Segments: Bottom-Up

This blog post looks at the segments you got from your ancestors. It will be an effort to outline what you should expect from various ancestors – to give you an overview of how you got your segments, and how they are arranged. Your DNA is like a big jigsaw puzzle, with many different and unique pieces – this will let you see a picture that might help you solve the puzzle. Of course, your picture will be somewhat different. DNA is very random, and there is wide variation in what you actually got. Nevertheless, there are averages, and there are some rules. I hope this post will give you an understanding of the big picture, as well as some detail, and help you work with atDNA segments.

There are two ways to look at your DNA: Top-Down and Bottom-Up.

Top-Down is the way you got your DNA segments – from your distant ancestors (from the top of your Tree), down to you (at the bottom of your Tree). This Top-Down explanation often includes many ancestors and DNA segments (lots of colors in the diagrams), and can get quite complex after a very few generations. I’ll attempt a simple version of the Top-Down look in a separate blog post.

Bottom-Up tends to be the way we look at our DNA – we start with all of our own DNA and divide it into maternal and paternal sides; and then determine the segments from grandparents and Great grandparents, etc. We work from ourselves (at the bottom of our Tree), up the Tree as far as we can go. This is the “look” that will be described below.

Before we start, there are three points to make:

  • We are talking about ancestral segments (see What is a segment). These are all segments you got from your ancestors.
  • We are not talking about shared segments with Matches. This discussion is only about you and your ancestors and the segments from them. There will be more about shared segments in later other blog posts.
  • DNA is random. This is a rough model; a general picture of our DNA segments. Please don’t get lost in the technical details or in an unusual situation. This is all about what you are likely to encounter – it’s definitely not a one-size-fits all description. Your DNA is different, but the general principles below will apply.

Before we discuss shared segments (in a later blog post), it’s important first to understand how our DNA is made up of segments from different ancestors, and generally what to expect about these segments – how they are arranged in each chromosome. This understanding of ancestral segments will help you understand shared segments later.

Three ground rules:

  • This discussion will be about autosomal DNA (atDNA) – the numbered chromosomes (chromosome 1 to 22)
  • This discussion will focus on one parent – Mother. The concepts apply equally to either parent.
  • Examples will usually use chromosome 5.

So let’s get started…


You get many large segments from your Mother. They are exactly the size of each chromosome – because each segment is a chromosome. Your Mother gave you one of each of the 22 autosomes (chromosomes 1 to 22). In each case this is a large segment from the beginning of a chromosome (the first base pair) to the end of each chromosome (the last base pair). You probably already knew that you got one set of chromosomes from your Mother, but you may not have thought about them as very large segments. But this is all about segment-ology. You get your DNA segments from your ancestors – your Mother is an ancestor, and she gave you the ultimate segments – entire chromosomes.  See the chromosome 5 example in Figure 1.

05A Figure 1

So where did your Mother get this large segment? She got it from her parents – your two grandparents – through a process called recombination. Read on…

Recombination and Crossovers

Here is a very brief overview of recombination and crossover for genealogists:

A parent takes parts (segments) of the two chromosomes from her parents, and creates one new chromosome which she passes on to a child. Basically, when recombination occurs, a parent starts with one of their parent’s chromosomes and then shifts, or crosses over, to the other parent’s chromosome. This recombination results in two segments separated by one crossover. This process may be repeated several times on one chromosome. We’ll talk more about the probability of recombination below.

Recombination is a very complex process that is the cornerstone of life and diversity. You can google “DNA recombination” for more, but this brief summary is all you really need to know for genetic genealogy. See the Figures below to see examples of how this works.

Three important points:

  • After recombination, the new chromosome is exactly the same size as each of the two chromosomes which were used to form it.
  • The segments are “heel-and-toe” – that is, they are adjacent. When one segment ends, the next segment starts. There is no gap between segments.
  • The crossover point marks the point between segments from two different ancestors. You change from one ancestor to another at this point. 


So let’s look at your maternal chromosome 5 at the grandparent level. That is chromosome 5 with segments from the two maternal grandparents. See the chromosome 5 example in Figure 2.

05A Figure 2

There is lots of information here:

  • Segments from the two grandparents “fill up” the entire chromosome.
  • The segments from the grandparents alternate.
  • There are three segments and two crossovers (we will look more into the number of segments and crossovers below).
  • There are no gaps between segments.
  • These segments tend to be large, and the crossover points tend to be widely separated.
  • Note: Only ancestors from grandparent 1 can contribute to the segments from grandparent 1. In other words the segments for grandparent 1 can only come from the ancestors of grandparent 1. Ditto for grandparent 2.

