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LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
Chapter 14
Mendel and the Gene Idea
Lectures by
Erin Barley
Kathleen Fitzpatrick
© 2011 Pearson Education, Inc.
Overview: Drawing from the Deck of Genes
• 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)
© 2011 Pearson Education, Inc.
• The “particulate” hypothesis is the idea
that parents pass on discrete heritable
units (genes)
• This hypothesis can explain the
reappearance of traits after several
generations
• Mendel documented a particulate
mechanism through his experiments
with garden peas
© 2011 Pearson Education, Inc.
Figure 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
© 2011 Pearson Education, Inc.
Mendel’s Experimental, Quantitative
Approach
• Advantages of pea plants for genetic study
– There are many varieties with distinct heritable
features, or characters (such as flower color);
character variants (such as purple or white
flowers) are called traits
– Mating can be controlled
– Each flower has sperm-producing organs
(stamens) and an egg-producing organ (carpel)
– Cross-pollination (fertilization between different
plants) involves dusting one plant with pollen
from another
© 2011 Pearson Education, Inc.
Figure 14.2
TECHNIQUE
1
2
Parental
generation
(P)
3
Stamens
Carpel
4
RESULTS
First filial
generation
offspring
(F1)
5
• Mendel chose to track only those
characters that occurred in two
distinct alternative forms
• He also used varieties that were
true-breeding
–produce offspring of the same
variety when they self-pollinate
© 2011 Pearson Education, Inc.
• 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 or crosspollinate with other F1 hybrids, the F2
generation is produced
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Figure 14.3-1
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
Figure 14.3-2
EXPERIMENT
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)
Purple
flowers
White
flowers
All plants had purple flowers
Self- or cross-pollination
Figure 14.3-3
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had purple flowers
Self- or cross-pollination
F2 Generation
705 purpleflowered
plants
224 white
flowered
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
• The factor for white flowers was not
diluted or destroyed because it
reappeared in the F2 generation
© 2011 Pearson Education, Inc.
• What Mendel called a
“heritable factor” is what we
now call a gene
© 2011 Pearson Education, Inc.
Table 14.1
Mendel’s Model
1 - Alternative versions of genes
account for variations in inherited
characters
• Alternative versions of a gene are
called alleles
• Each gene resides at a specific
locus on a specific chromosome
© 2011 Pearson Education, Inc.
Figure 14.4
Allele for purple flowers
Locus for flower-color gene
Pair of
homologous
chromosomes
Allele for white flowers
2. Second: For each character, an
organism inherits two alleles, one
from each parent
© 2011 Pearson Education, Inc.
3 - 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
© 2011 Pearson Education, Inc.
4 - The Law of Segregation
• Two alleles for a heritable character
separate (segregate) during gamete
formation and end up in different
gametes
• An egg or a sperm gets only one of the
two alleles that are present in the
organism
© 2011 Pearson Education, Inc.
Figure 14.5-1
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
pp
PP
p
Gametes:
P
Figure 14.5-2
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
pp
PP
p
Gametes:
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
1/
2 p
2 P
Figure 14.5-3
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
pp
PP
p
Gametes:
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
1/
2 p
2 P
Sperm from F1 (Pp) plant
F2 Generation
P
Eggs from
F1 (Pp) plant
p
3
P
p
PP
Pp
Pp
pp
:1
Useful Genetic Vocabulary
• Homozygous
• Heterozygous
– Not true-breeding
• Phenotype
• Genotype
© 2011 Pearson Education, Inc.
Figure 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
Testcross
• A way to tell the genotype of an individual
with the dominant phenotype.
• Such an individual could be either
homozygous dominant or heterozygous
• The answer is to carry out a testcross:
breeding the mystery individual with a
homozygous recessive individual
• If any offspring display the recessive
phenotype, the mystery parent must be
heterozygous
© 2011 Pearson Education, Inc.
Figure 14.7
TECHNIQUE
Dominant phenotype,
unknown genotype:
PP or Pp?
