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CAMPBELL
BIOLOGY
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
14
Mendel and the Gene Idea
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
© 2014 Pearson Education, Inc.
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)
• 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
Figure 14.1 What principles of inheritance did Gregor Mendel discover by breeding garden pea plants?
© 2014 Pearson Education, Inc.
Figure 14.1a
Mendel (third from right, holding a sprig of fuchsia) with his fellow monks.
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.
• Advantages of pea plants for genetic study:
– There are many varieties with distinct heritable features,
or characters (such as flower color); many character
variants (such as purple or white flowers) called traits
– Mating of plants can be controlled
– Each pea plant has sperm-producing organs (stamens)
and egg-producing organs (carpels)
– Cross-pollination (fertilization between different plants)
can be achieved by dusting one plant with pollen from
another
© 2014 Pearson Education, Inc.
Mendel started his experiments
with True-Breeding Peas
• Mendel tracked only those characters that varied
in an “either-or” manner, rather than a “more-orless” manner.
– He worked with flowers that were either purple or
white.
– He avoided traits such as seed weight, which varied
on a continuum.
• Mendel started his experiments with varieties
that were true-breeding
– When true-breeding plants self-pollinate, all their
offspring have the same traits as their parents.
© 2014 Pearson Education, Inc.
Terminology for Mendel’s Experiments
• In a typical breeding experiment, Mendel would
cross-pollinate two contrasting, true-breeding
pea varieties, a process called hybridization.
• The true-breeding parents are the P (parental)
generation
• Their hybrid offspring are the F1 (first filial)
generation
• Mendel would then allow the F1 hybrids to selfpollinate to produce an F2 (second filial)
generation
© 2014 Pearson Education, Inc.
Mendel’s Experiment- The Law of Segregation
Figure 14.2
1.Removed stamen’s from truebreeding purple flower.
2. Transferred pollen (sperm) from
stamens of true breeding white
flower to carpel of purple flowers.
3. The pollinated carpel matured into
a pod.
4. Planted the pod seeds.
Results:
The F1 offspring were all
purple.
When the purple flower’s
pollen was transferred to
white flower’s carpel all of
the F1 were also purple.
© 2014 Pearson Education, Inc.
F1 hybrid plants are allowed to self-pollinate
what traits do have in F2 generation
1. Repeated experiment
as before. All F1 are
purple.
2. Allowed F1 to crosspollinate itself.
Results:
F2 plants both purple
and white flowered
were produced in a
ratio of 3:1.
© 2014 Pearson Education, Inc.
Fig. 14.3
Dominant and Recessive Traits
• 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 characters in pea plant,
each represented by two trait
• What Mendel called a
“heritable factor” is what we
now call a gene
Copyright © 2014 Pearson Education Inc.
Mendel’s Model
• Mendel developed a hypothesis to
explain the 3:1 inheritance pattern he
observed in F2 offspring
• Four related concepts make up this
model
• These concepts can be related to what
we now know about genes and
chromosomes
© 2014 Pearson Education, Inc.
• The first concept is that alternative versions of
genes account for variations in inherited
characters
• For example, the gene for flower color in pea
plants exists in two versions, one for purple
flowers and the other for white flowers
• These alternative versions of a gene are now
called alleles
• Each gene resides at a specific locus on a
specific chromosome
• The DNA at that locus can vary in its sequence of
nucleotides.
• The purple-flower and white-flower alleles are two
DNA variations at the flower-color locus.
© 2014 Pearson Education, Inc.
Alleles, alternative versions of a gene
Enzyme
C T A A A T C G G T
Allele for
purple flowers
Locus for
flower-color gene
G A T T T A G C C A
CTAAATCGGT
Enzyme that helps
synthesize purple
pigment
Pair of
homologous
chromosomes
Allele for
white flowers
A T A A A T C G G T
T A T T T A G C C A
ATAAATCGGT
Figure 14.4
Absence of enzyme
One allele
results in
sufficient
pigment
• The second concept is that for each character an
organism inherits two alleles, one from each
parent
• Mendel made this deduction without knowing
about the role of chromosomes
• A diploid organism inherits one set of
chromosomes from each parent.
• Each diploid organism has a pair of homologous
chromosomes and, therefore, two copies of each
gene.
• The two alleles at a locus on a chromosome may
be identical, as in the true-breeding plants of
Mendel’s P generation
• Alternatively, the two alleles at a locus may differ,
as in the F1 hybrids
© 2014 Pearson Education, Inc.
