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LAB 10
Genetics: Simple and Complex Genetic Inheritance
I.
Objectives:
This lab will help students understand the differences between simple and complex
genetic inheritance and how the environment can influence gene expression. At the
end of today's lab, students should:
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Know how to distinguish between heritable traits that are under the control of
a single gene (simple) and those that are under the control of many genes
(complex)
Understand the difference between monozygotic and dizygotic twins and
appreciate the contribution twin studies have made to genetics
Be able to define the term concordance and determine whether a trait is
influenced more by genes or environment using % concordance
Understand the importance of Drosophila as a model organism in genetics
research
Be able to identify the stages of the Drosophila lifecycle
Be able to identify various mutations in live Drosophila using a dissecting
microscope
II. Safety Considerations

None
III. Introduction
Simple and Complex Genetic Inheritance
In humans, as well as all other organisms, some phenotypic traits are under the
control of a single gene while others are under the control of two or more genes.
Traits that are under the control of single genes are called simple traits because
they follow simple (Mendelian) rules of inheritance and they can usually be easily
and predictably followed through generations. Traits that are under the control of
many genes, on the other hand, are called complex traits because their inheritance
is much more complex and difficult to predict. In addition, the expression of genes
can be influenced by the environment. Therefore, some traits that are under the
control of a single gene do not behave according to Mendel's predictions. For
example, polydactyly (extra fingers and/or toes) is controlled by a single gene in
mammals, where the polydactyly allele is dominant to the normal allele. However, an
individual who inherits the polydactyly allele may or may not express it. The
environmental influences on the expression of this allele are not well understood, but
presumably occur at some point during embryogenesis.
BIO 2 Lab Manual, Spring 2010
Lab 10, Page 1
When a trait is controlled by more than one gene and the expression of those genes
is also influenced by the environment, the trait is said to be multifactorial. One
example of this kind of trait is human skin color. Skin pigmentation is under the
control of at least six genes. However, exposure to sunlight can influence the
expression of those genes, giving a normally light-skinned person a darker skin
color. Another example is Alzheimer's Disease (AD), which is under the control of at
least 5 genes. AD clearly runs through families, usually as a dominant illness.
However, persons without a clear genetic disposition to the illness also get the
disease, indicating that interplay between genes and the environment is important.
As you will see in the discussion below, this idea is strengthened by data gathered
from sets of identical and fraternal twins raised apart or together.
The Biology of Twinning
Twins can be either monozygotic (identical) or dizygotic (fraternal). Monozygotic
(MZ) twins arise from a single fertilized egg, which then begins to divide and splits
into two embryos sometime during the first weeks of gestation. MZ twins account for
about 28% of all twin births and are always the result of a random event that is not
under genetic control. Therefore, MZ twins do not "run in families." MZ twins share
the same genotype but do not have exactly the same phenotype. Therefore, they are
living examples of how the environment influences gene expression.
Dizygotic (DZ) twins originate from two independent fertilization events. The
mother ripens two eggs in a cycle instead of just one, and each egg is then fertilized
independently by different sperm. As a result, DZ twins are no more related to one
another than any two siblings (on average, they share about ½ of their genes) and
can therefore differ in sex (unlike MZ twins, who are always the same sex).
Moreover, differences between the phenotypes of DZ twins are due to both genetic
factors and environmental influences on gene expression. DZ twinning accounts for
about 72% of all twin births and can "run through families" because there are genes
that control the tendency of a woman to ovulate from more than one ovary (or
release more than one egg from a single ovary) during a menstrual cycle.
Twin Studies and Concordance
Studying twins has been a particularly useful way to quantify the relative influence of
gene action and the environment on human traits. Concordance is the percentage
of pairs (of twins) in which both twins express the same form of a trait. Comparing
the concordance values of a trait across many pairs of MZ and DZ twins can be
particularly useful, since differences in concordance values indicate the influence of
genes on variations in the trait. If the variation in a trait that is observed in a
population is strictly under the control of genes, then the concordance value should
be 100% for MZ twins and much less than that for DZ twins. (Although DZ twins
share 50% of their genes, the different inheritance patterns of genes prevents DZ
twins from having concordance values of 50% for all traits whose variation is strictly
under the control of genes.)
