Linkage mapping may be conceptually simple but of course the practicalities are a different matter.  In particular, heritable traits are often not as all-or-nothing as the examples of high and low rates of processes suggest.

This is illustrated for the case of chlorophyll breakdown in senescence by a simple animation.  We can assess variation in this character by measuring two features.  One is the time at which yellowing begins (which we imagine could be controlled by gene B).  The other is the rate at which pigment is lost once senescence is underway (specified by gene R).

As the animation shows, we could have alleles of B that determine early or late onset, or any time in between.  Similarly we could have fast, slow or intermediate alleles of R.  This sort of genetic variation is quantitative, the traits are quantitative traits and the genes responsible are called quantitative trait loci or QTL.

Most agriculturally, environmentally and medically important characters are controlled in this kind of quantitative way.  Mapping QTL is quite a challenge for geneticists and requires a lot of heavy-duty statistics and very careful experimental design.

As an example of senescence QTL, here are analyses of the staygreen trait in sorghum.  Chlorophyll loss in different varieties is well described by the simple B-R concept described above.  A linkage map produced by Andy Borrell and colleagues in Australia and USA shows the location of three QTL for the staygreen trait.