Chapter 03
Lecture Outline
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1
3.1 Mendel’s Study of Pea Plants



Why pea plants are suitable for genetic studies
The steps that Mendel followed to make crosses between
different strains of pea plants
The seven characteristics of pea plants that Mendel
chose to study
2
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Early Theories of Inheritance
• Before Mendel, people knew that parents passed traits
onto offspring – but they didn’t understand how it worked
• Some early theories of inheritance:
– Pangenesis
• Hippocrates
• “Seeds” produced by all parts of body, collected
and transmitted to offspring at conception
– Blending hypothesis
• Factors that control hereditary traits are malleable
• They blend together generation after generation
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• These theories were refuted by the work of Gregor Mendel
in the mid-1800’s
• Mendel’s work was novel
– He used quantitative analysis
– He developed general laws – rules to predict
• which phenotypes would appear in offspring
• ratios of phenotypes in the offspring
• His work was first ignored, then rediscovered in
the early 1900’s
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Mendel’s Choice of the Pea Plant
• The pea (Pisum sativum) has several advantages:
– Small, easily grown
– Each flower has male and female structures
• A plant can fertilize itself (selfing)
• OR, A plant can be crossed to another different plant
(cross fertilization)
– Many different varieties were available with different traits
5
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Petals
Pollen lands on the stigma
Keel
Sepal
Stigma
Anther
Ovule
Figure 3.2 a
(a) Structure of a pea flower
Style
Ovary
Anthers contain pollen
grains, where the male
gametes are produced
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Mendel Studied 7 Characters
• Variable characters of pea plants:
– Height
– Flower color
– Flower position
– Seed color and shape
– Pod color and shape
CHARACTER
CHARACTER
VARIANTS
Height
Tall
Dwarf
Purple
White
Axial
Terminal
Flower color
VARIANTS
Seed color
Yellow
Green
Flower position
Seed shape
Round
Wrinkled
Green
Yellow
Pod color
Pod shape
Smooth
Constricted
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• Mendel carried out two types of crosses
– Self-fertilization
• Pollen and egg are derived from
the same plant
White
Remove anthers
from purple flower.
Anthers
– Cross-fertilization
• Pollen and egg are derived from
different plants
• When plants with different traits
are crossed, this is hybridization
– progeny are called hybrids
• To cross-fertilize, Mendel
transferred pollen into the flower
of another plant
Parental
generation
Purple
Transfer pollen
from anthers of
white flower to
the stigma of a
purple flower.
Cross-pollinated flower
produces seeds.
Plant the seeds.
Firstgeneration
offspring
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True-Breeding Lines
• Mendel started with plants that “bred true” for different
character
• True-breeding lines – Plants that always produce
progeny with the same traits when self-fertilized
(or bred to the same strain)
• A note on terminology:
– Character – The type of characteristic that can vary,
such as “height”
– Trait, or variant – The version of the character, such
as “tall” or “dwarf”
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3.2 Law of Segregation



Mendel’s experiments with single-factor crosses
The law of segregation and how it is related to gamete
formation and fertilization
Predicting outcomes of single-factor crosses using a
Punnett square
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Mendel’s Approach
• Mendel did not start with a hypothesis to explain the
formation of hybrids
– But he believed that a quantitative analysis of crosses
may reveal a mathematical relationship
– This is called an empirical approach
– General findings from such an approach are called
empirical laws
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Mendel’s Crosses
• Mendel mated true-breeding plants with one trait to plants
with a different trait to create hybrids
– Matings looking at one character – single-factor cross
– Matings looking at two characters – two-factor cross
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Mendel’s Single-Factor Cross Experiments
• Mendel studied seven characters, each with two variants
– e.g., Plant height variants were tall and dwarf
• His first experiments crossed only two variants of one
character at a time
– Called a single-factor cross or monohybrid cross
• He followed the characters for two subsequent crosses
– P generation – Parental generation
– F1 generation – 1st Filial generation
– F2 generation – 2nd Filial generation
13
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Experimental level
1. For each of seven characters, Mendel
cross-fertilized two different
true-breeding strains. Keep in mind
that each cross involved two plants
that differed in regard to only one of
the seven characters studied. The
illustration at the right shows one
cross between a tall and dwarf plant.