Crossover Points

OK – before we continue, we need to look at the realistic number of segments and crossover points we should expect in each generation. Science has found that in one generation (Mother to you, for example), there are about 35 crossover points spread out over all 22 chromosomes. In fact the cM is defined by the probability of a crossover – such that there is a probability of a crossover every 100cM. So let’s look at Table 1:

05A Table 1

Each atDNA testing company shows a slightly different table of cMs for each chromosome. You can see a report of cM for various companies at Don’t get hung up on the exact numbers – it’s the overall concept that counts. The average number of crossovers is the cMs in that chromosome divided by 100. Since the number of crossovers per chromosome must be a whole number, I’ve shown several alternatives above. Although the average may be 1 or 2 or 3, possibilities may include 0 or 4 (or more sometimes). The point is that there are only a few crossovers expected for each chromosome, in one generation, and if more occur on some chromosomes, there tends to be fewer on some other chromosome. This process occurs in each generation – for instance when a parent recombines the grandparent’s chromosomes and passes a single chromosome to you. It also happens when a grandparent recombines the Great grandparent’s chromosomes and passes a single chromosome to your parent. So there are about 35 crossover points at each and every generation, and they add up, depending on which generation is under consideration. This will be described for each generation in more detail below.

Important points from the above info:

  • Recombination does not “puree” the DNA into tiny pieces (segments). Recombination tends to divide each chromosome into a few segments.
  • Clearly with only a few recombinations, the resulting segments in one generation will tend to be large.
  • Clearly with only a few crossover points, they tend to be distributed over the chromosome.
  • DNA is random, and the number of crossovers in your chromosomes may vary. If you have more than average on some chromosomes, you will probably have fewer than average on some other chromosomes.
  • Regardless of the number of crossovers, or their locations, all the resulting segments on a chromosome will fill the chromosome, with adjacent segments, from one end to the other.
  • When there is 0 crossover, this means there was no recombination. This means that chromosome was passed intact to the next generation. Given the probabilities in Table 1, there is a high probability that at least one of the smaller chromosomes will be passed intact with each generation.

Great Grandparents

So, given the probability that there are two additional crossovers in each generation for chromosome 5, let’s look at a probable scenario from the Great grandparent’s perspective in Figure 3.

05A Figure 3

Important information from Figure 3:

  • Again, segments from the four Great grandparents “fill up” the entire chromosome.
  • The “new” segments from the Great grandparents alternate; and they alternate within their respective child. That is Ggp1 and Ggp2 are parents of grandparent 1; Ggp3 and Ggp4 are parents of grandparent 2.
  • There are two new crossover points (shown by large vertical lines); and the previous crossover points are still there.
  • There are no gaps between segments.
  • Again, these segments tend to be large, and the crossover points tend to be widely separated.
  • Note: Only grandparent 1 ancestors (Ggp1 and Ggp2) can contribute within the segments from grandparent 1. In other words the segments for grandparent 1 can only come from the ancestors of grandparent 1. Ditto for grandparent 2.
  • We only had two new crossover points for chromosome 5, so they could only subdivide two of the three grandparent segments. Sometimes there may be 3 crossover points, but then, sometimes there may only be one crossover point. Even with 2 crossover points, they could have both occurred within one grandparent segment. When dealing with random DNA, there are many possibilities. We used the average of 2 crossover points to paint the best overall picture. You are invited to print Figure 3 and randomly place one, two, three or four crossover points anywhere you want. If you put two, or more, crossovers in one grandparent segment, be sure to alternate the Great grandparent’s segments: Ggp1-Ggp2-Ggp1…
  • Note that there is no crossover point through the last segment for grandparent 1. That means the second grandparent1 segment was passed down, intact, from one of the Great grandparents. There is a 50% probability for either one. But only for one. We sometimes refer to such a segment as a “sticky segment”, because it appears to stick together through a generation.
  • We now show five, rather large, segments at this Great grandparent level of chromosome 5.
  • Note that the first new crossover point divides the grandparent 1 segment into two segments, one for each parent of grandparent 1. On the other hand, the second crossover point is separating two segments from different parents of the grandparents. These two parents, labeled Ggp2 and Ggp3 are not related to each other by marriage or otherwise. So some adjacent segments may be for husband and wife; and some may be for distant, unrelated ancestors.