Predictions
If purple-flowered
parent is PP
Sperm
p
p
Recessive phenotype,
known genotype:
pp
or
If purple-flowered
parent is Pp
Sperm
p
p
P
Pp
Eggs
P
Pp
Eggs
P
p
Pp
Pp
Pp
Pp
pp
pp
RESULTS
or
All offspring purple
1/
2
offspring purple and
1/ offspring white
2
The Law of Independent Assortment
• Mendel identified his second law of inheritance by
following two characters at the same time
• Crossing two true-breeding parents differing in two
characters produces dihybrids in the F1
generation, heterozygous for both characters
• A dihybrid cross, a cross between F1
dihybrids, can determine whether two characters
are transmitted to offspring as a package or
independently
© 2011 Pearson Education, Inc.
Figure 14.8
EXPERIMENT
YYRR
P Generation
yyrr
yr
Gametes YR
F1 Generation
Predictions
YyRr
Hypothesis of
dependent assortment
Hypothesis of
independent assortment
Sperm
or
Predicted
offspring of
F2 generation
1/
Sperm
1/
2
YR
1/
2
2
YR
YyRr
YYRR
Eggs
1/
2
1/
4
YR
4
Yr
4
yR
4
yr
Eggs
yr
YyRr
3/
yyrr
1/
4
YR
1/
4
1/
Yr
4
yR
1/
4
yr
yr
1/
1/
4
1/
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
4
Phenotypic ratio 3:1
1/
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
• Law of Independent Assortment
– Each pair of alleles segregates
independently of each other pair of alleles
during gamete formation
– This law applies only to genes on different,
non-homologous chromosomes or those
far apart on the same chromosome
– Genes located near each other on the
same chromosome tend to be inherited
together
© 2011 Pearson Education, Inc.
Concept 14.2: The laws of probability
govern Mendelian inheritance
• Mendel’s laws of segregation and
independent assortment reflect the rules of
probability
• When tossing a coin, the outcome of one
toss has no impact on the outcome of the
next toss
• In the same way, the alleles of one gene
segregate into gametes independently of
another gene’s alleles
© 2011 Pearson Education, Inc.
The Multiplication and Addition Rules
Applied to Monohybrid Crosses
• The multiplication rule states that the
probability that two or more independent
events will occur together is the product
of their individual probabilities
• The addition rule states that the
probability of an event that can occur
two or more different ways is the sum of
the separate probabilities of those ways.
© 2011 Pearson Education, Inc.
Figure 14.9
Rr
Segregation of
alleles into eggs

Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
2
Eggs
4
r
2
r
R
R
1/
1/
r
2
R
R
1/
1/
1/
4
r
r
R
r
1/
4
1/
4
Multiplication
Rule
A AND B
Addition Rule
A OR B
Concept 14.3: Inheritance patterns are often
more complex than predicted by simple
Mendelian genetics
• The relationship between genotype and phenotype
is rarely as simple as in the pea plant characters
Mendel studied
• Most heritable characters are not determined by
only one gene with two alleles
• However, the basic principles of segregation and
independent assortment apply even to more
complex patterns of inheritance
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Degrees of Dominance
• Complete Dominance
• Incomplete Dominance
• Codominance
© 2011 Pearson Education, Inc.
Figure 14.10-1
P Generation
White
CWCW
Red
CRCR
Gametes
CR
CW
Figure 14.10-2
P Generation
White
CWCW
Red
CRCR
Gametes
CR
CW
F1 Generation
Gametes 1/2 CR
Pink
CRCW
1/
2
CW
Figure 14.10-3
P Generation
White
CWCW
Red
CRCR
CR
Gametes
CW
F1 Generation
Pink
CRCW
1/
Gametes 1/2 CR
2
CW
Sperm
F2 Generation
1/
2
CR
1/
2
CW
Eggs
1/
2
CR
1/
2
CW
CRCR CRCW
CRCW CWCW
The Relation Between Dominance and Phenotype
• A dominant allele does not subdue a
recessive allele; alleles don’t interact that way
• Alleles are simply variations in a gene’s
nucleotide sequence
• For any character, dominance/recessiveness
relationships of alleles depend on the level of
expression of the alleles
© 2011 Pearson Education, Inc.