This is called a P (parental) cross.
2. Collect the F1 generation seeds. The
following spring, plant the seeds and
allow the plants to grow. These are
the plants of the F1 generation.
Conceptual level
P plants
x
Tall
Dwarf
Note: The P
cross produces
seeds that are
part of the F1
generation.
F1 seeds
All Tt
F1 plants
Tt
All
tall
Selffertilization
3. Allow the F1 generation plants to
self-fertilize. This produces seeds that
are part of the F2 generation.
Selffertilization
F2 seeds
4. Collect the F2 generation seeds and
plant them the following spring to
obtain the F2 generation plants.
5. Analyze the traits found
in each generation.
TT x tt
TT + 2 Tt + tt
F2 plants
Tall
Tall
Dwarf
Tall
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DATA FROM MONOHYBRID CROSSES
P Cross
F1 generation
F2 generation
Ratio
Tall X
dwarf stem
All tall
787 tall
277 dwarf
2.84:1
Purple X
white flowers
All purple
705 purple
224 white
3.15:1
Axial X
terminal flowers
All axial
651 axial
207 terminal
3.14:1
Yellow X
Green seeds
All yellow
6,022 yellow
2,001 green
3.01:1
Round X
wrinkled seeds
All round
5,474 round
1,850 wrinkled
2.96:1
Green X
yellow pods
All green
428 green
152 yellow
2.82:1
Smooth X
constricted pods
All smooth
882 smooth
229 constricted
2.95:1
TOTAL
All dominant
14,949 dominant
5010 recessive
2.98:1
15
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Interpreting the Data
• For all seven characters studied
– The F1 generation showed only one of the two parental
traits
– The F2 generation showed an ~ 3:1 ratio of the two
parental traits
• These results refuted a blending mechanism of heredity
– The recessive trait “disappeared” entirely in the F1
– But reappeared unchanged in the F2
• The data suggested a particulate theory of inheritance
16
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• Dominant and recessive traits:
– The trait that is exhibited in the F1 is called dominant
– The trait that is masked in the F1 is called recessive
• In the F1, only the dominant trait appeared
• In the F2, the dominant trait plants outnumbered
recessive trait plants with a 3:1 ratio
17
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Mendel postulated:
1. For a given character, a pea plant contains two discrete hereditary
factors, one from each parent
2. The two factors may be identical or different
3. When the two factors of a single character are different
– One is dominant and its effect can be seen
– The other is recessive and is not expressed
4. During gamete formation, the paired factors segregate randomly
so that half of the gametes receive one factor and half of the gametes
receive the other
– This is Mendel’s Law of Segregation
18
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Mendel’s Law of Segregation
The two copies of a gene segregate (or separate)
from each other during transmission
from parent to offspring
19
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Terminology
– Genes – the modern term for Mendelian factors
– Alleles – different versions of the same gene
– Homozygous – an individual with two identical alleles
– Heterozygous – an individual with two different alleles
– Genotype – an individual’s specific allelic composition
– Phenotype – the outward appearance of an individual
20
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Tall
Dwarf
x
P generation
TT
tt
Segregation
Gametes
T
t
T
t
Cross-fertilization
Tall
F1 generation
(all tall)
Tt
Segregation
Gametes
F2 generation
Genotypes:
(1 : 2 : 1)
Figure 3.6
Phenotypes:
(3 : 1)
T
t
T
t
Selffertilization
TT
Tt
Tt
tt
Tall
Tall
Tall
Dwarf
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Punnett Squares Are Used to
Predict the Outcome of Crosses
• A Punnett square is a grid that enables one to predict the
outcome of simple genetic crosses