2Great Grandparents

OK – moving on to the next generation back – continuing our Bottom-Up look… Again, we will add two more crossover points to get Figure 4:

05A Figure 4

Important information from Figure 4:

  • As always, segments from the 2G grandparents “fill up” the entire chromosome.
  • The “new” segments from the 2G grandparents alternate; and they alternate within their respective child. For instance 2Ggp5 and 2Ggp6 are parents of Ggp3.
  • There are two new crossover points (shown by large vertical lines); and all the previous crossover points are still there.
  • There are no gaps between segments.
  • Again, these segments tend to be large, and the crossover points tend to be widely separated. But in this example the first new crossover point occurs fairly close to an existing crossover point, so a relatively small segment is created for 2Ggp3. It happens… and small segments are created.
  • Again, only segments from parents can contribute to a child’s segment. So 2Ggp1 is a parent of Ggp1; 2Ggp3 & 4; are parents of Ggp2; 2Ggp5 & 6 are parents of Ggp3; 2Ggp8 is a parent of Ggp4; and 2Ggp3 is a parent of Ggp2. Note that there are no segments for 2Ggp2 or 2Ggp7 shown. Those ancestors did not contribute any DNA to chromosome 5.
  • Note that there are no crossover points through the first, fourth and fifth segments at the Great grandparent look. That means these Great grandparent segments were passed down, intact, from one of the 2G grandparents. There is a 50% probability for either one, and I selected one of those in each case for example. Now we have two “sticky segments” from the previous generation; and one “sticky segment”, 2Ggp3, which survived three generations.
  • We now show seven, mostly rather large, segments at this 2G grandparent level of chromosome 5.
  • Again, note that the first new crossover point divides a segment into the two parents.

3Great grandparents

Let’s look at one more generation – to get the hang of it – and then draw some general conclusions about what to expect.

05A Figure 5

Important information from Figure 5:

  • As always, segments from the 3G grandparents “fill up” the entire chromosome.
  • The “new” segments from the 3G grandparents alternate; and they alternate within their respective child. For instance 3Ggp1 and 3Ggp2 are parents of 2Ggp1.
  • There are two new crossover points (shown by large vertical lines); and the previous crossover points are still there.
  • There are no gaps between segments.
  • Although at this generation going back (Bottom-up), these segments tend to be large, but with each generation, a few segments are split into smaller segments. Another relatively small segment has been created.
  • Now there are 8of the 16 3G grandparents missing (3Ggp3, 4, 5, 8, 9, 13, 14, and 15)
  • The last, 3Ggp6, segment has now survived, intact, from the 3G grandparent level down to you.
  • We now show nine segments at this 3G grandparent level of chromosome 5.
  • Again, note that the first new crossover point divides a segment into the two parents.

So what are the big-picture observations:

  • At each generation going back, each chromosome is made up 100% by segments from that generation.
  • The segments at each generation are adjacent to each other; there are no gaps.
  • On, average, there are only two new crossovers at each generation. So only two segments are subdivided at each generation.
  • From here on out, at each generation, most of the segments will remain the same size; and only a few will be subdivided.
  • Some segments, particularly the smaller ones, will appear to be “sticky” and survive for several generations without being subdivided.
  • More and more ancestors, at each generation, will drop out of the picture as you move to more distant ancestors. This applies only to the chromosome under consideration. These ancestors may well be found on other chromosomes. Because the DNA is random, many of your ancestors will be represented on some chromosomes, for many more generations.
  • This should dispel the common idea that all segments are cut in half with each generation. This may be true for averages, but in practice, we found above that only a very few segments are subdivided each generation.
  • “Sticky segments” are normal. In fact, in dealing with the smaller segments and comparing with a parent, you’ll often find you have virtually the same segment as your parent, or none at all. More on this in a later blog post on shared segments.
  • When a husband and wife have adjacent segments, then that crossover was created in their child.
  • Although this “picture” was developed for Mother, the same principles apply to the father’s side of autosomal DNA. And, of course, everyone’s version would be uniquely different. But, on a big picture level, it would be somewhat similar.
  • You can use Kitty’s chromosome mapping program to show your results at any generation up to 20 ancestors. Just list the ancestors of that generation in the MRCA column. See

Final Thoughts

Remember this whole discussion is based on your ancestral segments. Your ancestral segments are defined by the crossover points. The crossover points are locked into your DNA when you were conceived. They never change. They define the picture of your segments in each of your chromosomes. They define which ancestral lines contribute to which segments on your chromosomes. We’ll talk about this more in discussions about shared segments with Matches. We don’t really see the picture of our own segments in a chromosome browser. The browser doesn’t know where your crossover points are. What we see in a chromosome browser are shared segments. By grouping and Triangulating these shared segments, we can learn where the crossover points are. Much more in future blog posts….

