Welcome to IB 201, Part II
Genetics
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IB 201 Genetics and
Evolution
Part 2 Genetics
Dr. Kim Hughes
465B Morrill Hall
kahughes@uiuc.edu
Office hours: Tu/Th after class or by
appointment (email for appointment)
Required Texts
Readings from “Essential of Genetics”,
by Klug and Cummings
Today
1. Genetic Data Analysis
2. How to do well in this course
3. Review Questions
Genetic Data Analysis
Does a genetic “model” fit the data?
Genetic experiments are designed to
answer this question.
Data analysis and experimental design
are intimately related.
Design of Experiments
A friend claims that he will only drink “Sierra
Springs” bottled water, because it tastes the
best.
How would you decide if he really can tell the
difference between brands of bottled water?
Design of Experiments
How many “tests” would you conduct?
What if you conducted 10 tests, and he got it
right 7 times?
Could you reject the hypothesis that he was
guessing randomly?
Genetic Example
You have discovered a new mutation in a fruit fly and you think
it’s a recessive mutation. How do you test this hypothesis?
Female (true-breeding wild-type strain) X Male (true-breeding
mutant strain)
P:
AA x aa (presumed)
F1:
Aa
(presumed)
Testcross: Aa (presumed) x aa (presumed)
Expected offspring ratio: 1 wild type : 1 mutant
If your genetic model is correct, then you predict a 1:1 ratio in the
offspring of the testcross.
Genetic Example
If your genetic model is correct, then you predict a
1 : 1 phenotypic ratio.
What if the phenotypic ratio is 1.2 to 1?
How do you know if this deviation from an
expected result is due to chance or due to the
fact that your genetic model is wrong?
Simple Experiment
What if you were to repeat a simple experiment
many times?
Toss a fair coin 100 times and count how
many heads are thrown.
Repeat this experiment 100 times.
What is the most likely outcome of a
single experiment (if coin is fair)?
Will all 100 experiments have the same
outcome?
Times out of 100
100 Tosses of a Fair Coin
9
8
7
6
5
4
3
2
1
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
Number of Heads
The most likely outcome is the expected
value, which is the same as the average
or mean.
95
100
Times out of 100
100 Tosses of a Fair Coin
9
8
7
6
5
4
3
2
1
0
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
Number of Heads
What is the probability that you would get 55 or more
heads in a single experiment?
P = 18.4/100 = 0.184. So, 55 is different from
expected, but close enough that the differences is
likely due to chance.
What is the probability that you would get 65 or more
heads?
P = 0.18/100 = 0.0018. So, 65 is different from expected,
and the difference is unlikely to be due to chance.
95
100
Genetic example
Q: You cross a dark moth and a light moth, and the
F1 offspring are all dark. You then inter-cross the
F1 with each other to produce the F2. In the F2,
you get 70 of the dark moths and and 30 light
moths. Can you reject the hypothesis that the dark
parent was AA and the light parent was aa?
What is your hypothesized genetic model? AA x aa
Genetic example
Q: You cross a dark moth and a light moth, and the F1
offspring are all dark. You then inter-cross the F1 with each
other to produce the F2. In the F2, you get 70 of the dark
moths and and 30 light moths. Can you reject the
hypothesis that the dark parent was AA and the light parent
was aa?
What is your hypothesized genetic model? AA x aa
If this model is correct, what are the expected number of dark and
light moths in the F2?
3 : 1 ratio, or 75 dark : 25 light
Genetic example
Q: You cross a dark moth and a light moth, and the F1 offspring are all
dark. You then inter-cross the F1 with each other to produce the F2.
In the F2, you get 70 of the dark moths and and 30 light moths. Can
you reject the hypothesis that the dark parent was AA and the light
parent was aa?
AA x aa
What is your hypothesized genetic model?
If this model is correct, what are the expected number of dark
and light moths in the F2? 3 : 1 ratio, or 75 dark : 25 light
70 dark : 30 light
What is the observed result?
