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I am very new to this so please be patient with me. I had a DNA test via Ancestry which didn't tell me very much and so I uploaded the data to GEDmatch. My top 3 matches are:

  • A:11 chr matches, 372.6 cM, longest 62.6 cM
  • B:13 chr matches, 381.6, cM, longest 49.6 cM
  • C:10 chr matches, 348.9 cM, longest 48.7 cM

Now, I know who these people are. A and C are brother and sister and are my 2nd cousins. B is my father's full cousin, so my 1st cousin once removed. So far so good. We are closely related so would expect good matches.

My 4th best match according to GEDmatch is:

  • D:2 chr matches, 51.2 cM, longest 38.8 cM - I don't know who this person is.

So now we come to my question(s):

  • Do closer relatives match on more chromosomes or is this irrelevant?
  • Do the actual chromosomes that you match on make any difference in interpretation?
  • Does the longest matching cM mean anything above 7 cM? e.g. in the example above A and C are both related to me in the same way but the longest cM is quite different.

None of the sources I have read address these questions.

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    Hi Diane, welcome. Great question! I did a few formatting edits to your post because sometimes new lines don't appear properly at first. – Harry Vervet Feb 13 '16 at 19:31
  • Hi, Diane -- welcome to G&FH.SE! While you are waiting for an answer, take a look at some of the other questions tagged DNA. There is a section of resources in my answer to one of the other DNA questions which has links to some of the major DNA bloggers and the ISOGG Wiki. – Jan Murphy Feb 13 '16 at 20:50
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The closer your match generationally you will generally have more shared centimorgans (cMs).

There is a published shared cM relationship range chart on the ISOGG website that gives a good general 'guideline' for interpretation.. enter image description here ...but it is just a guideline as the further one gets out in relationship you may retrieve additional shared cMs from more than one shared ancestor or path to a common ancestor.

Though the general rule is you inherit about 50% of each parents DNA it is not exact (reference previous question Generational Loss in DNA testing) and will vary by sibling on what DNA segments are passed onto each child and it can vary.. I seem to recall easily up to a ~10% difference in shared cMs between two siblings in my very amateur research so far for some of my 1st cousins. This would account for the difference in your tests as well.

Where the chromosomes matching comes into play is that it can provide clues on what common ancestor you share.. which at close distances this does not matter so much but for more distant ancestors it for me is a clue of a potential common shared ancestor. Such as as I illustrated in the "Family Trait in a Shared Segment" question.. though I haven't found an exact reference as to what this is scientifically called.. I have learned that people who share a common ancestor sometimes retain a particular common trait unique to a particular chromosome. While far from bulletproof, I look for shared segments sometimes in a particular range in a chromosome to identify someone from that line. The biggest caveat with this method is I have to first establish a high confidence shared segment from multiple lines of a common known and documented ancestor from people who have tested.. but this helps me identify others from that line. I have done this with my primary research line who share a common couple born in 1815/1817.. and the 7 known and documented descendants (age 95 to 20) I have tested from 5 different lines of their children all have shared segments in a particular region of Chromosome 1. Beyond that shared segment they share no other segments over a length of whatever ftDNA's filter is (I think it is 5cms?).

On your third question, about longest matching cM.. yes each service uses a slightly different method for measuring what a match is as well as what a shared cM's minimum length is to be countered/aggregated on a match. Some services also do not count the X chromosomes in their calculation. Basically the important thing to know is that longer continuous cM segment is that the match is likely closer in nature because the segment is unbroken and been less diluted through the generations.

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  • Thank you. Is there any significance in having 2 or 3 cM matches on the same chromosome? – Diane Fairhall Feb 14 '16 at 21:05
  • @DianeFairhall Generally not unless there is much stronger matches elsewhere...but once you get out so far that may be all you have to work with (5th cousin+). If this answers your questions please be sure to upvote and accept the answer. – CRSouser Feb 15 '16 at 2:04
  • With some of my good matches detailed above I have several different matching strips on the same chromosome. – Diane Fairhall Feb 16 '16 at 3:34
  • @DianeFairhall As one would expect, is there or additional clarification as your question was technically 3 and I believe I have answered all , the rest comes from experience and asking new additional Qs. I would appreciate you confirming. – CRSouser Feb 16 '16 at 4:27
  • I would upvote your answers if I knew how! – Diane Fairhall Feb 17 '16 at 18:25
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Just to build on @CRSouser, "Though the general rule is you inherit about 50% of each parents DNA it is not exact", it's possible that two differently-sexed siblings will share no DNA at all, albeit statistically unlikely. Ignoring X/Y, siblings share about 25% on average, but within one family there are few enough samples that the figures could vary more wildly.

In addition to generational loss (usually single-pair mutations), some individual chromosomes may be partly from one parent and partly from the other due to chromosomal crossover:

https://en.wikipedia.org/wiki/Chromosomal_crossover

I expect that the level of interest in genealogy DNA will contribute to a better understanding of chromosomal crossover in the human genome.

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