1. An Experiment in Gene Linkage using Drosophila Melanogaster
Shafer Stellema
BIOL 242
Section 011
Abstract
This experiment is designed to use statistical analysis to determine the
chromosomal location of 2 genes in drosophila melanogaster. The mutations of interest
were ebony bodies and scarlet eyes. The scarlet eye flies also had thread like antenna and
a notch in their wing. Two types of crosses were set up for P1 : ebony females were
crossed with scarlet eye male and ebony males crossed with scarlet eye females. The F1
generation showed all wild type phenotype in both types of crosses. The F1 generation
crosses were then self-crossed to make an F2 generation. It was hypothesized that the two
mutations were both on autosomal chromosomes and further more, they are on the same
chromosome.
Introduction
In this experiment, statistical analysis is used to determine the type of
linkage between 2 mutations in drosophila melanogaster. Drosophila melanogaster
(more commonly know as the fruit fly) is a perfect organism to use due to their quick
reproductive rate, small size, and extensive knowledge that exist about their genome. In
the lab conditions, adult drosophila flies may live 8-10 weeks (Karcher).
There are 4 main life stages, which include egg, larva, pupa and adult.
The reproductive cycle of the fruit fly is about 14 days and after hatching, females are not
sexually mature for 6-8 hours (Karcher). This time window allows for selective breeding.
Drosophila have 4 chromosome, one of which is a sex chromosome. This genetic
simplicity makes it much easier to discover the origins of mutations. Furthermore,
drosophila morphology is easily distinguished, and the males do not undergo meiotic
recombination (Rong).
Wild type flies show red eyes with a tan body. The mutations give to use
include ebony flies that show a dark brown body and scarlet flies that showed a
bright red eye. Through reciprocal crosses and self crosses, the chromosomal
location of these mutations were hypothesized. Scarlet males were crossed with
Ebony females and Scarlet females were crossed with Ebony males. It should be
noted that the females were virgins before creating these crosses, ensuring that only
the desired males are inseminating them. The children of these crosses were then
self-crossed. Statistical analysis of the phenotypical results from the crosses can
then be used to determine the mode of inheritance. Some possible expected
phenotypical ratios include: If the traits are on separate chromosomes (unlinked)
the phenotypic ratio for the F2 generation will be 9 wild type: 3 single mutation A: 3
single mutation B: 1 double mutant. If the traits are on the same chromosome
(linked) the phenotypic ratio for the F2 generation will be 2 wild type: 1 single
mutation A: 2 single mutation B: 0 double mutants. Based on the observed data and
the expected ratios, it is hypothesized that, in drosophila melanogaster, scarlet eye and
ebony body genetic mutations are both located on the same chromosome.