Who do I got this DNA piece from? Chromosome mapping in genealogy

by Marta Sobota
Po kim mam ten fragment DNA?

Below you will find an article prepared for 9. Geneami Genealogical Conference (2022) as well as the recording from a Q&A session that took place a few days after the conference.

I got the DNA test results. What now? 

When we’re anxiously awaiting the results of our genealogical DNA test we often hope, that the relatives we will meet are also going to be passionate about genealogy, with beautiful genealogical trees already prepared, and that it will make it easier for us to find shared ancestors.

However, the truth is that many people don’t have a genealogical tree or they only have information on their parents and grandparents.

🧬 How to find out, what degree of kinship we have if we share DNA but the relative has no information on their genealogy?

🧬 Can I see, in which chromosomes I have DNA from my great great grandparents?

🧬 Did my grandma also have the gen for curly hair?

🧬 Who did I get my eye colour after?

To find answers to these and similar questions we will use chromosome mapping – a technique making genealogical research easier.

If we have a chromosome map, we can look at segments of our DNA that we share with a relative and we will be able to tell, who our shared ancestors were, even if this relative has no genealogical tree developed!

Sounds interesting? Let’s create a map!

Mini dictionary of genetic genealogy terms

At the beginning let’s remember the terms, that will be used later in the article. They are useful to get to know the basics of genetic genealogy and will help us understand the chromosome mapping method.

Centimorgan (cM) – Measuring unit for shared DNS 2

We can think of centimorgans in the same way. The rule is that the more centimorgans we share with someone, the closer we are related to that person. For example, we share around 3500 cM with each of our parents, around 1750 cM with each grandma/grandpa, and with a second-degree cousin (shared great grandparents) around 230 cM.

Match – a relative, with whom we share DNA.

Autosomal chromosomes – 22 pairs of chromosomes ie. 44 autosomal chromosomes. They contain information about around 6-8 generations of ancestors. Their numbering is according to side (no.1 is the biggest, no. 22 the smallest). For our chromosome map we will get data from autosomal tests: 23andMe, Ancestry, Family Tree DNA 3, My Heritage.

Sex chromosomes – this is the 23rd pair of chromosomes, which are known as X and Y. Women inherit two X chromosomes, one after the mother and one after the father. Men inherit one X chromosome after the mother and after the father, they inherit a Y chromosome. This means that men don’t give their X chromosome to their sons, only to daughters.

What is chromosome mapping? 

Chromosome mapping is a method of assigning DNA fragments, which we share with a relative to a certain ancestor or a pair of ancestors.

Marta Sobota

The infographic below depicts a simplified genealogical diagram of Marta and Michał, who are second-degree cousins (they share great-grandparents) and share 230cM of DNA.

So we know, that the parts of DNA that Marta and Michał share have been inherited from great-grandparents – Antoni and Krystyna.

On our map we can assign it to:

  1. A pair of ancestors – Antoni and Krystyna
  2. One ancestor, when making a map for Michał to his grandma Danuta and in Marta’s case to her grandpa Leonard.

There are two systems of assigning the segments – either to a pair of ancestors or to a single ancestor. Both are correct but I’d recommend assigning to one ancestor for two reasons:

  1. sometimes our family has lived somewhere for many generations and at the beginning, we might nów know, that we are related to someone in two different lines. F.ex. on our mother’s side through our great grandma and on our father’s side through a great great great grandfather.
  2. when we create a map for a person from an endogamous community, f.ex. a community of Ashkenazi Jews, where marriages of cousins happen in many generations, assigning segments to one ancestor also makes it clearer, from whom they were inherited.

How to create a chromosome map?

A chromosome map can be created in many ways: on a piece of paper, in an Excel sheet but for beginners I would recommend the Chromosome Map tool on DNA Painter website. Many will recognize this website from The shared cM project tool, which shows us possible degrees of kinship, depending on how many centimorgans of DNA we share with someone. Creating a map on the DNA Painter website is free.

The first step to creating a map is registering in the service. 4. Next, we can start building our map in four easy steps:

  1. Creating a new map or opening an existing one
  2. Choosing a person and source of data
    • As a source we can use data from portals like: 
      • 23andMe, 
      • Family Tree DNA, 
      • My Heritage
      • GEDMatch.
    • Ancestry doesn’t allow downloading information, which segments of our DNA we share with a particular person. To add information about a person, which is in Ancestry to our map we will need to ask them to upload their data to one of the aforementioned portals, and if possible to all. I also recommend uploading data to GEDMatch, because it allows us for a detailed chromosome X analysis and contains other useful tools. Instructions: how to download data and how to upload data
  1. We paint segments on the map, by copying and pasting data and choosing colours that we want to use.
  2. We save the information.

We repeat steps from 2 to 4 for each person, whose information we want to add to our map.

When creating a new map, we choose the sex of the person, whose DNA we will map. The difference between a map of a woman and a man is, that a woman’s map will have space for entering two X chromosomes, and for a man only one.

Below you can find a graphic representation of 22 pairs of autosomal chromosomes and a pair of X chromosomes.

In each pair one chromosome comes from the father (blue line) and one from the mother (pink line).

For each of the 23 pairs of chromosomes, we will assign information, whether the relative comes from our father or mother’s side. Next, we will have to assign these matches to our grandparents, great-grandparents, great-great-grandparents, and so on. How distant relative we will assign a particular segment to depends on how far we are related to the person, whose shared segments we are adding to the map.

For example, let’s look at chromosome pair no. 1. At the graphic below we can see that one chromosome of this pair I have inherited from my father and one from my mother.

When looking further we will see from which of their parents (id. my grandparents) my parents gave me DNA on chromosome no. 1.