How could we measure the difference between the expected
and observed results?
Genetic Model
P (phenotypes) dark x light
P (genotypes) AA x aa
F1:
Aa
F2:
1/4 AA 1/2 Aa 1/4 aa
F2 phenotypes: 3/4 dark : 1/4 light
Expected numbers (phenotypes)? 75 dark: 25 light
(100 offspring)
Observed result?
70 dark: 30 light
(Null) Hypothesis: observed results do not deviate
from expected results more than expected by chance.
2

Test
A way to tell if the difference between observed results
and expected results is too big to be due to chance.
Observed number (O) in each category is compared to
the expected number (E) in each category under the
null hypothesis (the hypothesized genetic model):
(Obs - Exp).
Use the square of the difference between observed and
expected numbers : (O - E)2
Correct for the size of the experiment by dividing by the
expected number in each category: (O - E)2 /E
2 Test
class
dark
light
O
70
30
E
75
25
( O-E)
5
-5
( O-E) 2 = d2
25
25
( O-E) 2 / E
25 / 7 5 = 0.2 2
25 / 2 5 = 1
 = 1.33
d.f. = (# classes -1 ) = 1
= [(O-E)2]/E = ∑ (d2/E)
# classes
(O-E) is the “difference” (d).
 symbol means to sum the values over each
category or “class” of the data.
2 Table
= 1.33, d.f. = 1
Probabilities
df
0.90
0.50
0.20
0.05
0.01
0.001
1
0.02
0.46
1.64
3.84
6.64
10.83
2
0.21
1.39
3.22
5.99
9.21
13.82
3
0.58
2.37
4.64
7.82
11.4
16.27
4
1.06
3.36
5.99
9.49
13.3
18.47
A Real Experiment
You have crossed two plants with purple flowers and have
166 offspring. Your hypothesis is that both plants are
heterozygous for a dominant allele at a single locus
controlling flower color.
Genetic Model
P: (Ww X Ww)
F1: 3/4 W- (purple) and 1/4 ww (white)
Expected: If model is true, you expect 3/4 purple and 1/4
white. Out of 166 offspring: 124.5 purple: 41.5 white.
Observed Data: 166 progeny: 110 purple & 56 white.
Q: Is the deviation from expected too big to be due to
chance?
 Test
Class
Purple
White
O
E
(O-E)
110 124.5 -14.5
56 41.5 14.5
(O-E)2 = d2
210.25
210.25
d2 /E
1.69
5.07
 = 6.76
df = 1
 Table
df
1
2
3
4
0.90
0.02
0.21
0.58
1.06
Probabilities
0.50
0.20
0.05
0.46
1.64
3.84
1.39
3.22
5.99
2.37
4.64
7.82
3.36
5.99
9.49
0.01
6.64
9.21
11.35
13.28
0.001
10.83
13.82
16.27
18.47
Problem: An ear of corn has a total of 381
grains, including 216 purple, smooth, 79
purple, shrunken, 65 yellow, smooth, and
21 yellow, shrunken.
Your Hypothesis: This ear of corn was
produced by a dihybrid cross (PpSs x
PpSs) involving two pairs of heterozygous
genes resulting in an expected ratio of
9:3:3:1. Purple dominant to yellow;
smooth dominant to shrunken.
Class
Purple
Smooth
Purple
Shrunk
Yellow
Smooth
Yellow
Shrunk
Obs
216
Exp
214
O-E
2
(O-E)2/E
4/214 = 0.019
79
71
8
64/71 = 0.901
65
71
6
36/71 = 0.507
21
24
3
9/24 = 0.375
d.f. = 3
2 = 1.80
2 = 1.80
d.f. = 3
Probabilities
df
1
2
3
4
0.90
0.02
0.21
0.58
1.06
0.50
0.46
1.39
2.37
3.36
0.20
1.64
3.22
4.64
5.99
0.05
3.84
5.99
7.82
9.49
0.01
6.64
9.21
11.35
13.28
0.001
10.83
13.82
16.27
18.47
How to do well in this course
Course Web Site
http://www.life.uiuc.edu/ib/201
Class schedule
Assigned readings
Homework assignments
Lecture notes
Powerpoint slides
Recommended problems
Other
Course Pre-requisites
IB 150: Organismal and Evolutionary
Biology
MCB 150: Molecular and Cellular Basis
of Life
Review material available on the course
web site: Basic Mendelian and
Molecular Genetics
How is this course different?
