Building a linkage map

A useful feature of Drosophila is that the adult has many distinctive features; these features can be used to trace the inheritance of a number of different traits.

These features include eye color and shape, bristle patterns, wing shape and structure. 

Many mutations have been identified that produce distinctive changes in one or more of these features.

Looking through a microscope at the somatic cells of Drosophila melanogaster it is possible to see four pairs of chromosome. Males have an "X" and a "Y", female two "X"s.

Both sexes have two copies of the autosomes 2, 3, and the rather tiny 4.  After the discovery of the white (w) mutation, a number of other recessive "sex-linked" mutations were discovered.

These included white (w) and vermillion (v) eye color , rudimentary wings (r), yellow (y) and lemon (lem) body color and bar-shaped eyes (B).

Experiment 2 Directions

• Using the Morgan applet,determine the genetic distance between the flower color trait and the pollen shape trait. 
• Record the results of your experiments (# of each type of individual).

We estimate how closely linked two traits are by calculating the percentage of recombinants.  We count the number of both classes of recombinants to get the total number of recombnants).

(total # of recombinants / total # of progeny) x 100 = genetic distance in centiMorgans (cM)

As should be evident, two traits that are unlinked (that is, they behave like Mendel's traits) appear to be 50 cM apart. 

It is possible to map these traits with respect to one another. 

If there are enough markers along the chromosome, it is possible to determine the order and spacing between markers, and to use that information to generate a genetic map of the chromosome.

We can add the genetic distances together, an intact chromosome is typically many 100's of cMs in length. 

Experiment 4: Directions

• Use the Morgan applet (when it is ready) to map the genetic distance between one pair of trait.  If it is less than 40 cM, continue by mapping new traits with repect to of the pairs of traitsmutations on the X chromosome. 
• Produce a linkage map of the X. 

• Is it possible that some of Mendel's traits were on the same chromosome?
• Can you imagine a case in which two traits are seperated by greater the 50 cM?