Virtual FlyLab Experiments, Set III

Working in groups of four assigned in discussion, analyze the following traits using the Virtual FlyLab. For each of these crosses you will be given three traits to analyze at a time. In addition to determining the inheritance pattern of each trait individually you need to explain the inheritance patterns that occur when all mutations are involved in the same cross (a trihybrid cross). In particular, be sure to determine the distance between genes on the same chromosome and the order and any interference if all three of them should be on the same chromosome. There is no crossing over in male Drosophila so you have to be very careful about how you design your crosses if you want to map the distance between the genes. Don't forget to use the chi-square test to test any hypotheses that you propose and to give phenotypic and genotypic diagrams of all relevant crosses with results. These problems are very difficult, especially problem 3, and you will work in groups of three to four students assigned in class and turn in a joint report. Problems 1 and 2 are worth five points and problem 3 is worth 15 points, your results are due in your discussion section, the week of March 5th. There is an updated scoring rubric for problems 1 and 2 and another one for problem 3.

Strategy hints:

  • The easiest way to do this is to first design a female fly with all three mutant traits and then cross this fly with a wild type male.
    • Any mutant traits that are present in the F1 females are dominant
    • Traits that appear in the males but not the females are X-linked recessive.
    • Lethal traits will have a 1:1 ratio in both males and females
  • If only some of the traits are X-linked use dihybrid crosses to map the linked genes as in FlyLab 2.
  • If all of the traits are either X-linked or autosomal then cross the trihybrid F1 female with a male fly homozygous for the recessive traits (design this fly). Examine the F2 results for evidence of linkage.
    • If none of the traits are linked you will see a 1:1:1:1:1:1:1:1 ratio of the eight different phenotype combinations.
    • If two of the traits are linked there will be two groups of numbers, one larger than the other. Half of the phenotype combinations (4) will have the large numbers, these four combinations contain the parental configuration for the two linked genes, and the other half will have the small numbers and are the recombinants
      • determine which gene is not linked to the other two (if you're confused, try some dihybrid crosses to check for the linked genes)
      • add all of the recombinants and divide by the total progeny to get the map distance
    • If all three genes are linked then you will get four different pairs of numbers
      • the largest pair should be the parental configuration
      • the smallest pair is the double recombinants, use this to determine which gene is in the middle
      • the other two pairs are the single recombinants between the middle gene and each flanking gene, use these numbers plus the double recombinants to get the map distances
      • check for interference by calculating an expected number of double recombinants from the two flanking distances and compare this to the actual number of double recombinants

1. Investigate the spineless bristle, dumpy wing and brown eye traits (5 pts).

2. Investigate the white eye, tan body and crossveinless wing traits (5 pts.).

3. Black body is located near the middle of chromosme 2. In Virtual Fly II and III you encountered five other genes located on chromosome 2. Determine which genes are on chromsome 2 and the linkage map of all the chromosome 2 genes (15 pts.). Include in your report only crosses used to determine the map, you can use dihybrids or trihybrids (trihybrids will be much faster). Also report any significant interference.

Bell CSU Chico Library
This document is copyright of Jeff Bell
Last Update: Monday, September 4, 2000