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1Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education .
Chapter 03
Lecture Outline
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
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2
• 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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3
Early Theories of Inheritance
• 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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4
• 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
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5
Mendel’s Choice of the Pea Plant
Figure 3.2 a
(a) Structure of a pea flower
Petals
Keel
Stigma
Anther
Style
Ovary
Sepal
Ovule
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6
Pollen lands on the stigma
Anthers contain pollen grains, where the male gametes are produced
• Variable characters of pea plants:– Height– Flower color– Flower position– Seed color and shape– Pod color and shape
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7
Mendel Studied 7 CharactersCHARACTER VARIANTS
Flower color
Purple White
Flower position
Axial Terminal
Height
Tall Dwarf
CHARACTER VARIANTS
Seed colorYellow Green
Seed shapeRound Wrinkled
Pod shape
Smooth Constricted
Pod color
YellowGreen
Remove anthersfrom purple flower.
Anthers
Transfer pollenfrom anthers ofwhite flower tothe stigma of apurple flower.
Cross-pollinated flowerproduces seeds.
Plant the seeds.
White
PurpleParentalgeneration
First-generationoffspring
• Mendel carried out two types of crosses
– Self-fertilization• Pollen and egg are derived from
the same plant
– 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
8
• 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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9
True-Breeding Lines
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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10
• 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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11
Mendel’s Approach
• 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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12
Mendel’s Crosses
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
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13
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Experimental level
P plants
Tall Dwarf
F1 seeds
Self-fertilization
Alltall
Self-fertilization
F1 plants
F2 seeds
F2 plants
Conceptual level
TT + 2 Tt + tt
All Tt
Tt
x
x
TT tt
Note: The Pcross producesseeds that arepart of the F1
generation.
Tall Tall TallDwarf
1. For each of seven characters, Mendelcross-fertilized two differenttrue-breeding strains. Keep in mindthat each cross involved two plantsthat differed in regard to only one ofthe seven characters studied. Theillustration at the right shows onecross 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.
3. Allow the F1 generation plants toself-fertilize. This produces seeds
thatare part of the F2 generation.
4. Collect the F2 generation seeds and plant them the following spring to obtain the F2 generation plants.
5. Analyze the traits foundin each generation.
14
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 purple224 white
3.15:1
Axial Xterminal flowers
All axial 651 axial207 terminal
3.14:1
Yellow X Green seeds
All yellow 6,022 yellow2,001 green
3.01:1
Round Xwrinkled seeds
All round 5,474 round1,850 wrinkled
2.96:1
Green X yellow pods
All green 428 green152 yellow
2.82:1
Smooth X constricted pods
All smooth 882 smooth229 constricted
2.95:1
TOTAL All dominant 14,949 dominant5010 recessive
2.98:1
DATA FROM MONOHYBRID CROSSES
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15
• 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
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16
Interpreting the Data
• 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
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17
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
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18
Mendel postulated:
The two copies of a gene segregate (or separate)
from each other during transmission
from parent to offspring
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19
Mendel’s Law of Segregation
– 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
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20
Terminology
Figure 3.6
P generation
Segregation
Segregation
Self- fertilization
Cross-fertilization
F2 generationGenotypes:(1 : 2 : 1)
Phenotypes:(3 : 1)
Gametes
F1 generation(all tall)
Gametes
tT t
t TT t
x
Tall
Tall
TT
Tt
Dwarf
tt
Tt TtTT tt
Tall Tall Tall Dwarf
T
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21
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
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22
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
Using a Punnett Square
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23
Punnett square of a cross between two heterozygotes for one character
TT Tt
Tt tt
Male gametes
Fem
ale
gam
etes
T
T
t
t
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24
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
TT Tt
Tt tt
Male gametes
Fe
ma
le g
am
ete
s
T
T
t
t
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
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26
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
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27
Figure 3.7
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P generation
Haploid gametes
RRYY
RY x
rryy