• 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 inherited
a purple-flower allele from one parent and a whiteflower allele from the other.
• In the flower-color example, the F1 plants had
purple flowers because the allele for that trait is
dominant
© 2014 Pearson Education, Inc.
• The fourth concept, now known as 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
© 2014 Pearson Education, Inc.
• Mendel’s segregation model accounts for
the 3:1 ratio he observed in the F2
generation of his numerous crosses
• The possible combinations of sperm and
egg can be shown using a Punnett square,
a diagram for predicting the results of a
genetic cross between individuals of known
genetic makeup
• A capital letter represents a dominant allele,
and a lowercase letter represents a
recessive allele
© 2014 Pearson Education, Inc.
Figure 14.5-3
P Generation
Appearance:
Purple flowers White flowers
Genetic makeup:
PP
pp
Gametes:
p
P
F1 Generation
Appearance:
Genetic makeup:
Purple flowers
Pp
Gametes:
1
2
P
1
2
p
Sperm from F1 (Pp) plant
F2 Generation
P
p
PP
Pp
Pp
pp
P
Mendel’s law of
segregation accounts
for the 3:1 ratio that
he observed in
the F2 generation
Eggs from
F1 (Pp) plant
p
3
:1
The F1 hybrids produce
two classes of gametes,
half with the purpleflower allele and half
with the white-flower
allele.
Useful Genetic Vocabulary
• An organism with two identical alleles for a character
is said to be homozygous for the gene controlling that
character
- For flower color in peas- white flowers are homozygous
recessive (pp) and purple flower are homozygous
dominant (PP) for the flower-color gene.
• An organism that has two different alleles for a gene is
said to be heterozygous for the gene controlling that
character
- For flower color in peas- purple flower are heterozygous
dominant (Pp)
• Unlike homozygotes, heterozygotes are not by truebreeding
© 2014 Pearson Education, Inc.
• Because of the different effects of dominant and
recessive alleles, an organism’s traits do not
always reveal its genetic composition
• Therefore, we distinguish between an organism’s
phenotype, or physical appearance, and its
genotype, or genetic makeup
• Two organisms can have the same phenotype but
different genotypes if one is homozygous
dominant and the other is heterozygous.
• In the example of flower color in pea plants, PP
and Pp plants have the same phenotype (purple)
but different genotypes (homozygous dominant
and heterozygous).
© 2014 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
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
© 2014 Pearson Education, Inc.
The Testcross is used to determine
genotype
Figure 14.7
• A Purple pea can be
either homozygous or
heterozygous. What
is the genotype?
• The purple pea must
be crossed with a
homozygous white
(recessive trait) pea.
© 2014 Pearson Education, Inc.
•
Mendel’s law of
segregation states that
the two alleles for a
heritable character
segregate (separate)
during gamete production
and end up in different
gametes.
•
If an organism has two
identical alleles for a
particular character, then
that allele is present as a
single copy in all
gametes. All offsprings
are puple.
•
If different alleles are
present, then 50% of the
gametes will receive one
allele and 50% will
receive the other mean
half puple and half white
flower.
© 2014 Pearson Education, Inc.
The Law of Independent Assortment
• Mendel’s first testcross experiments followed only a
single character, such as flower color.
• All the F1 progeny produced in these crosses were
monohybrids, heterozygous for one character.
• A cross between two heterozygotes is a monohybrid
cross.
• Mendel identified the 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
• In one such dihybrid cross, Mendel studied the
inheritance of seed color and seed shape.
• A dihybrid cross, a cross between F1 dihybrids, can
determine whether two characters are transmitted to
offspring as a package or independently
© 2014 Pearson Education, Inc.
A Dihybrid Cross:
Seed Color x Seed Shape
• From his earlier experiments Mendel knew that the allele for yellow
seeds is dominant (Y) and the allele for green seeds is recessive (y).
• For seed shape, the allele for round seed shape is dominant (R) and
the allele for wrinkled is recessive (r).
• Mendel crossed two true-breeding pea varieties that differ in both
color and seed shape:
– Yellow round seeds (YYRR) x Green wrinkled seeds (yyrr)
– The F1 will be dihybrids, heterozygous for 2 characters (YyRr)
Are these 2 characters transmitted from parents to offspring as a
package (Y and R alleles always stay together and y and r
always stay together) or are seed color and seed shape inherited
independently?
© 2014 Pearson Education, Inc.