• Tay-Sachs disease is fatal
– A dysfunctional enzyme causes an
accumulation of lipids in the brain
– At the organismal level, the allele is recessive
(Only tt have the disease)
– At the biochemical level, the phenotype is
incompletely dominant (Tt expresses functional
and non-functional enzyme but No Symptoms)
– At the molecular level, the alleles are
codominant (Since Tt expresses functional and
non-functional enzyme indicates TTTN)
© 2011 Pearson Education, Inc.
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 (Polydactyly)
© 2011 Pearson Education, Inc.
Pleiotropy
• Most genes have multiple phenotypic
effects, a property called pleiotropy
Multiple Alleles
• Most genes exist in populations in more
than two allelic forms
• Ex. Blood groups in humans
© 2011 Pearson Education, Inc.
Figure 14.11
(a) The three alleles for the ABO blood groups and their
carbohydrates
IA
Allele
Carbohydrate
IB
i
none
B
A
(b) Blood group genotypes and phenotypes
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
A
B
AB
O
Red blood cell
appearance
Phenotype
(blood group)
Epistasis
– A gene at one locus alters the
phenotypic expression of a gene at a
second locus
– For example, in Labrador retrievers 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
© 2011 Pearson Education, Inc.
Figure 14.12
BbEe
Eggs
1/
4 BE
1/
4 bE
1/
4 Be
1/
4
be
Sperm
1/ BE
4
1/
BbEe
4 bE
1/
4 Be
1/
4 be
BBEE
BbEE
BBEe
BbEe
BbEE
bbEE
BbEe
bbEe
BBEe
BbEe
BBee
Bbee
BbEe
bbEe
Bbee
bbee
9
: 3
: 4
Polygenic Inheritance
• Quantitative characters are those
that vary in the population along a
continuum
• Quantitative variation usually indicates
polygenic inheritance, an
additive effect of two or more genes on
a single phenotype
© 2011 Pearson Education, Inc.
Figure 14.13
AaBbCc
AaBbCc
Sperm
1/
1/
8
8
1/
1/
Eggs
8
1/
1/
8
8
1/
8
1/
1/
8
8
8
8
1/
8
1/
8
1/
1/
8
1/
8
1/
8
1/
8
Phenotypes:
Number of
dark-skin alleles:
1/
64
0
6/
64
1
15/
64
2
20/
64
3
15/
64
4
6/
64
5
1/
64
6
Nature and Nurture: The Environmental
Impact on Phenotype
• Phenotype for a character may depend on
environment as well as genotype
• The norm of reaction is the phenotypic
range of a genotype influenced by the
environment
• For example, hydrangea flowers of the same
genotype range from blue-violet to pink,
depending on soil acidity
© 2011 Pearson Education, Inc.
Figure 14.14
Concept 14.4: Many human traits follow
Mendelian patterns of inheritance
• Humans are not good subjects for genetic research
– Generation time is too long
– Parents produce relatively few offspring
– Breeding experiments are unacceptable
• However, basic Mendelian genetics endures as the
foundation of human genetics
© 2011 Pearson Education, Inc.
Pedigree Analysis
• A pedigree is a family tree that describes
the interrelationships of parents and
children across generations
• Inheritance patterns of particular traits can
be traced and described using pedigrees
• Pedigrees can be used to make
predictions about future offspring
© 2011 Pearson Education, Inc.
Figure 14.15
Key
Male
1st
generation
Affected
male
Female
Affected
female
Mating
1st
generation
Ww
ww
Ww
ww
2nd
generation
Ww
ww
3rd
generation
WW
or
Ww
Widow’s
peak
ff
ff
(a) Is a widow’s peak a dominant or
recessive trait?
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd
generation
ww
No widow’s
peak
ff
Ff
2nd
generation
FF or Ff
Ww ww ww Ww
Ff
Offspring
Attached
earlobe
Free
earlobe
b) Is an attached earlobe a dominant
or recessive trait?
