January 24, 2011
Introduction & Background
Fruit flies have made a huge contribution towards knowledge about genetics, but for most people, they are just annoying insects that are attracted to their fruit. Their scientific name is Drosophila melanogaster, and to scientists, they have been a key to understand many principles of heredity including sex linked inheritance, epistasis, multiple alleles, and gene mapping. Fruit flies were the first organisms to be used for genetic analysis in 1910 by Thomas Hunt Morgan, and ever since, they have been used for genetic experiments (Ashburner).
There is a huge advantage to using fruit flies instead of another organism. Fruit flies have simple food requirements, they occupy little space (they are contained in vials), they complete their life in about 8-14 days (depending on the temperature), they produce many offspring, and they can be “put to sleep” to examine them (Life). Although there are many factors that determine the life span of the fruit flies, the biggest one is the temperature of the environment they are in. When they are in room temperature, the life cycle lasts for about 10-12 days. It starts by the male depositing his sperm into the female. If fertilization occurs, the female lays about 500 eggs on top of either fruit or any decomposing organic matter. This is the first stage of fruit flies--the egg. The egg is oval and has two filaments at one end, and after about one day, they hatch into larva. The larva is divided into three different stages: the 1st instar, the 2nd instar, and the 3rd instar. The total amount of time that it is in the larva stage is between 3-5 days. During the first instar, the larva eats continuously, tunneling through the medium. It sheds its skin twice, each time, it increases in size. After it molts, it enters the second instar (after about a day). During the second instar, it eats all it can, like a caterpillar before making its cocoon. After eating for about two days, it enters the third instar; it gets to drier area and encapsulates itself in hard puparium. The fruit fly has now become a pupa, and after metamorphosis has occurred (about 4 days), the puparium disintegrates and the fruit fly emerges. At first, the flies are light in color and their wings are not expanded, but after a few hours, their body turns darker, and after 8-12 hours, the female fly is already receptive, meaning that they are willing to mate. By expanding their wings and vibrating them, the males perform a mating dance for the females, and for the next two days, they are sexually active, mating with each other for 30 minutes at a time (Johnson).
In this lab, we determined the mode of inheritance for a mutant gene in fruit flies. A mutant gene is a gene that is not shown in the wild type. The wild type phenotypes of fruit flies are red eyes and tan color, and if a fruit fly, like any organism, does not show the phenotypes of the wild type, they are considered a mutant. Fruit flies exhibit sexual dimorphism, meaning that the females appear differently than the males. The females are larger than the males (they are 2.5 mm long). The males also have a black patch at the end of their abdomen, which are also rounder than the tip of the female abdomen. Lastly, the males have sex combs on their forearms, and the females do not (Johnson).
Null hypothesis: In Cross A, a monohybrid cross, the genotypic ratio will be 3:1. Cross B, a dihybrid cross, will have a genotypic ratio of 9:3:3:1. In Cross C, a sex linked recessive cross, there will be a genotypic ratio of 1:1:1:1.
Alternative hypothesis: The F1 offspring of Cross A, the cross between a wild type male (red eyed) and a sepia eyed female, will all have red eyes, the wild type phenotype, because the wild type allele is dominant over the sepia colored eyes. In the F2 generation of this F1 generation, wild type male and wild type female, wild...