No announcement yet.

A Re-combination Question, Please

  • Filter
  • Time
  • Show
Clear All
new posts

  • A Re-combination Question, Please

    I recently read the following on a blog. Is any portion of this statement incorrect, or variable?

    "A 24 cMs segment is more likely to be passed down in its entirety (or not at all) than to be passed on in part."
    Last edited by Biblioteque; 16 July 2019, 11:01 AM.

  • #2
    To me, it sounds correct.
    If the writer meant "passed down" from parent to child.
    (strictly, it should be "cM", not "cMs", but this is only a detail)

    When you share a segment of 24 cM with a match, it is more likely that your child will share all of it, or nothing.
    It is less likely that your child shares only, say, 10 cM in that particular location, with that particular match.

    Over the thumb: a segment that is only 24 cM long, only has cca 1/4 chance to be cut (on its way from parent to child).

    What exactly looks wrong to you?


    • #3
      Emona, thank you for responding. That was very helpful. I thought a segment could be passed down in part, as well as all, or none. I understand now it is "less likely" to be passed down in part. I just wanted another opinion.

      Regarding centimorgans and the expression of cM versus cMs:

      1 cM = singular
      2 cMs, or more = plural

      But, perhaps in the scientific community they have there own expression.
      Last edited by Biblioteque; 16 July 2019, 04:58 PM.


      • #4
        Yes, I can guess that "cMs" are used as plural; and you are right, it was a scientific community rule that I had in mind:

        At school, we were taught that symbols: kg, km, W, ... stay as they are. No plural, no dual, no cases, no change at all. (to make things simple and clear, any language, worldwide)
        Never mind. As long as everything else is correct, the blog is probably worth reading. Can you please share the link with us?


        • #5
          A 24 cM segment is 24 cMs long. Do you run a 100 yard dash or a 100 yards dash? Do you buy a 10 foot tape measure or a 10 feet tape measure?


          • #6
            Jim, thank you for your finer point and clarification, as always.

            So, it seems we are discussing how it is used in the sentence (sentence structure) to determine whether to use cM versus cMs. In America, I was taught by using an example in a sentence. For example:

            George and I share 24 cMs.
            George and I share a segment of 24 cMs.
            George and I share a 24 cM segment.

            And, as Emona points out, this could be expressed differently in other languages.
            Last edited by Biblioteque; 17 July 2019, 12:09 PM.


            • #7
              Indeed, it seems that the rules are different on this side of the ocean. Both or you are using US measures, and US rules.
              I am from Europe. My tape measure is 3 m long (not 3 ms). Only when we write "three metres" with whole words, plural is used.
              The rule is not from any local language, it comes from
              The International System of Units:
              "Unit symbols are mathematical entities and not abbreviations. Therefore, they are not followed by a period except at the end of a sentence, and one must neither use the plural nor mix unit symbols and unit names within one expression, since names are not mathematical entities."
              (Chapter 5.1)

              We are lucky to have centimorgans in both US and Europe. Calculating feet and yards to metres is complicated enough.


              • #8
                Biblioteque, I too would be interested in a link to that blog


                • #9
                  Hi Fern and Emona - It was Leah Larkin's latest blog. Her blog is: the dnageek. My quote here was her response to me on July 14, 2019. Sorry, I did not keep the link.
                  Last edited by Biblioteque; 24 July 2019, 11:53 AM.


                  • #10
                    Thanks, now I have found it. Here is the link, and the quote is somewhere among the comments.


                    • #11
                      Thank you, Biblioteque and Emona I thought I'd seen it somewhere before; that must've been where.

                      Emona, would you able to clarify, without getting too mathematical, why a 24cM segment has only a 1/4 chance of being recombined ... or direct me to an explanation elsewhere? Many thanks


                      • #12
                        No complicated mathematics.
                        It was estimated "over the thumb", as suggested by Wikipedia.


                        "The probability of recombination is approximately d/100 for small values of d
                        and approaches 50% as d goes to infinity."

                        (d is distance measured in centimorgans)

                        24/100 is almost 1/4.

                        If you remember that a chromosome is typically cut once or twice when travelling from parent to child, and look at the length of chromosomes, the number is realistic enough.
                        So an average genealogist does not need to spend a lot of time with the definition of centimorgan, and with complicated math.


                        • #13
                          Thanks very much, Emona