– Proposed by the English geneticist, Reginald Punnett
• Must know the genotype of the parents
• We will illustrate the Punnett square approach using
the cross of heterozygous tall plants as an example
22
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Using a Punnett Square
1. Write down the genotypes of both parents
Male parent = Tt
Female parent = Tt
2. Write down the possible gametes each parent can make
Male gametes: T or t
Female gametes: T or t
3. Create an empty Punnett square
4. Fill in the possible genotypes of the offspring
23
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Punnett square of a cross between two heterozygotes for one character
Female gametes
Male gametes
T
t
T
TT
Tt
t
Tt
tt
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24
Female gametes
Male gametes
T
t
T
TT
Tt
t
Tt
tt
5. Determine proportions of genotypes and phenotypes
– Genotypic ratio
• TT : Tt : tt
• 1 : 2 : 1
– Phenotypic ratio
• Tall : dwarf
• 3 : 1
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25
3.3 Law of Independent Assortment

Mendel’s experiments involving two-factor crosses

The law of independent assortment

Predicting the outcome of two-factor crosses using a
Punnett square
26
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Mendel’s Two-Factor Cross Experiments
• Mendel also performed two-factor crosses
– Crossing individual plants that differ in two characters
• Example:
– Character 1 = Seed shape (round vs. wrinkled)
– Character 2 = Seed color (yellow vs. green)
• There are two possible patterns of inheritance for these
characters – either linked or independent assortment
• Refer to Figure 3.7
27
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P generation
RRYY
Haploid gametes
rryy
RY
ry
x
1/
2
RY
1/
2
ry
(a) HYPOTHESIS: Linked assortment
Figure 3.7
rryy
RY
ry
x
RrYy
F1 generation
Haploid gametes
RRYY
RrYy
Haploid gametes
1/
4
RY
1/
4
Ry
1/
4
rY
1/
4
ry
(b) HYPOTHESIS: Independent assortment
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Figure 3.8
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DATA FROM DIHYBRID CROSSES
P Cross
F1 generation
F2 generation
Round,yellow seeds
X wrinkled, green
seeds
All round, yellow
315 round, yellow seeds
101 wrinkled, yellow seeds
108 round, green seeds
32 green, wrinkled seeds
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Interpreting the Data
• The F2 generation contains seeds with novel combinations
not found in the parental generation
– Round and green
– Wrinkled and yellow
• These nonparentals are predicted if the genes are
segregating independently of each other
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Predicted phenotypic ratio in the F2 generation would be
9:3:3:1 if genes act independently of each other
P Cross
F1 generation
F2 generation
Round,
yellow seeds X
wrinkled, green
seeds
All round, yellow
315 round, yellow seeds
101 wrinkled, yellow seeds
108 round, green seeds
32 green, wrinkled seeds
Ratio
9.8
3.2
3.4
1.0
32
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Mendel’s Law of
Independent Assortment
During gamete formation, the segregation of any
pair of hereditary determinants is independent of
the segregation of other pairs
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Methods for
Independent Assortment Problems
• Like a one-factor cross, a two-factor cross can be displayed
as an array diagram
– Refer to Figure 3.9
• Punnett squares can also be used to predict the outcome
of crosses involving two independently assorting genes
– Refer to Figure 3.10
34
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RY
Ry
rY
ry
Four possible female
RY
gametes:
Ry
rY
ry
Four possible male
gametes:
RRYY RRYy RrYY RrYy RRYy RRyy RrYy
Rryy RrYY RrYy
rrYY rrYy
RrYy
Rryy
rrYy
rryy
By randomly combining male and female gametes, 16 combinations are possible.
