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Chapter 14
Mendel and the Gene Idea
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
What genetic principles account for the passing
of traits from parents to offspring?
• The “blending” hypothesis is the idea
that genetic material from the two parents
blends together (like blue and yellow
paint blend to make green)
• The “particulate” hypothesis is the idea
that parents pass on discrete heritable
units (genes)
• Mendel documented a particulate
mechanism through his experiments with
garden peas.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-1
Concept 14.1: Mendel used the scientific approach
to identify two laws of inheritance
•Mendel discovered the basic principles of
heredity by breeding garden peas in carefully
planned experiments.
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Mendel’s Experimental, Quantitative Approach
• Advantages of pea plants for genetic study:
– There are many varieties with distinct
heritable features
– Mating of plants can be controlled
– Cross-pollination (fertilization between
different plants) can be achieved by
dusting one plant with pollen from another
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-2
TECHNIQUE
1
2
Parental
generation
(P)
Stamens
Carpel
3
4
RESULTS
First
filial
generation
offspring
(F1)
5
•He also used varieties that were true-breeding (plants
that produce offspring of the same variety when they
self-pollinate)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• In a typical experiment, Mendel mated two
contrasting, true-breeding varieties, a
process called hybridization
• The true-breeding parents are the P
generation
• The hybrid offspring of the P generation are
called the F1 generation
• When F1 individuals self-pollinate, the F2
generation is produced.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The Law of Segregation
• When Mendel crossed contrasting, truebreeding white and purple flowered pea plants,
all of the F1 hybrids were purple
• When Mendel crossed the F1 hybrids, many of
the F2 plants had purple flowers, but some had
white
• Mendel discovered a ratio of about three to
one, purple to white flowers, in the F2
generation.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-3-1
EXPERIMENT
P Generation
(true-breeding
parents)

Purple
flowers
White
flowers
Fig. 14-3-2
EXPERIMENT
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)

Purple
flowers
White
flowers
All plants had
purple flowers
Fig. 14-3-3
EXPERIMENT
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)

Purple
flowers
White
flowers
All plants had
purple flowers
F2 Generation
705 purple-flowered 224 white-flowered
plants
plants
• Mendel reasoned that only the purple flower
factor was affecting flower color in the F1
hybrids
• Mendel called the purple flower color a
dominant trait and the white flower color a
recessive trait
• Mendel observed the same pattern of
inheritance in six other pea plant characters,
each represented by two traits
• What Mendel called a “heritable factor” is
what we now call a gene
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Table 14-1
Mendel’s Model
• Mendel developed a hypothesis to explain
the 3:1 inheritance pattern he observed in
F2 offspring:
1. Alternative versions of a gene exist - are
now called alleles. The position of the gene
is called the locus.
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Fig. 14-4
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers
2. For each character an organism inherits
two alleles, one from each parent.
• Mendel made this deduction without
knowing about the role of chromosomes!
• The two alleles at a locus on a chromosome
may be identical – homozygous.
• Alternatively, the two alleles at a locus may
differ – heterozygous.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
3.The third concept is that if the two alleles at
a locus differ, then one (the dominant allele)
determines the organism’s appearance, and
the other (the recessive allele) has no
noticeable effect on appearance
• In the flower-color example, the F1 plants
had purple flowers because the allele for
that trait is dominant
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
4. The law of segregation, states that the two
alleles for a heritable character separate
(segregate) during gamete formation and
end up in different gametes.
• Thus, an egg or a sperm gets only one of
the two alleles that are present in the
somatic cells of an organism
• This segregation of alleles corresponds to
the distribution of homologous
chromosomes to different gametes in
meiosis.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-5-1
Punnett
Squares
P Generation
Purple flowers White flowers
Appearance:
Genetic makeup:
PP
pp
Gametes:
P
p
Fig. 14-5-2
P Generation
Purple flowers White flowers
Appearance:
Genetic makeup:
PP
pp
Gametes:
p
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
2
P
1/
2
p
Fig. 14-5-3
P Generation
Purple flowers White flowers
Appearance:
Genetic makeup:
PP
pp
Gametes:
p
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
2
1/
2
P
Sperm
F2 Generation
P
p
PP
Pp
Pp
pp
P
Eggs
p
3
1
p
• phenotype, or physical appearance
• An organism’s phenotype also includes
internal anatomy, physiology, molecular
pathways, and behavior
• genotype, or genetic makeup
• In the example of flower color in pea plants,
PP and Pp plants have the same phenotype
(purple) but different genotypes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-6
3
Phenotype
Genotype
Purple
PP
(homozygous)
Purple
Pp
(heterozygous)
1
2
1
Purple
Pp
(heterozygous)
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
1
Important ratio:
for monohybrid cross, Aa x Aa
3:1 dominant to recessives
The Law of Independent Assortment
• A monohybrid cross follows one trait.
• A dihybrid cross follows two traits.
• Using a dihybrid cross, Mendel developed
the law of independent assortment
• The law of independent assortment states
that each pair of alleles segregates
independently of each other pair of alleles
during gamete formation
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
What does that mean in terms of meiosis?
• Strictly speaking, this law applies only to
genes on different, nonhomologous
chromosomes.
• So a AaBb parent can produce gametes:
AB, aB, Ab, ab
Predicted outcomes of heterozygous dihybrid
cross:
AaBb x AaBb
9 both dominant traits
3 one dominant, one recessive trait
3 other dominant, other recessive trait
1 both recessive traits
Fig. 14-8
EXPERIMENT
YYRR
P Generation
yyrr
Gametes YR