Dihybrid Cross between F1
• YyRr F1’s will have the yellow
round phenyotype no matter which
hypothesis is correct.
• What happens when the F1’s self
pollinate and produce F2 offspring?
• Dependent Assortment hypothesis2 classes of gametes produced:
YR and yr so F2 phenotype will be
in 3:1 (3 yellow round:1 green
wrinkled)
Independent Assortment
hypothesis4 classes of gametes produced:
YR, Yr, yR and yr
16 equally probable ways Y and R
can combine so F1 phenotype will
be in 9:3:3:1
9 yellow round
3 green round
3 yellow wrinkled
1 green wrinkled
© 2014 Pearson Education, Inc.
Figure 14.8
• 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.
• Strictly speaking, this law applies only to
genes on different, nonhomologous
chromosomes
• Genes located near each other on the same
chromosome tend to be inherited together.
© 2014 Pearson Education, Inc.
The laws of probability govern
Mendelian inheritance
• Mendel’s laws of segregation and independent
assortment reflect the same laws of probability that apply
to tossing coins or rolling dice.
• Values of probability range from 0 (an event with no
chance of occurring) to 1 (an event that is certain to
occur).
• The probability of tossing heads with a normal coin is
1/2.
• The outcome of one coin toss has no impact on the
outcome of the next toss. Each toss is an independent
event, just like the distribution of alleles into gametes.
• Like a coin toss, each ovum from a heterozygous parent
has a 1/2 chance of carrying the dominant allele and a
1/2 chance of carrying the recessive allele.
• The same probabilities apply to the sperm.
© 2014 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
– Probability in an F1
monohybrid cross can be
determined using the
multiplication rule
• Segregation in a heterozygous
plant is like flipping a coin:
Each gamete has a 1 chance
2
of carrying the dominant
allele
and a 1 chance of carrying the
2
recessive allele
Figure 14.9
© 2014 Pearson Education, Inc.
The Multiplication and Addition Rules
Applied to Monohybrid Crosses
• Each gamete has a 12 chance
of carrying the dominant allele
and a 12 chance of carrying the
recessive allele
• The probability that a
heterozygous pea plant (Pp)
will self-fertilize to produce a
white-flowered offspring (pp) is
the probability that a sperm
with a white allele will fertilize
an ovum with a white allele.
• So, the probability of a white F2 plant
is 12 X 12 = 1/4
© 2014 Pearson Education, Inc.
The Multiplication and Addition Rules
Applied to Monohybrid Crosses
•
•
•
•
We can use the addition rule to
determine the probability that an
F2 plant from a monohybrid cross
will be heterozygous rather than
homozygous.
The probability of an event that
can occur in two or more different
ways is the sum of the individual
probabilities of those ways.
The probability of obtaining an F2
heterozygote by combining the
dominant allele from the egg (P)
and the recessive allele from the
sperm (p) is 1⁄4.
The probability of combining the
recessive allele from the egg (p)
and the dominant allele from the
sperm (P) is also 1⁄4.
• Using the rule of addition,
we can calculate the
probability of an F2
heterozygote as 1⁄4 + 1⁄4 =
1⁄2.
© 2014 Pearson Education, Inc.
The rule of multiplication applies to
dihybrid crosses
• For a heterozygous parent
(YyRr), the probability of
producing a YR gamete is
1/2 × 1/2 = 1/4.
• We can now predict the
probability of a particular
F2 genotype without
constructing a 16-part
Punnett square.
• The probability that an F2
plant from heterozygous
parents will have a YYRR
genotype is 1/16 (1/4
chance for a YR ovum ×
1/4 chance for a YR
sperm).
© 2014 Pearson Education, Inc.
Solving Complex Genetics Problems with the
Rules of Probability- A Dihybrid Cross YyRr
•
•
•
Seed color: Remember, in a monohybrid
cross of Yy plants the probability of the
offspring (F2) genotypes are:
– ¼ for YY, ½ for Yy, and ¼ for yy
Seed Shape: The same probabilities apply
to the offspring (F2) genotype for seed
shape:
– ¼ for RR, ½ for Rr, and ¼ for rr
Use the multiplication rule to determine the
probability of each of the F2 genotypes in a
dihybrid YyRr cross:
– For YYRR = ¼ (YY) x ¼(RR) = 1/16
– For YyRR = ½ (Yy) x ¼ (RR) = 1/8
– For yyRR = ¼ (yy) x ¼ (RR) = 1/16
– For yyrr = ¼ (yy) x ¼ (rr) = 1/16
– For YyRr = ½ (yy) x ½ (Rr) = ¼
– For YYRr = ¼ (YY) x ½ (Rr) = 1/8
– For YYrr = ¼ (YY) x ¼ (rr) = 1/16
– For Yyrr = ½ (Yy) x ¼ (rr) = 1/8
– For yyRr = ¼ (yy) x ½ (Rr) = 1/8
© 2014 Pearson Education, Inc.
