Genetic distance sex differences?

E. Wijsman wijsman at
Tue Dec 28 12:54:36 EST 1999

Yes, there *are* large differences in the intermarker distances between
males and females.  This is not an anomaly.  It is true in general for
lots of species.  Just think about Drosophila melanogaster (the fruit fly
that has been used for so long for genetics experiements in the
laboratory) - in the fruit fly under normal conditions *all* the
recombination happens in the female since there is *no* recombination in
the male (i.e., in the male the distance between markers is 0 for all
pairs of markers on the same chromosome).  However, the fact that males
and females have different map lengths does not imply that there are huge
differences among people of the same sex (although there is some
indication that there may be a modest amount of inter-individual variation
even within-sex).

The difference in male:female maps has little bearing on what happens with
single-marker analyses since the map itself is not part of the analysis.
However, when a multipoint analysis is used, we have shown that use of a
sex-averaged map may be, if anything, slightly conservative when there
*is* a trait gene in the region, but can lead to false-positive results if
there is *not* a trait gene in the region.  Therefore, in cases where
positive evidence for linkage is obtained with a sex-averaged map, we
would advocate repeating the analysis with sex-specific maps.  Most of the
available model-based analysis packages (e.g., linkage, fastlink, vitesse,
Loki) can correctly use both the male and the female maps for analysis.
However, genehunter does not allow for different male and female maps, nor
(I believe) do the few model-free analysis programs that perform
multipoint analyses.

Ellen M. Wijsman                        EXPRESS MAIL ADDRESS ONLY:
Research Professor                      Ellen M. Wijsman
Div. of Medical Genetics and            1914 N 34th St., suite 209
Dept. Biostatistics                     Seattle, WA   98103
BOX 357720, University of Washington    (Note:  Use this address
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On Tue, 28 Dec 1999, unknown wrote:

> Looking over the genetic differences reported by the Marshfield web
> server (, there's a huge difference
> between male and female cM distances for STRP markers.  Why should this
> be, and doesn't this imply that genetic distance varies widely between
> individuals?  Other database servers only report one number, presumably
> the sex averaged number.  Does this really give a useful map that can be
> applied to numerous samples in mapping?
> Thanks,
> Mike Holloway
> holloway at

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