Memorization is less important.
Learning and applying concepts is more important.
You need to be able to think about and analyze
genetic data.
Most homework and exam questions will require you to
examine and analyze data, and to make
conclusions based on your analysis.
In this way, the work you do in this course is very
similar to work done by professional geneticists.
How to succeed in this course





Come to class!
Read the assigned material before class
Download lecture notes before class.
Bring a calculator and scrap paper to class.
Do recommended problems and the problems in
lecture notes/slides: you will see them again!
Posted to Lecture Schedule web page. Use your
resource groups, TAs, and me for help!
 Participate in class activities
Don’t
• Furiously take notes during class
• Put off the homework until the last
minute
• Put off doing recommended problems
until just before the exam
Review Questions
1. A round-seeded pea was crossed to a wrinkle-seeded
pea, and all the offspring were round. Diagram the cross
and predict the ratio of wrinkled to round offspring in the
F2 generation.
2. A round-seeded pea is crossed to a wrinkle-seeded pea.
Half the offspring were round and half were wrinkled.
What were the genotypes of the parents, the round F1
offspring, and the wrinkled F1 offspring?
3. True-breeding yellow and round seeded plants were
crossed to true breeding green and wrinkled ones. All F1
offspring had yellow and round seeds. What were the
genotypes of the parents and the F1 offspring? What
phenotypes, and in what proportions, do you expect in
the F2 generation?
4. A red snapdragon is crossed to a white snapdragon and
all the offspring are pink. This is an example of what
genetic property?
Review Questions
5. A woman homozygous for the A allele of the ABO blood
group marries a man who is homozygous for the B locus.
What is the blood group phenotype of their child? This is
an example of what genetic property?
6. What is the most likely mode of inheritance illustrated by
this pedigree?
7. If a couple want to have two children, what is the
probability that the first will be a girl, and the second will
be a boy?
8. If a couple want to have two children, what is the
probability that they will have one girl and one boy?
Answers
1.
2.
3.
4.
5.
6.
7.
8.
P: WW x ww
F1: Ww
F2: 3/4 round (W-) to 1/4 wrinkled (ww)
P: Ww x ww
F1: 1/2 Ww (round) and 1/2 ww (wrinkled)
P: GG WW x gg ww
F1: Gg Ww
F2: 9/16 yellow, round (G- W-); 3/16 yellow, wrinkled (G- ww); 3/16
green, round (gg W-); 1/16 green, wrinkled (gg ww)
Incomplete dominance
AB; Codominance
X-linked recessive
0.25
0.5
Terms you should know
Gene
Locus
Allele
Dominance
Recessiveness
Wild type
Mutation
Genotype
Phenotype
Homozygote
Heterozygote
Monohybrid
Dihybrid
Pleiotropy
Epistasis
Autosome
Sex chromosome
Hemizygous
Pleiotropy
Epistasis
Haploid
Diploid
Nuclear DNA
Mitochondrial DNA
Chloroplast DNA
Chromosome
Homologous
chromosomes
Sister chromatids
Tetrad
Mitosis
Meoisis
Independent
assortment
Recombination
Crossing over
Transcription
Translation
If you found these questions easy
You’re prepared to start this portion of the class
If these questions were not easy
You should attend the review
session and study review material
Review session: Today, 5 pm,
140 Burrill Hall