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
• 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
© 2014 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
© 2014 Pearson Education, Inc.
Degrees of Dominance
• Complete dominance occurs when
phenotypes of the heterozygote and
dominant homozygote are identical
– Mendel’s pea experiments.
• In incomplete dominance the phenotype of
F1 hybrids is somewhere between the
phenotypes of the two parental varieties in
Snapdragon.
• In codominance two dominant alleles affect
the phenotype in separate, distinguishable
ways in human blood cell.
© 2014 Pearson Education, Inc.
Incomplete Dominance
• Some alleles show incomplete dominanceheterozygotes show a distinct intermediate phenotype
not seen in homozygotes.
• This is not blending inheritance because the traits are
separable (particulate), as shown in further crosses.
• Offspring of a cross between heterozygotes show three
phenotypes: each parental phenotype and the
heterozygous phenotype.
• The phenotypic and genotypic ratios are
identical: 1:2:1.
• A clear example of incomplete dominance is the flower
color of snapdragons.
• A cross between a white-flowered plant and a redflowered plant produces all pink F1 offspring.
• Self-pollination of the F1 offspring produces 25% white,
25% red, and 50% pink F2 offspring.
© 2014 Pearson Education, Inc.
Figure 14.10-3
Incomplete
dominance
in snapdragon
flower color
How do we know that this is not an example of blending?
P Generation
Red
CRCR
White
CWCW
CR
Gametes
CW
F1 Generation
Pink
CRCW
Gametes
1
2
CR
1
2
CW
Sperm
F2 Generation
1
1
2
2
CR
1
2
CW
CR
Eggs
1
2
C RC R
C RC W
C RC W
CW CW
CW
Codominance
• For example, the M, N, and MN blood groups of
humans are due to the presence of two specific
molecules on the surface of red blood cells.
• People of group M (genotype MM) have one
type of molecule on their red blood cells, people
of group N (genotype NN) have the other type,
and people of group MN (genotype MN) have
both molecules present.
• The MN phenotype is not intermediate between
M and N phenotypes but rather exhibits both the
M and the N phenotype.
© 2014 Pearson Education, Inc.
The Relationship Between Dominance and
Phenotype
• It is important to recognize that a dominant allele
does not somehow subdue a recessive allele.
• When a dominant allele coexists with a recessive
allele in a heterozygote, they do not interact at all.
• 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
© 2014 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
– At the biochemical level, the phenotype (i.e., the
enzyme activity level) is incompletely dominant
– At the molecular level, the alleles are
codominant
© 2014 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
• 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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Multiple Alleles
•
•
•
•
•
•
•
•
The ABO blood groups in humans are
determined by three alleles: IA, IB, and i.
Both the IA and IB alleles are dominant to
the i allele.
The IA and IB alleles are codominant to
each other.
Because each individual carries two
alleles, there are six possible genotypes
and four possible blood types.
Individuals who are IAIA or IAi are type A
and have type A carbohydrates on the
surface of their red blood cells. Can
donate blood to A and AB.
Individuals who are IBIB or IBi are type B
and have type B carbohydrates on the
surface of their red blood cells. Can
donate blood to B and AB.
Individuals who are IAIB are type AB and
have both type A and type B
carbohydrates on the surface of their red
blood cells. Can donate to AB but
receive from all others. Type AB is
universal recipient.
Individuals who are ii are type O and have
neither carbohydrate on the surface of
their red blood cells. Can donate to all
others. Type O is universal donor.
© 2014 Pearson Education, Inc.
Figure 14.11
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
© 2014 Pearson Education, Inc.
Extending Mendelian Genetics for Two or
More Genes
• Some traits may be determined by two or
more genes
• Epistasis and polygenic inheritance are
situations in which two or more genes are
involved in determining phenotype.
© 2014 Pearson Education, Inc.
Epistasis
• In 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
© 2014 Pearson Education, Inc.
