Genetics of Organisms (Drosophila)
Objectives:
• Learn the life cycle for Drosophila melanogaster
(egg-larval-pupal-adult)
• Distinguish between male & female flies
• Study & examine mutational characteristics of:
Eye color (sepia-brown: ss vs. wild type: red SS)
# Bristles
Wing size (long vs short)
Antennae size (vestigial vs. normal
• Examine THREE genetic crosses:
-Monohybrid: ss (sepia) x SS (normal-red)
-Dihybrid: Normal eyes, vestigial wings x sepia, normal wings
SSvv x ssVV
-Sex-Linked: white eyed female x Normal red-eyed male
XrXr x XRy
Why use Drosophila???
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Simple food requirements
Occupies little space
Hardy (survives well)
Completes life cycle around 12 days (temp)
Produces large numbers of offspring
Easily immobilized (chilling on ice) to examine
Several hereditary variations
Small # of GIANT chromosomes (four pairs)
Chromosomes easily located in the salivary gland
cells
Key Factors of this Lab
• The flies live for about a month
• Females will store up sperm in her
receptacles & can continue to fertilize
eggs
• Virgins must be separated from males
early on (distinguishing males & females)
• Temperature is the most important factor
that determines the length of life cycle
Distinguishing between Male &
Females
Males
– smaller
– Dark, blunt abdomen
– Sex comb on forelimb
Females
– Larger
– Lighter pointed abdomen (ovipositer)
– Transverse stripes on abdomen
– No sex comb
Table 7.1: F1 Generation Data (pg. 82)
(Note: this page needs to be added to your packet)
Phenotype and Symbol
Red-eyed
White eyed
Females
Males
all XRXr
all XrY
**The table is larger than two rows. Don’t worry about the other rows,
just leave them blank.
F2 Generation: Sex-Linked trait (pg. 82)
Table 7.2 F2 Generation Data
Phenotype and Symbol
Females
Males
Red-Eyed (XR)
78 XRXr
69 XRY
White-Eyed (Xr)
62 XrXr
73 XrY
Assume that the following Data was obtained and determine if it is
significant by calculating a Chi-Square value. Do this in the margin space
available on pg 82. Remember to calculate the expected (theoretical)
values from the cross that produced the results above. The cross is written
below.
R r
r
(these came from F1 generation)
X X x XY
Red-Eyed heterozygous female crossed with white-eyed male
Cross between two individuals from F1 Generation: XRXr x XrY
r
X
R
X
R
r
X X
Red-eyed female
r
X
r
r
XX
white-eyed female
Y
R
X Y
Red-eyed male
r
XY
white-eyed Male
2nd Genetic Cross (pg. 83)
• Red-Eyed: Dominant
• Sepia eyes: Recessive
(3) Parental cross:
(3) F1 Cross:
S
s
SS X ss (show a punnett square in box)
Ss X Ss (show a punnett square in box)
**Fill in this info. For #4 Table
F1:
F2:
All Ss
3:1 (red:sepia)
#5 There should be no deviations in the F2 results.
Sometimes there could be fewer red-eyed individuals
than expected and more individuals with sepia eyes.
Data for Second Cross (Pg. 84)
#6.
The mutation will be Autosomal
The mutation will be recessive
The cross is monohybrid
#7
Ss x Ss
Phenotype # observed (o)
# Expected (e)
(o-e)
(o-e)2
(o-e)2
e
Red
140
Sepia
60
**Answer Questions “a” and “b”
Page 85
• Answer Questions #1-3 based on the
information written in the beginning of the
lab.
Other Crosses to do
• Pgs 85-86: Green & Albino seedlings
– Table 7.3
Gg x Gg
Green
Albino
# Expected
Calculate
Calculate
• Pg.88: Long & short winged flies
**complete and calculate Chi-Square values to
determine if the results are significant and
acceptable (statistically valid)
• Pg 89 Table 7.8