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Pea Plant Characteristics
Why Pea Plants?
• Easy, cheap, and quick to grow
• 7 easily distinguishable contrasting traits
• Normally self-pollinate due to tight enclosure
of the parts of the flower
• Can be cross-pollinated by cutting out the
stamens from one flower and using them to
pollinate another flower with a contrasting
trait
Fig. 14-2a
TECHNIQUE
1
2
Parental
generation
(P)
Stamens
Carpel
3
4
Mendel’s Crosses
• Mendel crossed plants that had bred true for a
certain trait for multiple generations with
plants that bred true for the contrasting trait
for multiple generations.
• He called these the P generation.
• The offspring from these crosses were called
the F1 generation.
• The offspring from crosses between the F1
plants were the called F2 generation.
Fig. 14-2b
RESULTS
First
filial
generation
offspring
(F1)
5
• Mendel observed:
• the F1 generation always had a single
phenotype
• the phenotype of the other parent
remained hidden.
Where does the recessive trait go during
the F1 generation?
He hypothesized that it is still present in
all the offspring, but is not expressed.
Mendel’s Laws
• The Law of Dominance:
• When two plants are crossed that are pure for
contrasting traits, then one of the traits, the
dominant one, will be expressed in the offspring.
The other trait, the recessive one, will not be
expressed.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Without knowing about meiosis and genes and
how gametes are formed, Mendel determined:
• The Law of Segregation:
• When an individual produces gametes, the
two “factors” responsible for a trait separate
so that a gamete will receive only one factor
or the other.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Mendel asked:
How will the recessive trait ever be
expressed if it is dominated by the other
trait?
• It can only be expressed when there is no
dominant trait present.
• He called this genotype homozygous
recessive.
When writing genotypes, a capital letter
represents the dominant trait and a lower-case
letter represents the recessive trait.
Tall – T
Short - t
The same letter is normally used.
Yellow seed – Y
Green seed - y
Gamete Formation
• Due to meiosis, only one chromosome from
each homologous pair is in each gamete.
• Thus, there will be one allele for each trait in
each gamete.
• Mendel tracked one or two alleles at a time.
Pure Tall Pea Plant X Pure Short Pea Plant
TT
T
X
T
tt
t
t
Hybrid Tall Pea Plant X Hybrid Tall Pea Plant
Tt
T
X
t
Tt
T
t
Punnett Squares
• A Punnett square makes it easy to track the
alleles which are passed on to offspring by the
fertilization of gametes.
Using a Punnett square:
Male gametes
A
A
Female
gametes
a
a
Possible Genotypes
A
a
A
AA
Aa
a
Aa
aa
Tracking Traits
• genotype
• the alleles of a given gene locus
• phenotype
• the physical expression of a gene
What are the possible genotypes for the offspring
of each of the following crosses?
1) AA x AA
2) Aa x aa
3) AA x aa
4) aa x Aa
5) AA x Aa
6) Aa x Aa
1) AA x AA?
2) Aa x aa?
3) AA x aa?
4) aa x Aa?
5) AA x Aa?
6) Aa x Aa?
100% AA
50% Aa; 50% aa
100% Aa
50% Aa, 50% aa
50% Aa, 50% AA
25% AA, 50% Aa, 25% aa
With only two alleles:
• there are a limited number of possible
combinations for any one gene
• these examples represent the simplest
circumstances
Sometimes there are genes with three or more
alleles
• Ex. blood types
Sometimes one allele does not have complete
dominance over the other.
Probability
If I flip a coin one time, what is the probability
that it will land on heads?
1 out of 2 chances, or 50%
Since I got heads (or tails) the first time, what
is the probability that I will get heads on my
next flip?
It is still 50%.
If I keep flipping the coin and getting heads,
does that affect the probability of the
outcome of my next flip?
No, the chances of getting heads or tails is
the same on each flip.
What is the probability that I will get heads on
two coin tosses in a row?
You are now asking for the probability of flipping
heads and flipping heads again.
When you have two separate probabilities and
both must occur, the two separate probabilities
must be multiplied to find the overall
probability—so the probability of heads and
heads occurring is
1/2 x 1/2 = 1/4
• The probability of two events combined
occurring will be much lower than one event alone
• the chance of two coin tosses both being of a
particular outcome is less likely than one coin
toss of a particular outcome.
Rule of Multiplication
• Multiply the probabilities of a combination of
possible outcomes.
• Multiply whenever there is a statement of
one probability “and” another probability.
What is the probability that a couple would
have three boys?
The separate probabilities are 1/2 for each boy,
so the combined probability would be
1/2 x 1/2 x 1/2 = 1/8
What is the probability that parents with the
genotypes Aa and Aa would have a child with
the genotype AA?
• The probability of the father giving his child a
gamete with the “A” allele is ½
• The probability of the mother giving her child a
gamete with the “A” allele is ½
1/2 x 1/2 = 1/4
If the probability of getting dealt an ace from a
deck of cards is 4 out of 52 (4/52), what is the
probability of getting dealt an ace or a king?
The probability of getting an ace is 4/52 and the
probability of getting a king is 4/52; to learn
what chance there is of getting dealt either
card, you must add the probabilities together to
get the total probability:
4/52 + 4/52 = 8/52
What is the probability of getting any face card?
Calculate this two ways:
• first, count up the total number of face
cards (12/52)
• verify this, using the rule of addition:
(kings) 4/52 + (queens) 4/52 + (jacks) 4/52 =
12/52
Rule of Addition
Adding the probabilities of a combination of
independent outcomes is called the rule of
addition.
Use this whenever there is a statement of
one probability “or” another probability.
What is the probability of my being dealt an ace
or a face card from one deck of cards?
The probability of getting an ace is 4/52 and
the probability of getting a face card is 12/52,
so the probability of either outcome occurring
is 4/52 + 12/52 = 16/52
If a child with a dominant gene A is affected by a
particular disease, what is the probability of a
couple having a child with this disease if they
have the genotypes of Aa (father) and aa
(mother)?
The probability of the father giving the child an
“A” allele is 1/2 and the probability of the mother
giving the child an “A” allele is 0, so the total
probability is 1/2 + 0 = ½
Verify this using a Punnett square.
The Testcross
• How can we 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
• 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
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
Fig. 14-7a
TECHNIQUE

