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Return to U.S.A. DNA Markers Return to the English DNA Markers All the “multi-copy” markers, that is, those marker sets that have the same marker number but with subsets “a”, “b” etc., require an explanation. Every marker value is simply a non-coding chemical group (base) that repeats itself several times on the “Y” chromosome chain, located among the genes, but are not genes. The lab calls them STR’s meaning “Short Tandem Repeats”. Each marker is different, that is, the chemical composition of the repeated base in each marker is different. The numbers assigned to each marker (marker value) during a DNA test tells us how many times these bases repeat themselves. Therefore, a value of 5 for a marker means that there are five identical copies of this base at this site on the chromosome. Most markers are unique, that is, there is only one grouping of any particular base on the “Y” chromosome. A very few however are “multi-copy” markers meaning there are two or more groups of a particular base residing at various sites on the “Y” chromosome. Laboratory procedures can determine the number of base repeats (marker value) at these different sites, even if one or more of these multi-copy markers have the same number of repeats. However, they cannot determine from which site on the chromosome each group came from. For example, we might have a duel marker “XXXa” & “XXXb” where “a” resides near the middle of the chromosome and “b” resides further toward one end. Laboratory procedures will indeed show two markers, one maybe with a value of 15 repeats and the other with a value of 17 repeats. By convention, the value 15 will be assigned to “XXXa” and the higher 17 will be assigned to “XXXb”. However, it could in reality be the other way around. The lab cannot tell. So, if one person has a 14 & 15 at these two markers and another person has a 15 & 16, the correct way to look at it is that they have a 15-15 match and a 14-16 mismatch. Whether or not you use this convention, the total number of mismatches is not usually changed. (but see paragraph below). What is changed is the comparison of differences relating to a single marker. An example might be that everyone in a surname group had a 13 at XXXa except one participant, and you might consider this exception as important. However, if this participant had a 13 at XXXb, then it is the same as having a 13 at XXXa and he would not be considered an oddity. Marker DYS464 is a multi-copy marker with 4 copies on the “Y” chromosome. They are labeled DYS464a through DYS464d. [Sometimes DYS464 may have more than four copies.] As described above, each marker in this set (a-d) contains a series of identical molecular groups (bases). Researchers know where on the “Y” chromosome these markers lie, but they cannot determine which marker value resides at which site. As per the convention described above the values are usually reported from lowest to highest. For example, typical values might be 15, 15, 17, & 18. To compare these values to another participant who has 15, 15, 16 & 17, the only mismatch between the two is between the 18 & the 16. This is because the absolute reported position of the marker value is immaterial. The 17 at the third position in the first group matches the 17 in the last position of the second group. Unfortunately, DYS464 gets even more quirky. The difference between any two mismatched values is always counted as one, no matter how far apart they are. In the above example the difference between the two groups is 1, not 2 as you would normally expect, meaning that the 18 versus 16 mismatch is counted as one mutation. It’s not very intuitive but those are the rules, at least for this muli-copy marker. Markers 389-i & 389ii (also called 389-1 & 389-2) are another strange pair. Surprisingly, marker 389 is not considered a multi-copy marker. That is why the pair is given the “i” and “ii” sub-values rather than “a” & “b”. It is a strange marker nevertheless because 389-i is a subset of 389-ii. Lab procedures always show either 389-i alone, or 389-i in combination with additional repeats that produce marker 389-ii. Those extra repeats in 389-ii are never found alone, only in combination with 389-i. The problem is that if there is a mutation in 389-i, it will also show up as a mutation in 389-ii, giving the impression that there were two mutations when in reality there was only one. To reduce confusion in these tables we created a fictitious marker 389A which is marker 389-ii minus the 389-i marker. Therefore marker 389A only contains the value for the extra repeats without the confusing influence of the marker 389-i repeats. |
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