05A Segmentology: Segments: Bottom-Up by Jim Bartlett 20150523


In my spreadsheets and analysis, I use Ahnentafel numbers. They are a standard numerical code for each ancestor: I am 1; my father is 2, my mother is 3; my 4 grandparents are 4-7; etc. They offer a unique shorthand for indentifying ancestors.  Here is a summary of the chromosome 5 charts in this post, using Ahnentafel numbers for ancestors. Starting with 3 for my mother…

05A Figure 6

49 thoughts on “Segments: Bottom-Up

  1. Jim, thank you so much for sharing your in depth knowledge of this process in such a clear yet concise way. It has significantly advanced my understanding and will greatly advance my analysis.


    • Thank you. At some point I will address pile-ups, but the short answer is that pile-ups are not an issue with segments over 7cM which Triangulate. In fact, in this case, pile-ups are helpful – they give you many more cousins to help determine the Common Ancestor. Pile-ups with short shared segments may go either way – TG “jury” is still out.


  2. Wow! The light bulb went from completely out to dim. Thank you.

    If I understand correctly, using chromosome 22 as an example. I may have 0 crossover points in which case I inherited the entire chromosome from either mother or father? With 1 crossover point, I may have father segment concatenated with mother segment OR mother segment concatenated with father segment? Can there be a case of chromosome 22 ever having 2 or more crossover points? I assume yes, but the probability is very slight.

    Can’t wait to see how to determine the crossover points.


    • Yes, you probably did get one or more of chromosomes 19-22 intact from one grandparent. And yes it is possible to have two crossovers in one generation sometimes. NB: with each generation, you are likely to get additional crossover points on these small chromosomes, so when you are looking at shared segments with distant cousins, and Triangulating, you’ll determine your several/many crossover points on those chromosomes.


  3. Thanks very much for the clear explanation, which prompted me to closer at the bottom levels. I compared two full siblings (myself and my sister) to the phased paternal genome and noticed that the very large segments the siblings have in common break at about the same rate — 0 to 4 times per chromosome. Will these shared segments tend to start and end on the crossover points for the grandparents or are they more likely to be random?


      • Jason

        This blog post includes one Table and several Figures. They are separate images inserted after the text. It appears OK when I publish the post, but many folks have different computers and software. Try using Google’s free Chrome browser.


      • I would love to see how this works. I have a sibling set of three as well as a number of first cousins on both sides. Would love to see how you might be ab,e to illustrate it. On my maternal grandmother’s side, her grandparents were all recent immigrants so finding tg groups is hard. I’m having a hard time conceptually pulling these siblingships apart into understanding where crossovers must be since they have areas of the chromosome where they match one parent, neither parent or both parents. I love this series and would love to see more with comparisons of siblings or an individual and close relation such as an uncle or first cousin.

        Again, great work,


  4. The diagram with Ahntafel numbers is a very clear way of presenting it/keeping track of it. Thanks!

    And thanks for this comment in the reply on the “three siblings is enough” point…time to get that extra kit…..


  5. I only have one full sibling. She has been tested. I also have a half-sibling. How much would getting him tested help in determining crossover points given that he also has his mother’s DNA in the mix?


  6. June,

    All close relatives are helpful. 1 – they usually share several segments; 2 – you already know the MRCA, which serves as a “pointer” for the CA of each TG; you also know the side for each TG they are in.

    Liked by 1 person

  7. Thank you for your “Bottom Up” post. I have a much better idea of atDNA now, which should prove helpful as I compare my test results with my matches.


    • Diane,

      Thank you. When you get some Common Ancestors worked out, try Kitty’s Chromosome Mapper to see the shared segments. For many that picture is a very helpful tool that highlight questionable areas which are probably more chopped up than you should expect.


  8. Pingback: Fuzzy Data, Fuzzy Segments – No Worry | segment-ology

  9. I’m so delighted that you decided to blog. Thank you for sharing your knowledge of this complex topic! Like several others, I found this post regarding crossovers to be especially helpful.


  10. Thanks for your clear description of cross-over points. I have one question please. Are the cross-over points on the chromosome you inherit from your father exactly the same as those on the same chromosome you inherit from your mother? For example, if the first cross-over point on my paternal chr1 is at location 13,100,409, will the first cross-over point on my maternal chr1 also be at location 13,100,409, and so on? Or is there no relationship whatsoever? Thanks.


    • Lennel – thanks for the question. The answer is that your maternal chromosomes and their crossover point are generated completely randomly in your mother, with no knowledge of or interaction with your father. The crossover points in each chromosome are independent and random.


