The difference between genetic and
physical scale is analogous to measuring the separation between New York and Boston
in either travel hours or kilometers.
The map on the top has an
X-axis scale based on frequency of recombinations events between markers (B
to D transitions, see slide 19 for a color-coded example). These so-called
genetic maps are scaled in centimorgan (recombinations per 100 gametes). In contrast,
the physical map shown below the genetic map has an X-axis scale based on DNA
length measured in nucleotides or base-pairs. Notice the large difference
between the two maps in the size of Chr 19 (large on the genetic scale but
small on the physical scale).
Also notice the large
difference in the width of the chromosome 7 QTL peak. In mice, recombinations
occur with higher frequency toward the telomeric side (right side) of each
chromosome. As a result, genetic maps are stretched out more toward the
telomere relative to a physical map. The QTL on distal Chr 7 is therefore
actually more precisely mapped than might appear looking at the genetic map.
The physical scale is
becoming more useful than the genetic scale primarily because many other data
types can be easily superimposed on a physical map. You will see more
examples in the next several slides.