The Behavior of Recessive Alleles
• Recessively inherited disorders show
up only in individuals homozygous for
the allele
• Carriers are heterozygous individuals
who carry the recessive allele but are
phenotypically normal; most individuals
with recessive disorders are born to
carrier parents
© 2011 Pearson Education, Inc.
Figure 14.16
Parents
Normal
Aa
Normal
Aa
Sperm
A
a
A
AA
Normal
Aa
Normal
(carrier)
a
Aa
Normal
(carrier)
aa
Albino
Eggs
• If a recessive allele that causes a disease is
rare, then the chance of two carriers meeting
and mating is low
• Matings between close relatives increase the
chance of mating between two carriers of the
same rare allele
• Most societies and cultures have laws or
taboos against marriages between close
relatives
© 2011 Pearson Education, Inc.
Cystic Fibrosis
• Cystic fibrosis is the most common lethal genetic
disease in the United States striking one out of
every 2,500 people of European descent
• The cystic fibrosis allele (recessive) results in
defective or absent chloride transport channels in
plasma membranes leading to a buildup of
chloride ions outside the cell
• Symptoms include mucus buildup in some internal
organs and abnormal absorption of nutrients in the
small intestine
© 2011 Pearson Education, Inc.
Sickle-Cell Disease: A Genetic Disorder with
Evolutionary Implications
• Sickle-cell disease affects one out of 400 AfricanAmericans
• The disease is caused by the substitution of a
single amino acid in the hemoglobin protein in red
blood cells
• In homozygous individuals, all hemoglobin is
abnormal (sickle-cell)
• Symptoms include physical weakness, pain, organ
damage, and even paralysis
© 2011 Pearson Education, Inc.
Fig. 14-UN1
• Heterozygotes (said to have sickle-cell trait)
are usually healthy but may suffer some
symptoms
• About one out of ten African Americans has
sickle cell trait, an unusually high frequency
of an allele with detrimental effects in
homozygotes
• Heterozygotes are resistant to the malaria
parasite so there is an advantage to being
heterozygous
© 2011 Pearson Education, Inc.
Dominantly Inherited Disorders
• Some human disorders are caused by
dominant alleles
• Dominant alleles that cause a lethal
disease are rare and arise by mutation
• Achondroplasia is a form of dwarfism
caused by a rare dominant allele
© 2011 Pearson Education, Inc.
Figure 14.17
Parents
Dwarf
Dd
Normal
dd
Sperm
D
d
d
Dd
Dwarf
dd
Normal
d
Dd
Dwarf
dd
Normal
Eggs
Huntington’s Disease: A Late-Onset Lethal
Disease
• The timing of onset of a disease significantly
affects its inheritance
• Huntington’s disease is a degenerative
disease of the nervous system
• The disease has no obvious phenotypic
effects until the individual is about 35 to 40
years of age
• Once the deterioration of the nervous system
begins the condition is irreversible and fatal
© 2011 Pearson Education, Inc.
Multifactorial Disorders
• Many diseases, such as heart disease,
diabetes, alcoholism, mental illnesses,
and cancer have both genetic and
environmental components
© 2011 Pearson Education, Inc.
Counseling Based on Mendelian Genetics
and Probability Rules
• Using family histories, genetic counselors help
couples determine the odds that their children will
have genetic disorders
• Probabilities are predicted on the most accurate
information at the time; predicted probabilities may
change as new information is available
© 2011 Pearson Education, Inc.
Video: Ultrasound of Human Fetus I
© 2011 Pearson Education, Inc.
Figure 14.19
(a) Amniocentesis
1
(b) Chorionic villus sampling (CVS)
Ultrasound monitor
Amniotic
fluid
withdrawn
Ultrasound
monitor
Fetus
1
Placenta
Chorionic villi
Fetus
Placenta
Uterus
Cervix
Cervix
Uterus
Suction
tube
inserted
through
cervix
Centrifugation
Fluid
Fetal
cells
Several hours
2
Several
weeks
Biochemical
and genetic
tests
Several
hours
Fetal cells
2
Several hours
Several weeks
3
Karyotyping