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Flow Chart

This page describes Y-DNA match interpretation in terms of a decision flow chart. The idea was suggested by Allster John March.

Incomplete: Work in progress

Steps: Main Flow

  1. Are the two sets of Y-DNA of the same haplogroup?

    decision: smae haplogroup?
    If not the same haplogroup, stop. No CMA w/in thousands of years.

    Note: Do not be misled by one haplogroup designator being more refined than the other. R1b1b2, for example, falls within R1b1.
  2. Count the number of markers compared.

  3. Count number of mismatching markers.

    (Each diagram is a link to a larger version.)
    <16 16-28 29-60 >60
    29+ marker match   60+ marker match

    If results of mismatch count are "Highly probable" or "Probable", continue. If results are "Possible", "Unlikely" or "Highly unlikely",  stop and await further persons to get tested & give better matches; Y-DNA can not -- at this time -- provide adequate answers to your genealogical questions.

  4. Do they share the same surname or a variant of it?

    decision: same surname?To NPE

    A rare surname may be more indicative of a CMA than a common surname.

  5. Do the lines trace back to the same time and place?

    decision: same time & placeTo more research

    Similar surnames found in the same place at the same time may be more indicative of a CMA.

  6. Are there indicators to make a match more or less likely for indicating CMA w/in recent times

    1. Off-modal, slow-mutating markers shared?
      Shared allele values for slow-mutating markers which depart from the haplogroup mode may make a recent CMA more likely.

    2. Close to Hg modal? If yes, consider possibly unrelated without additional evidence.
    3. Is the genetic distance small or large?

      decision: large or small genetic distance?To Large G.D.

      A large genetic distance in relation to the number of markers compared is, generally speaking, less indicative of a CMA than a small one. However, if the large genetic distance is concentrated in one or two markers, this suggests fewer mutations.

  7. Compare paper trails.

    Compare documentation to identify common male ancestor by name, dates and other specifics.

    compare treesTo more research

Departures from Main Flow

NPE -- Non-parental Event

A non-parental event (NPE) occurs when the child does not bear the surname of the biological father. It does not necessarily imply illegitimacy or adultery; other non-parental events are adoptions and name-changes. Evidence to definitively confirm or reject a NPE hypothesis is notoriously difficult to obtain. Indirect evidence, e.g., Y-DNA matches, may be the best available.

Weighing NPE evidence

Return to main flow.

More Research

The most common reason for the earliest known ancestors not including the CMA or tracing back to the same time and place is that one or the other of the trees does not go sufficiently far back in time. Another reason for time/place discrepancy is that siblings may not have been included.

It's possible that the CMA lies beyond the limits of the present state of the documentation. Perhaps, you haven't traced one or both of the lines sufficiently far back in time.

Use the match probability information and the documentation in both lines to focus your research.

Return to main flow.

Large G.D. -- Genetic Distance

Genetic distance (GD) is the sum of all absolute marker differences. (See note below.)  It may indicate many markers differing by small amounts or fewer markers differing by large amounts. We'll roughly define large genetic distance as 10% times the markers compared.

A large GD usually signals that that the two men who provided the DNA samples do not share a common paternal ancestor within genealogic time. The may be a CMA, but he lived before the time in which he could be identified by name, date, places or other characteristics. However, be alert to a null value (reported with a value of 0) in compared markers; it should be counted as GD=1 but may be reported as larger.

Note: FTDNA reports a combination GD. For most markers, the GD is the absolute difference; for other -- highly volatile -- markers, GD=1 for any difference.

Return to main flow.