Drosophila Genetics I Inheritance, Autosomal genes vs. Sex Linked genes, Monohybrid and Dihybrid Crosses *Lab manual protocol will not be used Lab Objectives To have a first hand look at how genes are inherited in the model organism Drosophila melanogaster (Fruit Fly) To develop a better understanding of genetics by conducting monohybrid and dihybrid crosses. To have a basic knowledge of working with Drosophila as a model organism Overview Review Mitosis and Meiosis Review Concepts of Genetics Drosophila melanogaster Background Information Applied Genetics I. Mitosis and Meiosis II. Genetics Review Allele-alternate forms of a given gene (brown or blue eyes) Independent assortment- random distribution of unlinked genes into gametes. Linked Genes- genes located on the same chromosome tend to be inherited together. The alleles do not assort independently. Sex linked genes- genes located on a sex chromosome (e.g. white eyes in Drosophila) Crossing over- reciprocal exchange of genetic material between non-sister chromatids during meiosis I (performed in pipe cleaner exercise during meiosis). Alleles on Chromosomes Mendel’s Law of Segregation The two alleles present for a gene will separate during gamete formation For example: AA will segregate into A and A Aa will segregate into A and a The Law of Independent assortment “Each pair of alleles segregates into gametes independently” (Campbell) For examples: AaBb will segregate and form 4 different gametes: AB, Ab, aB, and ab. Thomas Hunt Morgan State College of Kentucky (later the University of Kentucky), B.S. Johns Hopkins University, Ph. D. Nobel Prize for Physiology / Medicine in 1933. Chromosome theory of heredity Found that genes are located on chromosomes Discovered Sex-Linkage (module 10) Worked with Drosophila Why Drosophila? Short Generation Time Easy to Maintain 4 large chromosomes Easily identifiable markers (mutant phenotypes) For more information chapter 21 in Campbell. http://flymove.uni-muenster.de/ Drosophila life cycle Egg Larva Pupa Adult The whole life cycle is about 14 days at RTº CHROMOSOMES Female Male XX /XY II III IV Sex -----------Autosomal----------- Sex -chromosomes responsible for determining sex of an individual Autosomal- chromosome that is not directly involved in determining sex. Sexing adult flies (p.65) ‘white eye’ phenotype WT White eye mutant Example Phenotypes W.T. Ebony Vestigial Curled body wings wings Nomenclature Autosomal Genes (use + for wt allele) Convention dictates that we denote genotype in the following fashion: Genotype Phenotype +/+ wt (homozygous) vg / + wt (heterozygous) vg / vg mutant +_ wt (homo. or het.) Autosomal Gene Nomenclature Wild-type genes located on autosomal chromosomes are donated “+” by convention (note that a separate system is used for sex-linked genes--module 10) (Each allele is represented and is seperated by a “/”) Genotype Phenotype +/+ wt (homozygous) vg / + wt (heterozygous) vg / vg mutant Terminology Wild type- the normal or most common phenotype in a population. Mutant Phenotype- traits that are alternates to the wild type. Changes due to mutations in the wild type. wt ebony White body Monohybrid Cross Cross breeding organisms while tracking one trait. Example Monohybrid Cross ++ x vg vg (Wildtype) (Vestigial) F1 Genotypes vg vg + +vg +vg + +vg +vg F1 Phenotypic Ratio: wt We then cross two F1 progeny…… +vg x +vg (heterozygous) (heterozygous) F2 Genotypes + vg + ++ +vg vg +vg vgvg F2 Phenotypic Ratio: 3 wt 1 vg Refer to Assignment I Worksheet Dihybrid Cross Cross breeding organisms while tracking two traits Example Dihybrid Cross ++ ++ x bb vgvg (Wild type) (Black vestigial) This is not the cross that you will design on your worksheets. See worksheet for Assignment I for fly lab and complete the dihybrid cross. Test Cross A testcross is a valuable way to use a genetic cross to determine the genotype of an organism that shows a dominant phenotype but unknown gentoype. Example Test Cross +_ x vgvg (Unknown) (Vestigial) If the F1 progeny are all WT the genotype of the unknown fly is +/+. If the F1 progeny are a mix of wild-type and vestigial flies, the genotype of the unknown fly is +vg. Goals Observe Male/Female flies Observe various phenotypes of flies Comprehend: Test Cross Monohybrid Cross Dihybrid Cross Chi Square Test View these all in virtual flylab.