The Drosophila melanogaster
In this experiment, the species Drosophila melanogaster, the common fruit fly, was studied to examine the processes of natural selection and genetic drift in the laboratory. The fruit fly has a sex linked gene determining eye colour. Located on the X chromosome, a fruit fly can exhibit the phenotype white eye (homozygous recessive), or red eyed wild-type (dominant). Males of the species are XY, and as such only carry one eye colour determining X chromosome. Females are XX, and can therefore carry two eye colour determining genes. A heterozygous female will always display the dominant wild-type gene if present. The uncertainty of female genotype is cleared by the acquisition of an F0 stock which is homozygous dominant or recessive.
Aside from determining eye colour, recognising the sex of D. melanogaster was paramount to this experiment. This can be accomplished by observation of the unconscious flies through the dissecting microscope. Males are distinguishable by the presence of heavy dark bristles surrounding the male genitalia, bristles not present on the females. Examination of the front legs provided another distinction, as males carry a sex comb which females do not. Size and shape cannot be used to determine sex as both vary depending on age. The abdomen of an older male will be darker than that of an older female; however abdominal colour also varies with age, making it an unreliable method of sexing.
Observed throughout the experiment were four life stages of D. melanogaster. Eggs hatch a day after being laid by the female, and are in 1st instar larval form. Over the next 6 days, the larvae would molt to the 2nd and 3rd instar. The cuticle of the 3rd instar hardens into a pupa, and after 6 days in the puparium, metamorphosis is complete and the adult fly forces its way through the anterior end of the puparium. Older males would attempt to mate with very young females;...
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