Dominant phenotype, Recessive phenotype,
known genotype:
unknown genotype:
pp
PP or Pp?
Predictions
If PP
Sperm
p
p
P
P
Pp
Eggs
If Pp
Sperm
p
p
or
Pp
P
Eggs
Pp
Pp
pp
pp
p
Pp
Pp
Fig. 14-7b
RESULTS
or
All offspring purple
1/2
offspring purple and
offspring white
1/2
The Law of Independent Assortment
• Mendel derived the law of segregation by following
a single trait
• The F1 offspring produced in this cross were
monohybrids, individuals that are heterozygous for
one trait
• A cross between such heterozygotes is called a
monohybrid cross
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Mendel identified his third 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-8
EXPERIMENT
YYRR
P Generation
yyrr
Gametes YR

F1 Generation
yr
YyRr
Hypothesis of
dependent
assortment
Predictions
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
Yr
1/
4
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. 14-8a
EXPERIMENT
YYRR
P Generation
yyrr
Gametes YR

F1 Generation
yr
YyRr
Hypothesis of
independent
assortment
Hypothesis of
dependent
assortment
Predictions
Sperm
or
Predicted
offspring of
F2 generation
1/
4
Sperm
1/
2
YR
1/
2
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
1/
4
yr
1/
4
1/
2
YR
YyRr
1/
4
Yr
Eggs
yr
yyrr
YyRr
3/
4
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
Fig. 14-8b
RESULTS
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
• Using a dihybrid cross, Mendel developed the
• Law of independent assortment
• 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
We now know
• Mendel’s Law of Independent Assortment
• Applies only to genes on different,
nonhomologous chromosomes
• The traits he studied were, in fact, on
nonhomologous chromosomes
• Genes located near each other on the same
chromosome tend to be inherited together
Solving Complex Genetics Problems
with the Rules of Probability
• The multiplication and addition rules can predict
the outcome of crosses involving multiple
characters
– A dihybrid 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Inheritance patterns are often more complex than
predicted by simple Mendelian genetics
• Many 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Extending Mendelian Genetics for a Single Gene
• Inheritance of characters by a single gene may
deviate from Mendelian patterns:
– 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
• In incomplete dominance, the phenotype of F1
hybrids is somewhere between the phenotypes of
the two parental varieties
• In codominance, two dominant alleles affect the
phenotype in separate, distinguishable ways
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-10-1
P Generation
Red
CRCR
Gametes
White
CWCW
CR
CW
Fig. 14-10-2
P Generation
Red
CRCR
Gametes
White
CWCW
CR
CW
Pink
CRCW
F1 Generation
Gametes 1/2 CR
1/
2
CW
Fig. 14-10-3
P Generation
Red
CRCR
White
CWCW
CR
Gametes
Incomplete
dominance
in
snapdragon
color
CW
Pink
CRCW
F1 Generation
Gametes 1/2 CR
1/
CW
2
Sperm
1/
2
CR
1/
2
CW
F2 Generation
1/
2
CR
Eggs
1/
2
CRCR
CRCW
CRCW
CWCW
CW
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
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• The allele for this unusual trait is dominant to the
allele for the more common trait of five digits per
appendage
• In this example, the recessive allele is far more
prevalent than the population’s 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: IA, IB, and i.
• The enzyme encoded by the IA allele adds the A
carbohydrate, whereas the enzyme encoded by the
IB allele adds the B carbohydrate; the enzyme
encoded by the i allele adds neither
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-11
Allele
IA
IB
Carbohydrate
A
B
i
none
(a) The three alleles for the ABO blood groups
and their associated carbohydrates
Genotype
Red blood cell
appearance
Phenotype
(blood group)
IAIA or IA i
A
IBIB or IB i
B
IAIB
AB
ii
O
(b) Blood group genotypes and phenotypes
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Extending Mendelian Genetics for Two or More Genes
• Some traits may be determined by two or more
genes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
EpistasisOne gene
affects the
expression
of another
gene

