Drosophila Drosophila Genetics IGenetics I

Inheritance, Autosomal genes vs. Inheritance, Autosomal genes vs. Sex Linked genes, Monohybrid Sex Linked genes, Monohybrid

and Dihybrid Crossesand Dihybrid Crosses

**Lab manual protocol will not be usedLab manual protocol will not be used

Lab ObjectivesLab Objectives To have a first hand look at how genes are To have a first hand look at how genes are

inherited in the model organism inherited in the model organism Drosophila melanogaster Drosophila melanogaster (Fruit Fly)(Fruit Fly)

To develop a better understanding of To develop a better understanding of genetics by conducting monohybrid and genetics by conducting monohybrid and dihybrid crosses. dihybrid crosses.

To have a basic knowledge of working To have a basic knowledge of working with with Drosophila Drosophila as a model organismas a model organism

OverviewOverview

Review Mitosis and MeiosisReview Mitosis and Meiosis Review Concepts of GeneticsReview Concepts of Genetics DrosophilaDrosophila melanogastermelanogaster

Background InformationBackground Information Applied GeneticsApplied Genetics

I. Mitosis and MeiosisI. Mitosis and Meiosis

II. Genetics ReviewII. 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 ChromosomesAlleles on Chromosomes

Mendel’s Law of Mendel’s Law of SegregationSegregation

The two alleles present for a gene will The two alleles present for a gene will separate during gamete formationseparate during gamete formation

For example: For example:

AAAA will segregate into will segregate into A A and and AA

AaAa will segregate into will segregate into AA and and aa

The Law of Independent The Law of Independent assortmentassortment

““Each pair of alleles segregates into Each pair of alleles segregates into gametes independently” (Campbell)gametes independently” (Campbell)

For examples:For examples:AaBb AaBb will segregate and form 4 different gametes:will segregate and form 4 different gametes:

AB, Ab, aB, AB, Ab, aB, andand ab 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 Why DrosophilaDrosophila??

Short Generation TimeShort Generation Time Easy to MaintainEasy to Maintain 4 large chromosomes4 large chromosomes Easily identifiable Easily identifiable markers (mutant markers (mutant phenotypes)phenotypes) For more information For more information

chapter 21 in Campbell.chapter 21 in Campbell. http://flymove.uni-muenster.de/ http://flymove.uni-muenster.de/

DrosophilaDrosophila life cycle life cycle

EggEgg LarvaLarva PupaPupa AdultAdult

The whole life cycle is The whole life cycle is about 14 days at RTabout 14 days at RTºº

CHROMOSOMESCHROMOSOMES

Sex -Sex -chromosomes responsible for chromosomes responsible for determining sex of an individualdetermining sex of an individual

AutosomalAutosomal- chromosome that is not directly - chromosome that is not directly involved in determining sex.involved in determining sex.

Female

MaleXX /XY II III IV

-----------Autosomal-----------Sex

Sexing adult flies (p.65)Sexing adult flies (p.65)

‘ ‘white eye’ phenotypewhite eye’ phenotype

WT White eye mutant

Example PhenotypesExample Phenotypes

W.T. Ebony body

Vestigial wings

Curled wings

Nomenclature

Convention dictates that we denote genotype in the following fashion:

Autosomal Genes (use + for wt allele)

Genotype Phenotype

+ / + wt (homozygous)

vg / + wt (heterozygous)vg / vg+_

mutantwt (homo. or het.)

Autosomal Gene NomenclatureAutosomal Gene Nomenclature

Wild-type genes located on autosomal Wild-type genes located on autosomal chromosomes are donated “+” by conventionchromosomes are donated “+” by convention

(note that a separate system is used for sex-linked genes--module 10)(note that a separate system is used for sex-linked genes--module 10)

(Each allele is represented and is seperated by a “/”)(Each allele is represented and is seperated by a “/”)

Genotype Phenotype+ / + wt (homozygous)

vg / + wt (heterozygous)

vg / vg mutant

wt ebony White body

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.

Terminology

Monohybrid CrossMonohybrid Cross

Cross breeding organisms Cross breeding organisms while tracking while tracking oneone trait. trait.

Example Monohybrid CrossExample Monohybrid Cross

++ x v++ x vggvvgg

(Wildtype) (Vestigial)(Wildtype) (Vestigial)

F1 GenotypesF1 Genotypes

vgvg vgvg

++ +vg+vg +vg+vg

++ +vg+vg +vg+vg

F1 Phenotypic Ratio: wt

We then cross two F1 We then cross two F1 progeny……progeny……

+vg x +vg+vg x +vg

(heterozygous) (heterozygous)(heterozygous) (heterozygous)

F2 GenotypesF2 Genotypes

vgvgvgvg+vg+vgvgvg

+vg+vg++++++

vgvg++

F2 Phenotypic Ratio: 3 wt 1 vg

Refer to Refer to

Assignment I Assignment I WorksheetWorksheet

Dihybrid CrossDihybrid Cross

Cross breeding organisms while tracking Cross breeding organisms while tracking twotwo traits traits

Example Dihybrid CrossExample Dihybrid Cross

++ ++ ++ ++ xx bb v bb vggvvgg (Wild type) (Black vestigial)(Wild type) (Black vestigial)

This is not the cross that you will design This is not the cross that you will design on your worksheets. on your worksheets.

See worksheet for Assignment I for fly lab See worksheet for Assignment I for fly lab and complete the dihybrid cross.and complete the dihybrid cross.

A testcross is a valuable way to A testcross is a valuable way to use a genetic cross to use a genetic cross to

determine the genotype of an determine the genotype of an organism that shows a organism that shows a

dominant phenotype but dominant phenotype but unknown gentoype.unknown gentoype.

Test Cross

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.

GoalsGoals Observe Male/Female fliesObserve Male/Female flies Observe various phenotypes of fliesObserve various phenotypes of flies Comprehend:Comprehend:

Test CrossTest Cross Monohybrid CrossMonohybrid Cross Dihybrid CrossDihybrid Cross Chi Square TestChi Square Test

View these all in virtual flylab.View these all in virtual flylab.