BbEe
Eggs
1/
1/
1/
1/
Sperm
1/ BE
4
1/
BbEe
4 bE
1/
4 Be
1/
4 be
Figure 14.12
4 BE
4 bE
4 Be
4
BBEE
BbEE
BBEe
BbEe
BbEE
bbEE
BbEe
bbEe
BBEe
BbEe
BBee
Bbee
BbEe
bbEe
Bbee
bbee
be
9
: 3
: 4
Polygenic Inheritance
• Quantitative characters are those that vary in
the population in a gradual fashion
• Quantitative variation 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
•
A cross between two AaBbCc individuals (with
intermediate skin shade) produces offspring with a
wide range of shades.
© 2014 Pearson Education, Inc.
Figure 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 phenotypic range is broadest for polygenic
characters
• Traits that depend on multiple genes combined with
environmental influences are called multifactorial
• 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
© 2014 Pearson Education, Inc.
The Environmental Impact on Phenotype
• 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
© 2014 Pearson Education, Inc.
Integrating a Mendelian View of Heredity
and Variation
• An organism’s phenotype includes its
physical appearance, internal anatomy,
physiology, and behavior
• An organism’s phenotype reflects its
overall genotype and unique
environmental history
© 2014 Pearson Education, Inc.
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
© 2014 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
• Information about the presence or absence of a particular phenotypic
trait is collected from as many individuals in a family as possible,
across generations.
• The distribution of these characters is then mapped on the family tree.
– For example, the occurrence of a widow’s peak (W) is dominant to
a straight hairline (w).
• Phenotypes of family members and knowledge of
dominance/recessiveness relationships between alleles allow
researchers to predict the genotypes of members of this family.
– For example, if an individual in the third generation lacks a widow’s
peak but both her parents have widow’s peaks, then her parents
must be heterozygous for that gene.
– If some siblings in the second generation lack a widow’s peak and one of
the grandparents (first generation) also lacks one, then we know the other
grandparent must be heterozygous, and we can determine the genotype of
many other individuals.
© 2014 Pearson Education, Inc.
Figure 14.15
Key
Male
Female
1st generation
(grandparents)
Ww
Affected
male
ww
2nd generation
(parents, aunts,
Ww ww ww Ww
and uncles)
Affected
female
ww
Ww
Ww
ww
Offspring, in
birth order
(first-born on left)
Mating
Ff
FF or Ff ff
Ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd generation
(two sisters)
WW
or
Ww
ww
Widow’s peak
No widow’s peak
(a) Is a widow’s peak 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
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant
or recessive trait?
Recessively Inherited Disorders
• Many genetic disorders are inherited in a
recessive manner.
• These range from relatively mild to life-threatening
• Recessively inherited disorders show up only in
individuals homozygous for the recessive allele
• Carriers are heterozygous individuals who carry
the recessive allele but are phenotypically normal
• 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
© 2014 Pearson Education, Inc.
Recessively Inherited Disorders
Albinism
• Albinism is a
recessive
condition
characterized by
a lack of
pigmentation in
skin and hair
• Most recessive
homozygotes are
born to parents
who are carriers
of the disorder,
but themselves
lack the disorder
Figure 14.16
© 2014 Pearson Education, Inc.
Recessively Inherited Disorders 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
– 1 in 25 people (4%) of European descent are
carriers
• The cystic fibrosis allele 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
© 2014 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
• 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 less susceptible to the malaria
parasite, so there is an advantage to being
heterozygous
© 2014 Pearson Education, Inc.
Recessively Inherited Disorders
Sickle-Cell Disease
• The disease is caused by the substitution of a
single amino acid in the hemoglobin protein in red
blood cells. Glutamine is replaced by Valine.
• The non sickle allele is incompletely dominant to
the sickle-cell allele.
• Carriers are said to have sickle-cell trait, about
1/10.
• These individuals are usually healthy, although
some may suffer symptoms of sickle-cell disease
under blood-oxygen stress.
© 2014 Pearson Education, Inc.
Figure 14.17
Sickle-cell alleles
Low O2
Sickle-cell
hemoglobin
proteins
Part of a fiber of
sickle-cell hemoglobin proteins
Sicklecell
disease
Sickled red
blood cells
(a) Homozygote with sickle-cell disease: Weakness, anemia, pain and fever,
organ damage
Sickle-cell allele
Normal allele
Sicklecell
trait
Very low O2
Sickle-cell and
normal hemoglobin proteins
Part of a sickle-cell
fiber and normal
hemoglobin proteins
Sickled and
normal red
blood cells
(b) Heterozygote with sickle-cell trait: Some symptoms when blood oxygen is
very low; reduction of malaria symptoms
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
– Heterozygous individuals have the dwarf phenotype.
