Download Example Dihybrid Cross

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
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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
gametes independently” (Campbell)
For examples:
AaBb will segregate and form 4 different gametes:
AB, Ab, aB, and ab.
into
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
Sex
 Sex
II
III
IV
-----------Autosomal-----------
-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
body
Vestigial
wings
Curled
wings
Nomenclature
Autosomal Genes (use + for wt allele)
Convention dictates that we denote genotype
in the following fashion:
Genotype
+/+
vg / +
vg / vg
+_
Phenotype
wt (homozygous)
wt (heterozygous)
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
+/+
vg / +
vg / vg
Phenotype
wt (homozygous)
wt (heterozygous)
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
++
(Wildtype)
x
vg vg
(Vestigial)
F1 Genotypes
vg
vg
+
+vg
+vg
+
+vg
+vg
F1 Phenotypic Ratio: wt
We then cross two F1
progeny……
+vg
(heterozygous)
x
+vg
(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
++ ++
(Wild type)
x
bb vgvg
(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
+_
(Unknown)
x
vgvg
(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:
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Test Cross
Monohybrid Cross
Dihybrid Cross
Chi Square Test
View these all in virtual flylab.