RrYyF1 generation
Haploid gametes RY ry
ry
(a) HYPOTHESIS: Linked assortment
1/21/2 Haploid gametes
RRYY
RY x
rryy
RrYy
ry
Ry ryrYRY
(b) HYPOTHESIS: Independent assortment
1/41/4
1/41/4
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P Cross F1 generation F2 generation
Round,yellow seeds X wrinkled, green seeds
All round, yellow 315 round, yellow seeds101 wrinkled, yellow seeds108 round, green seeds 32 green, wrinkled seeds
DATA FROM DIHYBRID CROSSES
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30
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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31
P Cross F1 generation F2 generation Ratio
Round,yellow seeds X wrinkled, green seeds
All round, yellow 315 round, yellow seeds101 wrinkled, yellow seeds108 round, green seeds 32 green, wrinkled seeds
9.83.23.41.0
Predicted phenotypic ratio in the F2 generation would be 9:3:3:1 if genes act independently of each other
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32
During gamete formation, the segregation of any pair of hereditary determinants is independent of
the segregation of other pairs
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33
Mendel’s Law of Independent Assortment
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
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34
Figure 3.9
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Four possible malegametes:
Four possible femalegametes:
RY Ry ry
ry
rY
rYRyRY
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 : 1 rrYY : 2 rrYy : 1 rryy
Phenotypes:
rrYy
9 round,yellow seeds
3 round,green seeds
3 wrinkled,yellow seeds
1 wrinkled,green seed
35
Figure 3.10
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Cross: TtYy x TtYy
TY
TY
Ty
tY
ty
Genotypes: 1 TTYY : 2 TTYy : 4 TtYy : 2 TtYY :
TTYY TTYy TtYY TtYy
Tall, yellow Tall, yellow Tall, yellow Tall, yellow
TtyyTtYyTTyyTTYy
TtYY TtYy ttYY ttYy
ttyyttYyTtyyTtYy
Tall, yellow Tall, yellow
Tall, yellow
Tall, yellow Tall, yellow Tall, greenTall, green
Tall, green
Dwarf, yellowDwarf, yellow
Dwarf, yellow Dwarf, green
Phenotypes:
1 TTyy : 2 Ttyy
9 tallplants with
yellow seeds
3 tallplants with
green seeds
3 dwarfplants with
yellow seeds
1 dwarfplant with
green seeds
1 ttYY : 2 ttYy 1 ttyy
Ty tY ty
36
• 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
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37
Three-factor crosses
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38Figure 3.11a, b
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39Figure 3.11c
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
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40
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
42
Inheritance patterns of traits can be explained by transmission patterns of chromosomes
during meiosis and fertilization
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43
Chromosome Theoryof Inheritance
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44
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
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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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 Patternsin Humans
The features of a pedigree
Analysis of a pedigree to determine if a trait or disease is dominant or recessive
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49
Pedigree Analysis
• 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
Female
Male
Sex unknown or notspecified
Miscarriage
Deceased individual
Unaffected individual
Affected individual
Presumed heterozygote(the dot notation indicatessex-linked traits)
Consanguineous mating(between related individuals)
Fraternal (dizygotic) twins
Identical (monozygotic) twins
I -1 I-2
III -4 II -5II -1 II -2 II -3
III -3III -1 III -2 III -6 III -7III -4 III -5
(a) Human pedigree showing cystic fibrosis
(b) Symbols used in a human pedigree
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
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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
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52
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
53
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
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54
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
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55
Probability
• The probability of an outcome is the chance, or likelihood, that the outcome will occur
• Probability =
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Total number of possible outcomes
• For example, in a coin flip
Number of times an outcome will occur
(1 heads + 1 tails) = 1/2 = 50%Pheads = 1 heads
56
• In our pea genetics example:
• Probability =
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Total number of individuals
Expected number of individuals with a given phenotype
(3 tall + 1 dwarf) = 3/4 = 75%Ptall = 3 tall
57
(3 tall + 1 dwarf) = 1/4 = 25%Pdwarf = 1 dwarf
• 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
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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
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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?
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• Applying the product rule
– Step 1: Calculate the individual probabilities• This can be obtained via a Punnett square
1/4 (25%)P(congenital analgesia) =
– 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
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