Drosophila Genetics I

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Drosophila Genetics I Powered By Docstoc
					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.

				
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