This page explains the basics of Drosophila genetics. Without this information and knowledge of the terminology and constructions outlined below, it will be very difficult to understand the crossing schemes and many of the other pages.
About the Drosophila Genome
The Drosophila genome is diploid, and organized into four groups of chromosomes: the sex chromosomes (X and Y), and 3 autosomal groups (chromosomes 2,3,4). All cells except germ cells that have undergone meiosis in the germ line have a pair of sex chromosomes (either XX or XY), and a pair of each of the three autosomes. For all practical reasons, chromosome 4 can be ignored, because it is extremely small- a “little stump” in the words of Lutz. The Y chromosome can also be ignored, because it carries very few genes. The three autosomes are denoted by their Roman numerals (i.e. chromosome 2 would be called II). Below is a diagram of Drosophila’s four chromosomes.
The entire Drosophila genome is 139 X 106 base pairs long, and codes for approximately 15 thousand genes. This number is bound to change in the future as further research is done. The main driving force behind current Drosophila research is the goal of deciphering the function of each individual gene, and studying the functional genetic networks in tissues and developmental time. Understanding these characteristics of Drosophila is key to our understanding of human biology and medicine, as fruit flies are wonderful model organisms for studying human cells and genes. The ultimate goal of the genetic side of the StanEx project is to utilize enhancer traps and epiflourescent microscopy to determine the function and location of specific genes on the Drosophila genome.
Genetic Naming Conventions
Drosophila genes have transitioned from being named after loss-of-function phenotypes, to emphasizing the functional group the gene belongs to. For example, the ortholog of human EGFR (Epidermal Growth Factor Receptor) is called DER (Drosophila Epidermal Growth Factor Receptor). Orthologous gene pairs frequently share a decent degree of amino acid similarity. Another useful convention to keep in mind is that genes are always written in lowercase, italicized letters, even if they begin a sentence.
Most genes have abbreviations or acronyms. The white gene, for example, which carries a mutation, is abbreviated simply as w. The wild type white gene is abbreviated as w+. Generally, wild type genes are not written into the genotype at all, meaning that everything that is not in a mutated genotype (such as w), is wild type. The mutated form can either be denoted as w, as above, or as w-. For even more specificity, the specific allele name may be integrated in the superscript, as in the case of w1118.
When the entire genotype of a fly strain is written out, different chromosomal groups are separated by semicolons, and alleles on the same chromosome are separated by commas. Because Drosophila are diploid organisms, and contain two copies of each chromosome, each chromosome is separated into a bottom half and top half by a long division line (see example below). The two copies of each chromosome need not be identical, especially when two different strains of fly mate to produce offspring. However, in the case that the two chromosomes ARE identical, the division line disappears, and the genotype of both chromosomes is written out once. Finally, if one copy of a chromosome has mutant alleles, but the second copy does not, the allele on the wild type chromosome is written as “+”.
Example 1: white mutant fly (side note: w is located on the X)
w-/w- or w or simply w (if it is a female) or w-/Y (if it is a male)
w
Example 2: white heterozygote female
w-/+ or w
+
So far, we have only looked at recessive markers, which require that both copies be mutated in order for the phenotype to be shown. But, there are also dominant alleles, who display their phenotype even when they are only present on one chromosome (and the allele on the second copy is wild type). Alleles that code for dominant phenotypes are always capitalized. Curly (causes curly wings) and Lobe (causes an eye reduced in size), two genes that are central to our work, are both dominant alleles on the second chromosome. New rule: Alleles that cause dominant phenotypes are written first letter in caps. We will need a couple of them in the course of this work. One is Cy (Curly, causes
Example 3: white mutant that carries L on one of its second chromosomes, and Cy on the other:
w-/w-; L/Cy OR w ; L (female) w-/Y; L/Cy OR w ; L (male)
w Cy Y Cy
With this information, you should be able to decipher the crossing scheme. For further information on genetic naming conventions for Drosophila, please read chapter 1 of “Fly Pushing” by Ralph Greenspan.
Genetic Markers Used in the Crossing Scheme
Below are images that show how to distinguish between the various genetic markers used in the crossing scheme. It is important to master the differences between these markers, because the phenotypic characteristics will indicate the specific genotypes of the flies. Based on the presence or absence of specific markers, we will select for or against flies as each generation hatches.
[All images are from the Genesis "Learning to Fly" poster: http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1526-968X/homepage/free_posters.htm]
The image to the left shows the difference between the wild type eye (wt), the white mutant eye (w-), and the Lobed eye (L).
The image to the left shows the difference between wild type wings (wt) and Curly wings (Cy). Curly wings are also denoted by Cyo.
The image to the left shows the difference between the wild type shoulder hair (wt), and the extra shoulder hair phenotype "Humeral" (Hu). Humerals are a specific kind of bristle on the first thoracic segment of the fly.
The image to the left shows the difference between wild type coat color (wt), recessive coat color marker ebony (e), and the dominant bristle marker Stubble (Sb), which causes shorter hairs. Both Stubble and ebony are on the third chromosome.