F1 Generation
YyRr
Hypothesis of
dependent
assortment
Predictions
yr
Hypothesis of
independent
assortment
Sperm
or
Predicted
offspring of
F2 generation
1/
4
Sperm
1/ YR 1/
2
2 yr
1/
4
1/
2
YR
1/
4
1/
4
Yr
yR
1/
4
yr
YR
YYRR YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
YYRR
Eggs
1/
2
YyRr
1/
4
Yr
Eggs
yr
YyRr
3/
4
yyrr
1/
4
yR
1/
4
Phenotypic ratio 3:1
1/
4
yr
9/
16
3/
16
3/
16
1/
16
Phenotypic ratio 9:3:3:1
RESULTS
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
Fig. 14-UN11
REMEMBER!
Each gamete has to
have one of each allele!
Fig. 14-UN6
The laws of probability govern Mendelian
inheritance
• Probability of an event occurring = expected
over possibilities
EX: Probability of throwing a tail with a coin is?
1/2
Segregation in a heterozygous plant is like
flipping a coin: Each gamete has a 1/2 chance
of carrying the dominant allele and a 1/2 chance
of carrying the recessive allele
Fig. 14-9

Rr
Segregation of
alleles into eggs
Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
R
1/
2
r
R
R
Eggs
4
r
2
r
2
R
1/
1/
1/
1/
4
r
r
R
r
1/
4
1/
4
• Mendel’s laws of segregation and
independent assortment reflect the rules of
probability
• The multiplication rule states that the
probability that two or more independent
events will occur together is the product of
their individual probabilities
What is the probability of getting an F2 (Aa x Aa)
offspring that is homozygous (aa) for both traits?
½ x ½
=
1/4
• The rule of addition states that the
probability that any one of two or more
exclusive events will occur is calculated by
adding together their individual
probabilities
• The rule of addition can be used to figure
out the probability that an F2 plant from a
monohybrid cross will be heterozygous
rather than homozygous
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Using the addition rule:
• In the cross of Rr x Rr, what is the
probability of Rr in offspring?
• If sperm is R and egg is r = ½ x ½ = ¼
• If sperm is r and egg is R = ½ x ½ = ¼
• So adding those together is ¼ + ¼ = 1/2
Solving Complex Genetics Problems with the Rules of Probability
• We can apply the multiplication and
addition rules to predict the outcome of
crosses involving multiple characters
• A dihybrid or other multicharacter cross is
equivalent to two or more independent
monohybrid crosses occurring
simultaneously
• In calculating the chances for various
genotypes, each character is considered
separately, and then the individual
probabilities are multiplied together
Examples
• AaBb
X
Aabb
What is probability of obtaining a Aabb
offspring?
Aa = ½
bb = ½
Answer = 1/4
AaBbCc x aabbCc
Probability of obtaining AabbCc?
For Aa from Aa x aa = ½
For bb from Bb x bb = ½
For Cc from Cc x Cc = ½
The answer is ½ x ½ x ½ = 1/8
AaBbcc x AabbCC
a) Probability of offspring that are A_B_C_?
A_ = ¾, B = ½, C = 1/1 answer 3/8
b) A_bbC_
aaB_C_
A_B_C_
c) aabbC_
The Testcross
• Used to tell the genotype of an individual with
the dominant phenotype
• Such an individual must have one dominant
allele, but the individual could be either
homozygous dominant or heterozygous
• A testcross: breeding the mystery individual
with a homozygous recessive individual
• If any offspring display the recessive
phenotype, the mystery parent must be
heterozygous.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-7
TECHNIQUE