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
Polygenic Inheritance
• Quantitative characters vary in the population
along a continuum
• usually indicates polygenic inheritance, an additive
effect of two or more genes on a single phenotype
• Skin color in humans is an example of polygenic
inheritance
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-13

Polygenic
Inheritance
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
Height in Human Beings
Frequency Distributions in Polygenic Inheritance
Nature and Nurture: The Environmental Impact on
Phenotype
• 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
– For example, hydrangea flowers of the same
genotype range from blue-violet to pink, depending
on soil acidity
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-14
Himalayan Rabbit
When an ice pack was kept on a shaved
portion of the rabbit’s back, the new fur grew
back black instead of white.
Temperature affected the expression of the
gene for fur color.
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
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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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-15a
Key
Male
Female
Affected
male
Affected
female
Mating
Offspring, in
birth order
(first-born on left)
Fig. 14-15b
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
Ww
ww
ww
Ww ww ww Ww
Ww
Ww
ww
3rd generation
(two sisters)
WW
or
Ww
Widow’s peak
ww
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
Fig. 14-15c
1st generation
(grandparents)
Ff
2nd generation
(parents, aunts,
and uncles)
FF or Ff ff
Ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd generation
(two sisters)
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?
• Pedigrees can also be used to make predictions
about future offspring
• We can use the multiplication and addition rules to
predict the probability of specific phenotypes
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Recessively Inherited Disorders
• Many genetic disorders are inherited in a recessive
manner
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The Behavior of Recessive Alleles
• Carriers are heterozygous individuals who carry the
recessive allele but are phenotypically normal (i.e.,
pigmented)
– Albinism is a recessive condition characterized by a lack
of pigmentation in skin and hair
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 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
• Consanguineous matings (i.e., 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Cystic Fibrosis
• Cystic fibrosis
– the most common lethal genetic disease in the
United States, striking one out of every 2,500 people
of European descent
– results in defective or absent chloride transport
channels in plasma membranes
– Symptoms include mucus buildup in some internal
organs and abnormal absorption of nutrients in the
small intestine
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Sickle-Cell Disease
• Sickle-cell disease affects one out of 400 AfricanAmericans
– caused by the substitution of a single amino acid in
the hemoglobin protein in red blood cells
– Symptoms include physical weakness, pain, organ
damage, and even paralysis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-17
Parents
Dwarf
Dd

Normal
dd
Sperm
D
d
d
Dd
Dwarf
dd
d
Dd
Dwarf
Eggs
Normal
dd
Normal
Huntington’s Disease
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Multifactorial Disorders
• Many diseases, such as heart disease and cancer,
have both genetic and environmental components
• Little is understood about the genetic contribution
to most multifactorial diseases
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Genetic Testing and Counseling
• Genetic counselors can provide information to
prospective parents concerned about a family
history for a specific disease
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Tests for Identifying Carriers
• For a growing number of diseases, tests are
available that identify carriers and help define the
odds more accurately
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fetal Testing
• In amniocentesis, the liquid that bathes the fetus is
removed and tested
• In chorionic villus sampling (CVS), a sample of the
placenta is removed and tested
• Other techniques, such as ultrasound and
fetoscopy, allow fetal health to be assessed visually
in utero
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-18a
Amniotic fluid
withdrawn
Centrifugation
Fetus
Placenta
Uterus
Cervix
Fluid
Fetal
cells
BioSeveral chemical
hours
tests
Several
weeks
Several
weeks Karyotyping
(a) Amniocentesis
Fig. 14-18b
Fetus
Placenta
Biochemical
tests
Karyotyping
Suction tube
inserted
through
cervix
Chorionic
villi
Several
hours
Fetal
cells
Several
hours
(b) Chorionic villus sampling (CVS)
Newborn Screening
• Some genetic disorders can be detected at birth by
simple tests that are now routinely performed in
most hospitals in the United States
– Phenylketonuria – inability to metabolize the amino
acid phenylalanine
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-UN2
Degree of dominance
Description
Complete dominance
of one allele
Heterozygous phenotype
same as that of homozygous dominant
Incomplete dominance
of either allele
Heterozygous phenotype
intermediate between
the two homozygous
phenotypes
Example
PP
Pp
C RC R
Codominance
Multiple alleles
Pleiotropy
Heterozygotes: Both
phenotypes expressed
In the whole population,
some genes have more
than two alleles
One gene is able to
affect multiple
phenotypic characters
C RC W C W C W
IAIB
ABO blood group alleles
IA , IB , i
Sickle-cell disease
Fig. 14-UN3
Relationship among
genes
Epistasis
Example
Description
One gene affects
the expression of
another
BbCc
BbCc
BC bC Bc bc
BC
bC
Bc
bc
9
Polygenic
inheritance
A single phenotypic
character is
affected by
two or more genes
AaBbCc
:3
:4
AaBbCc