  11. Pingback: Segments: Top-Down | segment-ology

  12. Pingback: The Porcupine Chart | segment-ology

  13. Pingback: The Porcupine Chart | segment-ology

  14. Hi Jim…question for you..I have my grandmother can I safely say that any gaps in our shared chromosomes belong to my grandfather?..Thank you!!!


  15. Kim
    1. Your DNA came from 4 grandparents – roughly 1/4 of your DNA from each.
    2. I’ve worked hard for 5 years to map my chromosomes to my 4 grandparents, and I still have 10% which is not mapped – I don’t know which grandparent will be linked to each of the remaining segments.
    3. I doubt that your current Matches cover all of the segments from your grandmother.
    4. The bottom line is you cannot assume the remaining Matches go to one grandparent.


  16. Pingback: Does Triangulation Work? | segment-ology

  17. First of all I’d like to tell you that I’ve loved all of your posts! They’ve helped me tremendously and I re-read them quite often. I was reviewing Segments: Bottoms-Up and have been trying to break down my chromosomes into these segments. I’m hoping you might clarify something for me as I want to ensure that I’m not making errors from the start. Just when I think I understand this, I get a bit jumbled up.

    My mother shares one 150 cM segment on chromosome 1 with her maternal aunt. Would it be correct to assume that this 150 cM segment came from only one of my great aunt’s parents?


    • No. An Aunt is too close – she shares a lot of DNA with your mother’s mother which came from their parents. Since they are so close, and have long stretches of identical DNA, it’s possible to get 150cM that is split between the parents of your mother’s mother and your mother’s aunt. In many respects we need to treat an aunt or uncle just as we would a parent. Perhaps think of parents with two children (you mother’s mother and her aunt) – they each get big blocks of DNA from their parents – some overlaps, some does not – but the part that overlaps, could be part from one parent and part from the other. Use triangulated groups to sort out the 150cM stretch of DNA.


  18. Pingback: My Father In Law’s Grandparents’ DNA – Hartley DNA & Genealogy

  19. One question, my daughter has 3-4 chromosomes where the entire segment matches only one of my parents. I understand the crossover, but I guess I don’t understand why one parent would give all the DNA, though my mother, cousin and I all share an entire chromosome in common as well.


    • Jennifer, A parent has two of each chromosome – they “recombine” these into two new chromosomes and one is passed to the child. The recombination process usually involves one to three crossovers (resulting in two to four large segments), but sometimes there are no crossovers (resulting in one of the two chromosomes being passed down intact), or four or five crossovers (resulting in more, but smaller segments). On average, we should expect one or two of the chromosomes to be passed along intact in each generation. Usually it will be different for each child, but it doesn’t have to be.


  20. My brother tested Y-DNA in 2008 37 marker he matches 4 Rock surnames, and 1 Bartlett John Joseph P Bartlett/Barrtlett b 1909 (was adopted) my grandfather was not adopted but raised by foster parents b abt 1868 Indiana no records; I have done the autosomal DNA fmtree-June- 2016 what I am asking here this, will I also show a Rock and Barnett connection? I have found none so far but I have 6 matches on segments 4 @ 19 1 @ 20 and 1@21 just what does this mean?

    Also I have a connection with Marilyn Hutton her gr grandfather and mine are brothers- we have good paper (proof) but yet we have not connected with DNA; why is that?


    • Eleanor, I would take it as a strong clue that your paternal surname is ROCK. About 1/2 of your autosomal DNA Match are probably on his side and half on your maternal side. You will match others with a blood (biological) relationship – that’s the path the DNA follows. You should get not real matches with any foster parents (unless there is some distant blood relationship). When starting out with autosomal DNA, I would work with (contact and shareinfo with) your closest Matches first – they are the ones who are listed first.
      If you don’t share any DNA segments with a known 2nd cousin, it may be that there is a break in the blood line – either on your side and/or on Marilyn’s side. I would highly recommend you both register at and upload your raw DNA files there. You can then get an independent check of a Match – a 2nd cousin should almost always match.


      • Mtdna is not generally successful when fishing for relatives – it seldom works out. You’d probably get some matches, but they’d probably be 10-30 generations back. My recommendation would be to get atDNA test for your closest cousins – they’ll help sort your Matches out.


  21. Pingback: Understanding and Using TGs | segment-ology

  22. Pingback: First 1st Cousin DNA Results: Part 2 – Trying to Explain Aunt/Nephew Matches – Hartley DNA & Genealogy

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s