Elements of Drosophila Genetics
Genetics can profitably be applied to the study of the development of tissues and tissue-specific gene expression as well as behavior, vision, muscle and nerve function in Drosophila. The Drosophila genome of about 1.65 × 108base pairs of DNA is contained in four chromosomes. Chromosomes II, III, and IV are normally found as pairs, while in males the constitution of the fourth chromosome is XY and in females it is XX. Unlike yeast, there is no simple way to make haploid fruit flies, and therefore the genes which are located on chromosomes II, III, and IV are more difficult to study. The genes located on chromosome X may easily be studied, for males are haploid for the X chromosome and females are diploid. Therefore recessive mutations in genes located on the X chromosome will be expressed in males, and the ability of X-located genes to complement can be tested in females.
Once a mutation is generated in the DNA of a bacterium, one of the daughter cells usually is capable of expressing the mutation. The analog is also true in Drosophila. However, the equivalent of a cell division in bacteria is the next generation in Drosophila. An adult fly can be mutagenized, but many of its genes are expressed only during development. Therefore this adult will not express the mutation. Mutagenized adults must be mated and their progeny must be examined for the desired mutation. One straightforward way to mutagenize flies is to feed them a 1% sucrose solution containing ethylmethanesulfonate (EMS). If male flies are mutagenized and mated with females, four types of
progeny are obtained. In the first generation only females could contain a mutagenized X chromosome. If these females are collected and mated again with unmutagenized males, then in the second generation half of the males have a chance of receiving and expressing a mutagenized X chromosome. Much tedious sorting of males and females could be required for the detection of rare mutants.