The top line depicts in a graphic way chromosome no.1 inherited from my father. We can see that two recombinations occurred and we have three different segments. The first part I inherited from my grandpa (b&w picture), the second from my grandma, and the third also from my grandpa. This means, that in the parts I have inherited from my grandpa, I cannot have any DNA inherited from my grandma’s ancestors.

The bottom linę depicts in a graphic way the chromosome no. 1 inherited from my mother, where four recombinations occurred and we can see four segments: the first from grandpa, second from grandma, third from grandpa, and the fourth again from grandma.  

What conclusion can be drawn from the mapping of chromosome no. 1 above?

If I find a relative, with whom I have shared DNA at the beginning of chromosome no.1, then definately we aren’t related through my grandma, because both on my mother’s and father’s side the beginning of chromosome no. 1 I inherited from grandfathers. So I will look for shared relatives in my grandfathers genealogy.

By checking, with which relatives already added to the map, a particular person also shares DNA, we can see whether this is through grandfather on the mother’s or father’s side. If a relative has a shared segment with me at the beginning of chromosome 1, then their DNA will match the DNA of people already added to the map, either on the father’s or mother’s side.

On the left, we see a clear map at the beginning of work. On the right, we see a map that I have been creating for several months. We can see many different segments and information about relatives, to which these were assigned.

A detailed process of adding segments from different portals is presented in the recording from the 9. genealogical conference Geneami 2022. You will find the video under this article.

Additional information:

DNA Painter gives us the possibility to find out:

1. Which parts of our DNA contain medical information or distinctive features, such as hair color, eye color, muscle build, and color-blindness tendency. From the example below I can read, that on the 15th chromosome, I inherited the genes responsible for my eye color from my grandfather on my father’s side and the grandmother on my mother’s side (left black line).

2. Where are the so-called ‘pile up areas’, which I like to call ‘ armed segments’ – these are DNA regions, where there are no frequent mutations. This may lead to false positive results, ie. showing a relationship with people, white to whom we aren’t really related or we were but many generations back, outside the reach of metrical genealogy. ‘Armed segments’ may be characteristic of certain communities.

Downloading the map

We can easily download the map by opening settings (gear icon at the top of the website and clicking Download a picture of this map


  1. We don’t upload our DNA to DNA Painter – we only place the information, on which segments we have matched with which relatives and which ancestor is our shared point. This means that we don’t place there any sensitive data. In the case of DNA sensitive data means what nucleotides we have on certain positions (f.ex. AT, CC, etc.)
  1. Creating a chromosome map gives us the possibility to compare people in different databases. By placing information from different sources in one place, we can learn about kinship by seeing which segments overlap. F.ex. by playing information about one person from My Heritage and a different person from 23andMe, we will see that these two people have a shared segment on the same chromosome and trójkować with the same relatives – this means that we are related through one ancestor.
  1. By mapping chromosomes, we understand the basics of DNA better. We work with detailed information about different segments and we visualize graphically, how the DNA was passed on from our ancestors. We will find regions, where we have more matches than usual, while at the same time not finding any genealogical ties. This may mean a segment, which is frequent in certain communities.5 (‘armed segment’) or it can mean a discrepancy between genetics and paper genealogy.
  1. We can see that in different family lines, we inherit different amounts of DNA. From our parents, we get an even 50% from each because every pair of our chromosomes consists of one chromosome from our father’s side and one chromosome from our mother’s side but it’s not true that we get an even 25% from each grandparent and an even 12.5% from each great-grandparent. This is because of recombination. When we have a chance to get our grandparents tested we can see that f.ex. we inherited 23% of our DNA from our grandma and 27% from our grandpa. The difference between a perfect or an expected number is bigger the further the degree of kinship. It may happen that we won’t have any DNA from our ancestors from six generations back.
  1. DNA Painter enables easy edition. In a simple way, we can introduce changes by assigning segments to different people. In the beginning, we make different mistakes while making the map but the DNA Painter makes it possible to fix them in an easy way.
  2. We can check from which ancestors we inherited particular genes, f.ex. hair color, eye color or muscle built.  

If you have any questions feel free to comment. I wish you a lot of fun mapping your chromosomes and interesting genealogical finds!


  1. Advanced Genetic Genealogy: Techniques and Case Studies,
    editor: Debbie Parker Wayne, Wayne Research, 2019
  2. Guide to DNA Testing and Genetic Genealogy,
    Blaine T. Bettinger, Penguin Random House, 2019
  3. DNAPainter, Blaine T. Bettinger, 2017
  4. DNA Painter – Chromosome Sudoku for Genetic Genealogy Addicts,
    Roberta Estates
  5. Why map your chromosomes?,
    Jonny Pearl, 2020

Recording of the lecture ‘Chromosome mapping in genealogy – Who do I got this DNA piece from?’

A lecture prepared for the 9. Genealogical Conference Geneami in Brzeg, 24 September 2022.

Recording of the Q&A session about chromosome mapping

A Q&A session for the lecture ‘Chromosome mapping in genealogy’, which took place on 27 September 2022.

You will find a detailed list of questions in the video description on YouTube.

  1. More in the article What is a centimorgan/efn_note]. Let’s think of it as a centimeter. If our DNA is a measuring tape 7000 cm long then 3500 cm we inherited from our mother and 3500 from our father. 1 Even the abbreviation is very similar to the centimeter, the difference being that the centimorgan abbreviation we spell with a capital ‘M’ (cM)
  2. In the case of Family Tree DNA I mean their autosomal test – Family Ancestry (before: Family Finder), but also available in their offer chromosome Y and mtDNA tests
  3. At the end of this article you will find a film about chromosome mapping, containing detailed instructions, that will take you through the whole registration process.
  4. Such region is f.ex. the beginning of chromosome 15 in MyHeritage, where many people report a false positive match.
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