What is a segment?

A DNA segment is a block, chunk, piece, string of DNA on a chromosome. It is typically determined by a start location and an end location on a chromosome. A segment refers to all the DNA in between and including the start and end locations.

We use the term segment in at least two fundamentally different ways:

  1. An ancestral segment is one which is passed down from an ancestor. Ancestral segments are passed to you from your parents, who got them from their ancestors. Each of your chromosomes are made up of ancestral segments – much more on this later.
  2. A shared segment is one which both you and a match have. Both you and your match have segments which are identical from start to end. Also sometimes called an HIR (Half-Identical Region). Also sometimes referred to as a matching segment. Note: a shared segment is determined by a computer algorithm – it may or may not come from a common ancestor – much more on this later.

IBD: When a shared segment comes from a common ancestor, we say it is IBD (Identical By Descent). Both you and your match have these identical segments on a chromosome because these segments came from the same ancestor.

Note that IBD shared segments are based on ancestral segments. The ancestral segment that you and/or your match received from an ancestor, may be (and often is) larger than the shared segment. What you see in the DNA match lists, reports, tables, or chromosome browsers, is the overlapping portion of your ancestral segment and your match’s ancestral segment. This overlap is the identical part that is reported as a shared segment.

02 Overlapping Segment graphic pic5

 IBC: Sometimes a “shared segment” does not come from an ancestor. The computer algorithm creates the apparent shared segment from parts of the DNA which are not all from one ancestor. This can happen in your “segment”, your matches “segment”, or both. Thus, the fact that they appear to be identical to the algorithm is by chance. We refer to these segments as IBC (Identical By Chance) or IBS (Identical By State). In any case these shared segments do not exist on one chromosome for you and/or your match, and they both therefor are not IBD – much more on this later. The “shared segment” or “matching” segment is therefor IBC.

02 Overlapping IBS Segment pic7

23 thoughts on “What is a segment?

  1. So whats the deal?
    Are you saying if me and a match have a 20cm shared segment, then it might not be from a common ancestor???

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    • Wes, A segment as large as 20cM (actually over 15cM) is always IBD (Identical By Descent) and comes from a Common Ancestor down a line to descent to one of your parents and then to you. Jim

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      • Thanks so much Jim!
        That definately makes me feel much better!
        After being new to this awesome field of genetic geneology, after recieving my Dna results two weeks ago, and boy were their some surprises!

        It is exciting!
        But with all the different info floating around, your answer was is the concrete that I needed!
        That if I’m sharing a 15cm plus, and definately a 30cm to 40cm plus segment to someone then we are definately on the right track of a recent ancestor!

        Just in my brief experience at Gedmatch, my known 3rd and 4tg cousins are sharing a good size SEGMENT.
        With some of them only sharing a single segment with me of 40cm and a total cm of 40… which i know these are know relatives, but its good to hear that i can take solace in that long segment, even sometimes Moreno than the total shared cm..

        Also one last question…
        If I am both 3rd and 4th cousins with someone, with the shared Cm be inflated!?

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      • Wes,
        An individual segment cannot be more than the total cM. The total is the sum of individual segments – which is usually just one shared segment.
        The total shared cM is never “inflated” – it is what it is. If you share only one segment it is probably from one of the two Common Ancestors – usually the closer one. I you share more than one segment, they can be from a mix of Common Ancestors – eg. one from the 3C CA and one from the 4C CA. And always be mindful, you might also be a 5C or 6C and get a segment from that relationship. The DNA is random, so there is no hard rule about which segments go with which CAs – the larger segment (of two) *could* be from the more distant CA. Again, no firm rules in this regard. Jim

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      • Thanks Jim!
        One last question
        Is a single shared segment from one ancestor.
        Or can it be from an ancestral couple?

        Also if I am 3rd and 4th cousins with someone, Will that inflate the CM?

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      • Wes.

        A segment cannot come from an ancestral couple. We often say it comes from a couple because we don’t know which one. But the husband’s DNA is in the sperm and the wife’s DNA is in the egg – very much separated. Your body has one set of Chr 1-22 from Dad and one set of Chr 1-22 from Mom – they are 44 autosomes that never interrelate (except in the process of passing them to your children). When you and a 3C share a segment of DNA, you got it through one child of that 2xG grandparent couple, and your Match got is through a different child of that couple. Each child got (at least) that segment from only one of the parents. Jim

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  2. Chromosome 19 has a beginning location of 211912 and an ending location of 63788972. It just happens that these are the parameters for my mother. Does this mean that the whole chromosome is one big segment?

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  3. On my chromosome 19, my mother starts at 211912 and ends at 63788972. How can this be one big segment as 211912 is the beginning location for chromosome 19 and nothing before this location. 63788972 is the very end of the chromosome and nothing after this. Would this mean that the whole chromosome 19 is one long segment? New to this, help.

    I have changed my e-mail address to oliverbeckersandy@Yahoo.com. If anyone can help, I would appreciate it.

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    • Sandy,
      To you, each chromosome from a parent looks like one long segment. 23 chromosome segments from each parent. When you compare with a 1C, you’ll “see” some segments that are from your grandparents, which will be part of the DNA a parent passed to you. Jim

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  6. Just found you today from DNA explained—–looking forward to learning–Thank you in advance for sharing your experience/knowledge.

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  10. Jim, looking forward to your following blog articles explaining your “much more on this later”. You create the suspense of a thriller 🙂

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