BIO 2 Lab Manual, Spring 2010
Lab 10, Page 2
% Concordance Values for Some Human Traits
(MZ and DZ twin pairs reared together)
Trait
MZ twins
DZ twins
Acne
14
14
Alzheimer's Disease
78
39
Anorexia nervosa
55
7
Autism
90
4-5
Bipolar disorder
33-80
8
Cleft lip and/or palate
40
3-6
Hypertension
62
48
Schizophrenia
40-50
10
Blood type
100
66
Eye color
99
28
Mental retardation
97
37
Hair color
89
22
Handedness (left or right)
79
77
Epilepsy
72
15
Diabetes
56
22
Interpreting the Date in the Chart:
It is important to note that it is the difference between the concordance rate in MZ
and DZ twins that is of paramount importance in determining how much influence
genes have over the variation observed in a trait. As an example, German Measles is
highly contagious (environmental) but MZ and DZ twins both show a concordance
rate of close to 100%. This isn't because getting German measles is due to the
influence of genes but because both twins are likely to be exposed!
Pre-lab Question 1: For the following traits, use the % concordance table to predict
whether each is influenced more by genes, the environment or both (i.e., genes and
environmental factors influence the trait in roughly equal amounts)?
1. Acne - ___________________________________
2. Autism - __________________________________
3. Blood type - _______________________________
BIO 2 Lab Manual, Spring 2010
Lab 10, Page 3
Introduction to Drosophila Melanogaster
Drosophila Melanogaster, otherwise known as the common fruit fly, is an important
model organism for studying genetics and developmental biology. It’s short life cycle
(approximately 10 days from egg to adult) and the fact that each fly produces many
offspring (as many as 500 eggs) makes it particularly convenient. Furthermore, it is
relatively easy to introduce and observe mutations, as well as culture flies in the lab.
The fruit fly goes through 4 stages in its short lifespan of approximately 3-4 weeks
(for flies reared in the lab). Eggs are laid and hatch into larvae one day later. The
larva molt twice before the larval cuticle hardens into a dark brown pupa. During
metamorphosis, which occurs during the pupal stage, larvae tissues degenerate and
reorganize to form an adult fly. The adult fly reaches full maturation several hours
after emerging from the puparium (the casing where metamorphosis occurs) and is
capable of laying eggs only 2 days later.
The fruit fly has been an important genetic model since the early 1900s, with
Thomas Hunt Morgan’s experiments that used physical, chemical and radiational
methods to mutate drosophila genes. He and his students would then cross-breed
the flies and look for heritable mutations that resulted in observable phenotypes.
Many spontaneous mutations have also be found and characterized in drosophila.
The normal fly is called a "wild type" and any fly exhibiting a phenotypic mutation is
called a "mutant". Mutant flies are given names that generally denote the type of
mutation the fly exhibits. For example, the mutant "ebony" has a much darker body
than the wild type fly.
In 2000, the drosophila genome was sequenced, making it the second multicellular
genome to be completely mapped (the first being C. elegans). It has since served as
an important organism for comparative genomics.
IV. Things to Do
PART A. DETERMINING YOUR PHENOTYPE AND POSSIBLE GENOTYPE FOR
SIMPLE TRAITS
Pre-lab Question 2:
A) Under what condition(s) is it possible to determine a person's genotype
unambiguously?
B) In what case(s) is it impossible to know?
C) What is being symbolized by the upper-case and lower-case letters?
1. Each lab group has been provided with pictures that illustrate various simple traits in
humans. Using these tools, score yourself and your lab partners for phenotype and
(possibly) genotype.
BIO 2 Lab Manual, Spring 2010
Lab 10, Page 4
2. Record your results in the table above and include it in your notebook. We will
discuss the pooled class results.
3. Do you know your blood type? If you do, can you determine your genotype knowing
your phenotype? What pattern of inheritance does ABO blood type display? (Record
these answers in your lab notebook.)
