Introduction to Drosophila Genetics

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DROSOPHILA CULTURE We will study basic principles of Mendelian inheritance with the use of the fruit fly, Drosophila melanogaster [the name means “black-bodied fruit-lover”]. Drosophila was one of the first organisms to be studied genetically: its small size, short life cycle (10 ~14 days at 25oC), high reproductive rate (an adult female can lay 400-500 eggs in 10 days), and ease of culture and genetic manipulation have made it perhaps the best understood animal genetic system. Many different species, and a large number and wide variety of naturally-occurring and artificially-induced genetic variants are available. The partial genetic map in Appendix B describes the location of all the mutations used in crosses and lab questions. VIRGIN FEMALES All female flies used in controlled genetic crosses must be “virgins”. Female flies are capable of mating as early as 8 hours after emerging from the pupae stage and are polyandrous, that is, capable of mating with several males. Once mated, females can retain viable sperm for several days and this will confuse the results of a subsequent controlled mating. To prevent this, all adult flies are removed from the culture bottle about 7 hours prior to lab time, so that all newly hatched flies will remain virgin. BASIC GENETICS The karyotype of Drosophila comprises four pairs of chromosomes, of which three pairs are autosomes and one pair are sex chromosomes. Female Drosophila are XX, and males XY. A gene is a heritable factor that controls the expression of some trait, which may be morphological, behavioural, molecular, etc. Each such gene occupies a specific physical locus (pl. loci) on a particular chromosome. Variant forms of these loci are termed alleles. Gene, locus, and allele are often used more or less interchangeably, and this can lead to confusion. Gene is the popular and most general term, and is most appropriate when the inherited basis of a trait is emphasized, e.g., a “gene” for eye colour. Locus is most appropriate when the physical nature or position of a gene, especially with respect to other genes, is emphasized, as for example in gene mapping and linkage studies. Allele is most appropriate when the particular form(s) of a gene found in any particular individual or chromosome is(are) emphasized: e.g., there are “brown” and “blue” alleles of the eye colour gene It is therefore inaccurate to say, for example, “He has the gene for sickle-cell anemia,” and more accurate to say “He has two HbS alleles at the beta-globin locus on Chromosome 6.” We all have the “gene” for every genetic condition, some of us have the particular allele(s) that result in the condition being expressed. In the technical literature, “locus” and “allele” are probably more common than “gene”. Drosophila, like most species we will deal with in this course, are diploid, with two sets of chromosomes and therefore two alleles at each autosomal locus. If both alleles are identical, the 1

individual is a homozygote and is described as homozygous. If the alleles differ from each other, the individual is a heterozygote and is described as heterozygous . If the gene occurs on a sex chromosome, females may be either homozygous or heterozygous, but a male fly with only one allele at a locus will be a hemizygote and would be described as hemizygous. Drosophila of typical appearance are said to show the “wild-type” forms (phenotypes) of genetically-controlled traits for body colour, eye colour, wing shape, etc. Naturally-occurring or artificially-induced genetic variants (mutations) of the alleles that control these traits produce flies with different morphologies, according to the dominant or recessive nature of the alleles involved in the genotype . Such mutant alleles are designated by symbols that are typically abbreviations of the mutant name. For example, the typical body colour phenotype is grey. One mutant produces an ebony (shiny black) body colour. Because this...
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