Dominant phenotype, Recessive phenotype,
unknown genotype:
known genotype:
PP or Pp?
pp
Predictions
If PP
Sperm
p
p
P
Pp
Eggs
If Pp
Sperm
p
p
or
P
Pp
Eggs
P
Pp
Pp
pp
pp
p
Pp
Pp
RESULTS
or
All offspring purple
1/2
offspring purple and
1/2 offspring white
Extending Mendelian Genetics for a Single Gene
• Inheritance of characters by a single gene
may deviate from simple Mendelian
patterns in the following situations:
– When alleles are not completely dominant
or recessive
– When a gene has more than two alleles
– When a gene produces multiple
phenotypes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Degrees of Dominance
• Complete dominance occurs when
phenotypes of the heterozygote and
dominant homozygote are identical
• In incomplete dominance, the phenotype of
F1 hybrids is somewhere between the
phenotypes of the two parental varieties
• In codominance, both alleles are expressed
fully
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Important ratio
Aa x Aa IF INCOMPLETE DOMINANCE
1:2:1
1 dominant
2 mix
1 recessive
The Relation Between Dominance and
Phenotype
• A dominant allele does not subdue a recessive
allele; alleles don’t interact
• Alleles are simply variations in a gene’s
nucleotide sequence
• For any character, dominance/recessiveness
relationships of alleles depend on the level at
which we examine the phenotype
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Tay-Sachs disease is fatal; a dysfunctional
enzyme causes an accumulation of lipids in
the brain
– At the organismal level, the allele is
recessive
– At the biochemical level, the phenotype (i.e.,
the enzyme activity level) is incompletely
dominant
– At the molecular level, the alleles are
codominant
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Inheritance of Tay-Sachs
Organism level
Imperfect enzymes
• The disease is caused from mutations on
chromosome 15 in the HEXA gene.
This gene defect would be codominant with
a normal gene.
Frequency of Dominant Alleles
• Dominant alleles are not necessarily more
common in populations than recessive
alleles
• For example, one baby out of 400 in the
United States is born with extra fingers or
toes, the result of a dominant allele
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Multiple Alleles
• Most genes exist in populations in more
than two allelic forms
• For example, the four phenotypes of the
ABO blood group in humans are determined
by three alleles for the enzyme (I) that
attaches A or B carbohydrates to red blood
cells: A, B, AB, O. O is recessive.
• The enzyme encoded by the A allele adds
the A carbohydrate, whereas the enzyme
encoded by the B allele adds the B
carbohydrate; the enzyme encoded by the O
allele adds neither
Fig. 14-11
Allele
A
B
Carbohydrate
A
You may see IA ,
IB or i for the
alleles.
B
O
none
(a) The three alleles for the ABO blood groups
and their associated carbohydrates
Genotype
Red blood cell
appearance
Phenotype
(blood group)
AA or AO
A
BB or BO
B
AB
AB
OO
O
(b) Blood group genotypes and phenotypes
If Mary has A blood and Ben has B blood, can
they have a child with O blood?
Pleiotropy
• Most genes have multiple phenotypic
effects, a property called pleiotropy
• For example, pleiotropic alleles are
responsible for the multiple symptoms of
certain hereditary diseases, such as cystic
fibrosis and sickle-cell disease
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Epistasis
• In epistasis, a gene at one locus alters the
phenotypic expression of a gene at a second
locus
• For example, in mice and many other mammals,
coat color depends on two genes
• One gene determines the pigment color (with
alleles B for black and b for brown)
• The other gene (with alleles C for color and c for
no color) determines whether the pigment will be
deposited in the hair
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-12
BbCc