Trait and Alleles
Hair Line
W= peak
w = no peak
Ear Lobes
L = free
l = attached
Darwin's Ear
D = has point
d = no point
Freckles
F - freckles
f = no freckles
Hair Form
C = curly
C' = strait
Mid-digital Hair
H = hair on second
(middle) joint of one or
more fingers
h = hair absent on all
fingers
Color vision
V = normal
v = red green color blind
Taster (PTC)
T = taster
t = non-taster
Tongue Rolling
R = can roll
r = cannot roll
Excretion of methyl
mecarptan (from
asparagus)
E = excretor
e = non-excretor
Hand clasping
H = left thumb on top
h = right thumb on top
Eye color
B = non-blue
b = blue
BIO 2 Lab Manual, Spring 2010
Possible
Genotypes/
Phenotype
WW
(peak)
Possible
Genotypes/
Phenotype
ww
(no peak)
Possible
Genotypes/
Phenotype
Ww
(peak)
LL
(free)
ll
(attached)
Ll
(free)
DD
(point)
dd
(no point)
Dd
(point)
FF
(freckles)
ff
(no freckles)
Ff
(freckles)
CC
(curly)
C'C'
(straight)
C'C
(wavy)
HH
(hair on one
or more
fingers)
hh
(hair absent)
Hh
(hair on one
or more
fingers)
male XVY
female XVXV
(normal)
TT
(taster)
male XvY
female XvXv
(color blind)
tt
(non-taster)
female XVXv
(normal)
RR
(can roll)
rr
(cannot roll)
Rr
(can roll)
EE
(excretor)
ee
(nonexcretor)
Ee
(excretor)
HH
(Left on top)
hh
(right on
top)
bb
(blue)
Hh
(Left on top)
BB
(non-blue)
Your
Genotype/
Phenotype
Tt
(taster)
Bb
(non-blue)
Lab 10, Page 5
PART B. EXHIBITS: TWINS
Several exhibits have been placed at the side of the room that concern twinning and
twins. Look at the photos and record your observations for each exhibit in your
notebook (answering the questions for each).
Exhibit A is a series of pictures of a set of identical twins, Molly and Betsy, at
various ages. Compare their phenotypes as infants, children, and adults.
What do you observe? How do you interpret your observations?
Exhibit B is a photograph of fraternal twins whose parents are both half AfricanAmerican and half Caucasian. How do you explain the differences in
phenotypes between these two twins? Do you think the differences are mostly
genetic or environmental? Why?
Exhibit C is a photograph of seven sets of identical twins. When the photograph was
taken, the twins were asked to sit beside their twin. No other instructions
were given. Carefully examine and compare the twins, looking at hair color,
skin color, facial expression, posture, hand position, and other characteristics.
What does this photograph seem to indicate about the influence of genes on
phenotype in humans?
Exhibit D shows a pair of identical twins, Sherry and Terry. Sherry is taller than
Terry and has always been more athletic and a better student than her sister.
In interviewing Sherry and Terry, Terry says she has always felt like the
"shadow twin" of her sister. What do you think might account for the
differences in these twins? (They were reared together and still live together
as college students.)
PART C. DROSOPHILA LIFECYCLE
1. Observe the contents of the jar containing live drosophila on your workbench.
2. Identify individual flies in the following stages of their lifecycle: 1) egg, 2) larva,
3) pupa, and 4) adult. Use the manual on your bench for assistance. Draw and
describe each stage.
BIO 2 Lab Manual, Spring 2010
Lab 10, Page 6
PART D. DROSOPHILA MUTANTS
In this section you will examine wildtype and mutant flies using a dissecting scope.
The mutant flies have a variation of a specific gene that results in an observable
phenotypic change. You will look for the following mutations:
- singed bristles (on top of pronotum)
- sepia (brownish) eyes
- vestigial wings (short wings)
- dumpy wings (shortened and angled)
- white eyes
- mini (smaller than normal size)
- apterous (no winges)
1. Examine the slide containing wildtype drosophila. Determine the sex using your
manual for assistance and record this in your lab notebook.
2. Obtain each of the slides with mutant drosophila (labeled A-G). Examine each
under the microscope and identify the mutation from the list above. You can use
your manual for assistance.
3. Fill out the following chart and include it in your lab notebook.
Labeled slide
Type of Mutation Observed
A
B
C
D
E
F
G
Wildtype
Indicate Male or Female
V. Lab Clean-Up

Make sure all of the visual tools you used for phenotyping and genotyping are
neatly laid out for the students in the next lab section.

Make sure microscopes are put up according to your instructor’s directions.
BIO 2 Lab Manual, Spring 2010
Lab 10, Page 7