The relatedness of A to B is defined as the limit, as an allele's frequency in the population tends to 0, of the probability that allele is in A, given it is in B. Since probabilities are at most 1, so are the limits of series of them, where said limits are well-defined (which it can be shown is applicable here). I suppose if we replace "The relatedness of A to B is defined as the limit, as an allele's frequency in the population tends to 0, of the probability that allele is in A, given it is in B. Since probabilities are at most 1, so are the limits of series of them, where said limits are well-defined (which it can be shown is applicable here). I suppose if we replace "probability that allele is in A" with "mean number of copies of that allele is in A" and subdivided the condition into "it is in B at least once", "it is in B exactly once" and "it is in B twice", you may be on to something, but I reckon the limit part of the definition would still spoil it.