BbCc
Sperm
1/
4 BC
1/
4 bC
1/
4 Bc
1/
4 bc
Eggs
1/
1/
1/
1/
4 BC
BBCC
BbCC
BBCc
BbCc
BbCC
bbCC
BbCc
bbCc
BBCc
BbCc
BBcc
Bbcc
BbCc
bbCc
Bbcc
bbcc
4 bC
4 Bc
4 bc
9
: 3
: 4
• Interactive Question 14.7
MmBb x MmBb
Phenotype
Genotype
Black
M_bb
Ratio
Polygenic Inheritance
• Quantitative characters are those that vary in
the population along a continuum
• Polygenic inheritance results in an additive
effect of two or more genes on a single
phenotype which results in quantitative
variation.
• Skin color in humans is an example of
polygenic inheritance
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-13

AaBbCc
AaBbCc
Sperm
1/
Eggs
1/
8
1/
8
1/
8
1/
8
1/
1/
8
1/
1/
8
8
1/
8
1/
64
15/
8
1/
1/
8
8
8
1/
8
1/
8
1/
8
8
1/
Phenotypes:
64
Number of
dark-skin alleles: 0
6/
64
1
15/
64
2
20/
3
64
4
6/
64
5
1/
64
6
Nature and Nurture: The Environmental Impact
on Phenotype
• Another departure from Mendelian genetics
arises when the phenotype for a character
depends on environment as well as
genotype
• The norm of reaction is the phenotypic
range of a genotype influenced by the
environment
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-14
For example, hydrangea flowers of the same
genotype range from blue-violet to pink,
depending on soil acidity
• Norms of reaction are generally broadest
for polygenic characters
• Such characters are called multifactorial
because genetic and environmental factors
collectively influence phenotype
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The risk of heart disease follows a
continuum. It is multifactorial due to both
genetic and environmental influences.
Role of environment?
• An organism’s phenotype reflects its
overall genotype and unique environmental
history
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
•
EXPERIMENTAL DATA
•
In a cross of true-breeding green seed coat peas with true-breeding yellow seed
coat peas, all of the F1 peas had green-seed coats. In the F2, you counted 122
green ones and 33 yellow seeded peas. The total number of individuals you
counted, N, is 155.
•
What is the dominant trait? ____________________________
•
What is your hypothesis for the F2 population? Show a Punnett square to show
the proposed results.
•
Fill in the tables below with your observed data, calculate the expected result
according to your hypothesis. Determine the Χ2 value for each experiment, and
use the table of probabilities to accept or reject the null hypotheses.
•
Monohybrid cross
•
Phenotypes Observed
Expected
•
d2
d2/e
Total (X2 )_____
•
X2 = _____
•
Range of probability: ______________
•
Accept or reject null hypothesis? __________
•
d = o –e
degrees of freedom: _____
Experimental Data for Chi Square Determination
• In a cross of true-breeding rough seed coat peas
with true-breeding smooth seed coat peas, all of the
F1 peas had rough-seed coats. In the F2, you
counted 79 rough coats and 33 smooth coats. The
total number of individuals you counted is 112.
• What is the dominant trait?
____________________________
• What is your hypothesis for the F2 population?
Show a Punnett square to show the proposed
results.
Fill in the tables below with your observed data, calculate the
expected result according to your hypothesis. Determine the Χ2
value for each experiment, and use the table of probabilities to
accept or reject the null hypotheses.
Monohybrid cross
Phenotypes Observed (o) Expected (e)
Rough
79
Smooth
33
d=o–e
d2
Total
Χ2 = _____
degrees of freedom: _____
Range of probability: ______________
Accept or reject null hypothesis? __________
d2/e
_____