Totals:
1 RRYY : 2 RRYy : 4 RrYy : 2 RrYY : 1 RRyy : 2 Rryy
Phenotypes:
Figure 3.9
9 round,
yellow seeds
3 round,
green seeds
: 1 rrYY :
2 rrYy : 1 rryy
3 wrinkled,
yellow seeds
1 wrinkled,
green seed
35
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Cross: TtYy x TtYy
TY
Ty
tY
ty
TTYY
TTYy
TtYY
TtYy
Tall, yellow
Tall, yellow
Tall, yellow
Tall, yellow
TTYy
TTyy
TtYy
Ttyy
Tall, yellow
Tall, green
Tall, yellow
Tall, green
TtYY
TtYy
ttYY
ttYy
TY
Ty
tY
Tall, yellow
TtYy
Tall, yellow Dwarf, yellow Dwarf, yellow
Ttyy
ttYy
ttyy
ty
Tall, yellow
Genotypes:
Phenotypes:
Figure 3.10
Tall, green Dwarf, yellow Dwarf, green
1 TTYY : 2 TTYy : 4 TtYy : 2 TtYY : 1 TTyy : 2 Ttyy
9 tall
plants with
yellow seeds
3 tall
plants with
green seeds
1 ttYY : 2 ttYy
1 ttyy
3 dwarf
1 dwarf
plants with
plant with
yellow seeds green seeds
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Three-factor crosses
• In crosses involving three or more independently assorting
genes, a single Punnett square becomes cumbersome
– Would need 64 squares for three genes!
– Can use three Punnett Squares plus
the multiplication method
– Refer to Figure 3.11a, b
• A second alternative is the forked-line method
– Refer to Figure 3.11c
37
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Figure 3.11a, b
38
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Figure 3.11c
39
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3.4 Chromosome Theory of
Inheritance


The key tenets of the chromosome theory of inheritance
The relationship between meiosis and Mendel’s laws of
inheritance
40
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Chromosome Theory of Inheritance
• A major breakthrough in our understanding of genetics
• Established the framework for understanding how
chromosomes carry and transmit genetic determinants
• Explains the patterns of inheritance seen by Mendel
41
• Chromosome Theory of Inheritance resulted from
three lines of evidence:
1. Mendel’s breeding experiments
2. Nägeli and Weismann
•
•
•
A substance in living cells is responsible for inherited traits
Parents contribute equally to determine traits of offspring
Hertwig, Strasburger, and Flemming suggested that
chromosomes are the carriers of the genetic material
3. Boveri and Sutton
•
•
Saw similarity between segregation of traits and behavior
of chromosomes during meiosis
Proposed the chromosome theory of inheritance
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Chromosome Theory
of Inheritance
Inheritance patterns of traits can be explained
by transmission patterns of chromosomes
during meiosis and fertilization
43
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Chromosome Theory of Inheritance
1. Chromosomes contain the genetic material
2. Chromosomes are replicated and passed
from parent to offspring
•
Also from cell to cell during development
•
Chromosomes retain individuality during transmission
3. Nuclei of most eukaryotic cells contain chromosomes
in homologous pairs (they are diploid)
•
Gametes, however, are haploid
4. In the formation of haploid cells, chromosomes
segregate independently
5. Each parent contributes one set of chromosomes
44
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Law of Segregation is Explained
by Separation of Homologs
• Mendel’s Law of Segregation can be explained by the
separation of homologous chromosomes during meiosis
• Consider a situation where one homolog carries a
dominant allele (Y, yellow seeds) and the other carries
the recessive allele (y, green seeds)
– The gametes of the heterozygote may contain the
dominant allele or the recessive allele, but not both
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46
Law of Independent Assortment is Explained by
Random Alignment of Homologs
• Mendel’s Law of Independent Assortment can be
explained by the random alignment of homologous
chromosomes during meiosis
• Consider a situation where a double heterozygote
carries the dominant and recessive alleles for two genes,
each gene on a different chromosome
– The chromosomes with dominant alleles may end up together
in a gamete, or not
– All four combinations are possible in the gametes
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3.5 Studying Inheritance Patterns
in Humans


The features of a pedigree
Analysis of a pedigree to determine if a trait or disease is
dominant or recessive
49
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I -1
Pedigree Analysis
II -1
III -1
• When studying human traits,
it is not ethical to control
parental crosses (as Mendel
did with peas)
– So we must infer gene
properties from analysis
of family trees or
pedigrees
I-2
II -2
III -2
II -3
III -3
III -4
III -4
