Position of the gene


can anyone tell the position of the gene on the right or left side of the chromosome if I say the gene’s Genetic position?

For example, if I say the genetic position of C30A5.3 gene on chromosome III is -.35 , so is it present on the right or left portion of the chromosome?


I don’t think so. Position zero on the genetic map should not be expected to coincide with exactly half way along the chromosome. I seem to remember that the 0 position is sometimes mapped to the centromere, but C. elegans is holocentric, and so does not have a single centromeric locus. My brain is trying to tell me that Jonathan Hodgkin (curator of the genetic map) once made some arbitrary decisions as to where 0 might be. I may be wrong about that.

I’d imagine that for higher map positions, you might have more confidence that they are on the left/right halves of a chromosome.

The zero positions of the genetic maps of the linkage groups correspond to (arbitrarily selected, so far as I know) centrally located visible markers (in order of linkage group number, dpy-5, dpy-10, unc-32, dpy-13, dpy-11, and dpy-6). These markers provided useful points from which to build the genetic maps, but unsurprisingly they have turned out not to be precisely in the middles of the genetic maps of their chromosomes; for example, the genetic map of LGI goes from at least -21.4 to at least 28.1 (because each chromosome arm should get one crossover, and C. elegans chromosomes only have one arm, each should be approximately 50 map units in total). Also, if you look at genetic maps from different years, there are slight changes in the map positions of various markers as additional data became available and the maps were refined - but the markers at zero stay at zero, because they are anchoring the maps.

So, if a marker is at -0.35, you can’t immediately say whether it’s in the left or the right half of the genetic map of the chromosome (nor would that information necessarily tell you whether it’s in the left or right half of the physical chromosome). Still, the marker is left of the defined zero point, and so is often spoken of as being (just) on the left side even though, as Keith points out, if you were to divide the chromosome in half this marker might well end up in the right half.

Thanks a lot for providing valuable information…
Really appreciate that…