– The 99.99% of the population who are not
achondroplastic dwarfs are homozygous recessive for
this trait.
– Achondroplasia is another example of a trait for which
the recessive allele is far more prevalent than the
dominant allele.
© 2014 Pearson Education, Inc.
Figure 14.18
Parents
Dwarf
Dd
Normal
dd
Sperm
D
d
d
Dd
Dwarf
dd
Normal
d
Dd
Dwarf
dd
Normal
Eggs
Dominantly Inherited Disorders
• A lethal dominant allele can escape elimination if it causes death at a
relatively advanced age, after the individual has already passed on
the lethal allele to his or her children.
• One example is Huntington’s disease, a degenerative disease of
the nervous system (1:10,000 people)
– The dominant lethal allele has no obvious phenotypic effect
until the individual is about 35 to 45 years old.
– Then the deterioration of the nervous system is irreversible
and inevitably fatal.
• Any child born to a parent who has the allele for Huntington’s
disease has a 50% chance of inheriting the disease and the
disorder.
• Recently, molecular geneticists have used pedigree
analysis of affected families to track the Huntington’s
allele to a locus near the tip of chromosome 4.
• The gene has now been sequenced.
• This has led to the development of a test that can detect the presence
of the Huntington’s allele in an individual’s genome.
© 2014 Pearson Education, Inc.
Multifactorial Disorders
• Many diseases, such as heart disease,
diabetes, alcoholism, mental illnesses,
and cancer have both genetic and
environmental components
• No matter what our genotype, our
lifestyle has a tremendous effect on
phenotype
• Little is understood about the genetic
contribution to most multifactorial
diseases
© 2014 Pearson Education, Inc.
Counseling Based on Mendelian Genetics and
Probability Rules
• Genetic counselors can provide information to prospective parents
concerned about a family history for a specific disease
• Using family histories, genetic counselors help couples determine the
odds that their children will have genetic disorders
• It is important to remember that each child represents an independent
event in the sense that its genotype is unaffected by the genotypes of
older siblings
• For a growing number of diseases, tests are available that identify
carriers and help define the odds more accurately
• The tests enable people to make more informed decisions about
having children
• However, they raise other issues, such as whether affected individuals
fully understand their genetic test results
• Probabilities are predicted on the most accurate information at the
time; predicted probabilities may change as new information is
available
© 2014 Pearson Education, Inc.
In Utero Fetal Testing
• In amniocentesis, the liquid that bathes the fetus is
removed and tested usually between 14-16 weeks
– Amniocentesis can be used to assess whether the fetus has a
specific disease.
• Fetal cells extracted from amniotic fluid are cultured and
karyotyped to identify some disorders.
• Other disorders can be identified from chemicals in the amniotic
fluids.
• 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
© 2014 Pearson Education, Inc.
(a) Amniocentesis
Ultrasound
monitor
(b) Chorionic villus sampling (CVS)
Figure 14.19
1
Amniotic
fluid
withdrawn
Fetus
Placenta
Uterus
Cervix
Fluid
Fetal
cells
Ultrasound
monitor
Fetus
Placenta
Chorionic
villi
Uterus
Centrifugation
Several
hours
Biochemical
2 Several
and genetic
tests
weeks
1
Cervix
Several
hours
Suction
tube
inserted
through
cervix
Fetal cells
2
Several
weeks
Several
hours
3
Karyotyping
Figure 14.19
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
• One common test is for
phenylketonuria (PKU), a recessively
inherited disorder that occurs in one of
every 10,000–15,000 births in the
United States
© 2014 Pearson Education, Inc.
Making a histogram and analyzing a distribution pattern of
GENE
Relationship among
alleles of a single gene
Complete dominance
of one allele
Description
Heterozygous phenotype
same as that of homozygous dominant
Incomplete dominance Heterozygous phenotype
intermediate between
of either allele
the two homozygous
phenotypes
Codominance
Both phenotypes
expressed in
heterozygotes
Example
PP
Pp
CRCR CRCW CWCW
IAIB
Multiple alleles
In the whole population, ABO blood group alleles
some genes have more
IA, IB, i
than two alleles
Pleiotropy
One gene is able to affect Sickle-cell disease
multiple phenotypic
characters
© 2014 Pearson Education, Inc.