III -5
II -5
III -6
(a) Human pedigree showing cystic fibrosis
Female
Male
Sex unknown or not
specified
Miscarriage
Deceased individual
Unaffected individual
Affected individual
Presumed heterozygote
(the dot notation indicates
sex-linked traits)
Consanguineous mating
(between related individuals)
Fraternal (dizygotic) twins
Identical (monozygotic) twins
(b) Symbols used in a human pedigree
III -7
Pedigree Analysis
• Pedigree analysis is commonly used to determine the
inheritance pattern of human genetic diseases
• Genes that play a role in disease may exist as
– A normal allele
– A mutant allele that causes disease symptoms
• Diseases can follow a simple Mendelian pattern of
inheritance that is either dominant or recessive
51
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• Recessive pattern of inheritance
– Two unaffected heterozygous individuals will
on average have 25% affected offspring
– Two affected individuals will have 100% affected offspring
– Can “skip generations”
• Dominant pattern of inheritance
– Does not skip generations
– Affected individual will have at least one affected parent
• However, disease may also result from a new mutation
52
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Example: Cystic fibrosis (CF)
– A recessive disorder of humans
– Affected gene is the cystic fibrosis
transmembrane conductance regulator
(CFTR)
– The mutant CFTR protein causes ion
imbalance
• Leads to abnormalities in many tissues
and organs– pancreas, skin, intestine,
sweat glands and lungs
• Buildup of sticky mucus in the lungs
makes breathing difficult
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3.6 Probability and Statistics



Definition of probability
Predicting the outcome of crosses using the product rule
and binomial expansion equation
Evaluating the validity of a hypothesis using a chi square
test
54
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Probability and Statistics
• The laws of inheritance can be used to predict the
outcomes of genetic crosses
• For example:
– Animal and plant breeders are concerned with the
types of offspring produced from their crosses
– Parents are interested in predicting the traits that their
children may have
• This is particularly important in the case of families
with genetic diseases
55
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Probability
• The probability of an outcome is the chance, or likelihood,
that the outcome will occur
• Probability =
Number of times an outcome will occur
Total number of possible outcomes
• For example, in a coin flip
Pheads = 1 heads (1 heads + 1 tails) = 1/2 = 50%
56
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• In our pea genetics example:
• Probability =
Expected number of individuals
with a given phenotype
Total number of individuals
Ptall = 3 tall (3 tall + 1 dwarf) = 3/4 = 75%
Pdwarf = 1 dwarf (3 tall + 1 dwarf) = 1/4 = 25%
57
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• The larger the size of the sample, or number of times the
experiment is performed, the more closely the observed
results will match the expected outcomes
• This is due to random sampling error
– Random sampling error is large for small samples,
and small for large samples
• For example
– If a coin is flipped only 10 times, it is not unusual to get
70% heads and 30% tails
– If the coin is flipped 1,000 times the percentage of heads
will be fairly close to the predicted 50% value
58
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Product rule
The probability that two or more independent events will
occur is equal to the product of their respective probabilities
• “Independent events” are those in which the occurrence of
one does not affect the probability of another
59
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• Consider the disease congenital analgesia
– Recessive trait in humans
– Affected individuals can distinguish between sensations
• However, extreme sensations are not perceived as
painful – they do not perceive pain
– Two alleles
• P = Normal allele
• p = Congenital analgesia
• Question:
– Two heterozygous individuals plan to start a family
– What is the probability that the couple’s first three children
will all have congenital analgesia?
60
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• Applying the product rule
– Step 1: Calculate the individual probabilities
• This can be obtained via a Punnett square
P(congenital analgesia) = 1/4 (25%)
– Step 2: Multiply the individual probabilities
1/4 X 1/4 X 1/4 = 1/64 = 0.016 = 1.6%
• This is the probability that the first three offspring
will all exhibit the disease
61
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