Biology 211 - Lab Seven: Genetics – Of Flies and Men
Reminder: Bring your textbook, a calculator, and goggles!
Pre-Lab Homework:
1. Understand the experimental basis for Morgan’s discovery of sex-linkage. Name
three visible traits that you will use to tell male fruit flies from female fruit flies.
2. Name THREE human traits you will be examining in lab today. What is the
Hardy-Weinberg equation and how will you be using it in lab today?
3. Understand how to draw and read human pedigrees.
In-Lab Activity Overview
a. Sex-Linked Traits and Working With Fruit Flies
b. Some Mendelian Human Traits and Population Genetics
c. Analyzing Pedigrees
d. Morgan Recombination
Sex-Linked Traits and Working With Fruit Flies (Drosophila)
In this experiment, you will all be little Morgans, studying fly eye color – which is sexlinked (red eyes = wild type, white = mutant). As with the problem above, your job will
be to figure out parental genotypes in the following “mystery cross.” Because flies are
harder to take a cursory look at, we will tell you in advance that the offspring contain
some red-eyed and some white-eyed flies… but we won’t tell you the gender.
Logic – Set-Up Your Hypothesis/Predictions First – Work In Pairs
1. Using human sex-linkage (XNY) nomenclature, write out ALL possible genotypes
for all possible parents. Be careful – you should come up with FIVE different
genotypes, which means that 1 box below has 2 options!
Red-Eyed Females
White-Eyed Females
Red-Eyed Males
White-Eyed Males
2. Using the 6-square box on your worksheet, determine and record a complete
Punnett Square for each (phenotype, genotype, AND gender!).
3. Given our hint that “mystery offspring” are a mix of red- and white-eyed flies,
place an X next to all crosses that DO NOT GIVE such a mix.
4. For the remaining crosses, make sure you understand how each is different…
because they are – and those differences are important!
Modified Immobilization Procedures, Analyzing Flies - Work in Pairs
1. Obtain a vial of Drosophila, place in ice (covered up to the cotton plug) for 5 min.
Biology 211 Lab Manual, Lab Seven, page 1
2. Pour immobilized flies onto Petri plate that is nested within a lid that is filled with
ice. Manipulating them with a paintbrush, toothpick, or pipe-cleaner.
3. View the flies using a dissecting microscope, sorting males to one side and
females to the other. You will also need to record all fly eye color: red vs. white.
Key Traits Associated With Gender
There are many GREAT websites with images if you want to see this in advance!
Males
Females
Overall Size
Smaller
Larger
Dorsal Abdomen Banding Less striped, more solid black
More striped
Sex Combs
Present – Check Foreleg
Absent
Ventral Genitals
Darker genital arch/penis
Lighter plates
4. If your flies begin to wake up, cover with the lid from another Petri plate until the
flies have calmed down. When finished, transfer unused flies back into the vial.
Some Mendelian Human Traits and Population Genetics
Several human traits are controlled by single genes that are inherited in a Mendelian
manner. In this exercise, we will – as a class – record our population genetics for
several of these genes. We will also learn about a predictive population genetics
equation that can be used to estimate carrier numbers.
Procedures – Collecting Class Data – Work As Table, Then Class
Characteristic
Your
Phenotype
Your
Genotype(s)
Total # Class
Recessives
Percent
Recessives
Dimpled Chin (D)
No Dimple (d)
Free Ear Lobe (F)
Attached (f)
Widow’s Peak1 (W)
No Peak (w)
Tastes PTC2 (T)
Cannot Taste (t)
Left Thumb Top3 (L)
Right Top (l)
Bent Little Finger4 (B)
Straight (b)
1
Widow’s Peak: look at hairline in center of forehead – pointy or straight?
2
PTC: phenylthiocarbamide, which tastes bitter to some people (dominant)
3
Thumb Position: when interlocking your fingers, is left or right thumb on top?
4
Little Finger: when holding little fingers next to each other, straight or bent?
Biology 211 Lab Manual, Lab Seven, page 2
Use Hardy-Weinberg Equation For Worksheet Problems:
p2 + 2pq + q2 = 1
p2 = % homozygous dominant genotypes
2pq = % heterozygous genotypes
2
q = % homozygous recessives genotypes
p = % dominant alleles; q = % recessive alleles; and p + q = 1
Analyzing Pedigrees
One of the things you also may be asked to do (e.g. on the MCAT) is to analyze
pedigrees and figure out the mode of inheritance (e.g. sex-linked vs. autosomal,
dominant vs. recessive). Solve the worksheet problems about pedigrees.
Morgan Recombination
The following dry lab data are based on a Morgan-style autosomal linkage cross
between AaBb and aabb. The observed offspring are as follows: 64 AaBb, 65 aabb,
36 aaBb, 35 Aabb. Use this information to solve the % parentals, % recombinants,
and distance (map units) between these genes.
Biology 211 Lab Manual, Lab Seven, page 3
Lab Seven: Genetics - Turn In As Pairs At End of Lab
Pair Member Names:
1. Fly/Sex-Linkage Experiment
a. 1.5 pts. Six Possible Crosses.
b. 2 pts. Fly Data.
Tally Class Data Here
Record Fly Data Here – sex, eye color
Fly 1
Fly 6
Fly 2
Fly 7
Fly 3
Fly 8
Fly 4
Fly 9
Fly 5
Fly 10
#, % Female Red
#, % Male Red
#, % Female White
#, % Male White
2. 1.5 pts. Based on CLASS fly data, write the most reasonable parental genotypes
and indicate the Punnett Square that supports this hypothesis.
Biology 211 Lab Manual, Lab Seven, page 4
3. 3 pts. Complete summary analysis for the human traits assessed in class.
Characteristic
% Recessives in Class
Estimated % Carriers
Chin Dimpling
Ear Lobe
Hairline/Widow’s
PTC Tasting
Thumb Position
Little Finger
4. 1 pt. ALL OR NOTHING. Analyze ANY TWO of the following human pedigrees.
Cross out the one you are choosing not to try and solve.
a. Circle One From Each Category
DOMINANT or RECESSIVE
AUTOSOMAL or SEX-LINKED
b. Circle One From Each Category
DOMINANT or RECESSIVE
AUTOSOMAL or SEX-LINKED
c. Circle One From Each Category
DOMINANT or RECESSIVE
AUTOSOMAL or SEX-LINKED
6. 1 pt. Using dry lab data provided for the Morgan cross, calculate the following:
___________________________ % parentals
___________________________ % recombinants
___________________________ distance (map units) between these two genes
Biology 211 Lab Manual, Lab Seven, page 5