Saturday, January 22, 2011

Genetic mapping

In yesterday's course meeting for my new second-year genetics course (which I'm now thinking of as "21st Century Genetics"), I mentioned that the syllabus doesn't include the classical technique of genetic mapping.  The others were shocked!

My students will learn how meiosis works.  They'll learn about segregation and independent assortment.  I've never really seen clear explanations of the meanings of these widely used terms, but segregation means that each daughter cell gets one version of the two homologous chromosomes (never two or none), and independent assortment means that which version of each pair a particular cell gets is random and independent of the version it got of each other pair.  They'll learn how crossing-over between parts of a pair of homologous chromosomes makes new combinations of the alleles.

The students will learn how to find out if genes are linked (close enough together on the same chromosome that their alleles aren't randomized by meiotic assortment and crossing-over).  They'll also learn that the frequency of crossing-over between any two genes gives a rough estimate of how far apart they are.  They might even learn how to compare these frequencies to tell which gene is in the middle of a group of three linked genes (maybe as a homework problem).  BUT, they won't learn to use three-factor crosses to determine 'map distances'.  (Here's a web page with a fill-in-the-boxes version showing how such mapping analysis is done.)

Why not?  Because they won't have any use for this skill.  Even if 1000 students take the course each year, I would be very surprised if even one ever needed to map genes using crosses, except as an exercise in an old-fashioned upper-level genetics course.

Here's a page arguing that even real geneticists didn't do this - that the idealized three-factor mapping cross was largely an exercise for students.  I don't think that's necessarily true, but it's certainly true that real geneticists rarely do this any more.  Genetic mapping in general, and mapping by three-factor crosses in particular, is fast becoming an archaic technique.  If one of my students should ever find that they need to do this (and I'm having a hard time coming up with an example where they would), there are lots of textbooks to show them how.

I think that the main reason genetics courses have always included three-factor mapping is that (i) this used to be how accurate gene maps were made, and (ii) this provides a tidy way to test whether students understand the consequences of crossing-over.

I think I will teach the students the difference between a physical map and a genetic map of a chromosome, and I'll expect them to be able to explain why the two kinds of map might not be identical - because recombination frequencies are influenced by DNA sequences (chromosomes have hotspots and cool spots), and because the data from the crosses may have flaws (low numbers, phenotypic problems that limit detection of recombinants).  But I won't expect them to be able to do the mapping.

1 comment:

Chang said...

One phrase: Double crossovers.