless she comes back on I that I also have to leave it alone. I can’t do anything with her like gedmatch, I am just wondering if you think 1524 cm over 58 segments is strong for half siblings? I understand if you wouldn’t know without any further tests. Thank you.

  29. Hi Kitty,

    I’m working with a group that is looking to prove or disprove our ancestors were either brothers or other close relationship back in the mid 1700’s.
    My 1st cousin who descends from line A and a descendant of line B have a match on chromosome 16 at 11.5 cM’s. They both have others that match them on that chromosome but none that triangulate, at least none that we’ve found so far.
    How big of a leap is it to identify a chromosome to a particular ancestor without a third person? We’ve pretty much ruled out any other common line of descent.

    • Dee –
      That far back it is hard to use autosomal DNA to prove the relationship plus triangulation is not particularly likely. So go ahead and assign that piece of DNA to those ancestors even though it is not for sure

      If you can do a Y test on a male line descendant from each brother that is your best chance to prove that they were brothers or from the same family line. A Family Tree DNA 37 marker test would be a start.
      Here is my affiliate link: https://affiliate.familytreedna.com/idevaffiliate.php?id=1529_0_3_5

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