Lab 6- Drosophila Genetics: Linkage
In this lab, you will observe flies with phenotypes that are different from those observed in the previous lab. Moreover, the traits studied in this lab are influenced by genes that are linked. You will use the ratios of different phenotypic classes to determine linkage relationships among those genes. There are three genes of importance in this lab:
black (b)- black on body and wing veins, which darkens with age;
brown (bw)- eyes pale red-brown, which also darkens with age; and
vestigial (vg)- wings and halteres reduced.
All of these genes are on chromosome 2, (Drosophila has four chromosomes) and each shows typical Mendelian inheritance for recessive mutations. That is, heterozygotes or homozygotes show the wild-type (or +) phenotype. B-, Bw- or Vg- flies show the wild-type phenotypes for body color, eye color, and wings, respectively. For example, a vgvg fly will have vestigial wings, and a Vgvg fly will have normal wings. By examining the results of a testcross, you will determine: (a) the order of these genes on chromosome 2 and (b) the relative map (or linkage) distances between them.
1. Working with your partner, obtain several true-breeding triple-mutation flies: bwbvg/bwbvg. Observe the phenotypes and contrast them with wild-type flies. You will need to distinguish flies of mixed genotypes, for example, a wild-type body color from a black body. (This trait, in particular, requires careful observation.)
2. Obtain several flies from the testcross mating:
female: bwbvg/+++ x male: bwbvg/bwbvg
The flies in your vial are the offspring from the testcross; the testcross parents were removed before the pupae began to hatch. Observe the flies and score them for phenotype. To do this, you must know how many and which phenotypes to look for. (See Assignment steps 1-3 below.) Discard scored flies only after you are certain that you have separated them correctly by phenotypic class. Let your partner and the instructor double check your phenotype assignments.
3. Teams will take turns scoring flies in the vials with the blue medium. When it is your teamís day, come to lab, anesthetize the flies, and score for phenotype. Keep your own data, and we will combine data at the end of scoring. If there are many flies (>15), do not score more than 15; leave the rest for the next team to score later.
Complete the following tasks and questions and type your responses.
1. Although you never saw them, what were the phenotypes of the two flies used in the test cross? What were the phenotypes and genotypes of their parents (i.e., the P generation)?
2. Draw the chromosomes for the two flies used in the testcross. Place the various alleles along the chromosomes as they might be arranged on the real chromosomes. (Until you have data that suggest an actual order, your initial placement represents only a working hypothesis.)
3. Draw the chromosomes that would result from all possible single and double crossovers. How many phenotypes should result from the testcross and what are they?
4. Generate a table showing the phenotypic classes and numbers of each phenotype, as collected by your team and by the class. Using that information about phenotype, determine the order of the three genes in this experiment and the map distances separating them.