Chapter 14 - Mendel and the Gene Idea

Transcription

Figure 14.1
CAMPBELL
Drawing from the Deck of Genes
BIOLOGY
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
14
Mendel and the Gene Idea
!  What principles account for the passing of traits
from parents to offspring?
!  The “particulate” hypothesis is the idea that
parents pass on discrete heritable units (genes)
!  The “blending” hypothesis is the idea that genetic
material from the two parents blends together
(like blue and yellow paint blend to make green)
!  Mendel documented a particulate mechanism
through his experiments with garden peas
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
2
© 2014 Pearson Education, Inc.
3
Concept 14.1: Mendel used the scientific
approach to identify two laws of inheritance
Mendel’s Experimental, Quantitative Approach
4
Figure 14.1a
!  Mendel discovered the basic principles of heredity
by breeding garden peas in carefully planned
experiments
!  Mendel’s approach allowed him to deduce
principles that had remained elusive to others
!  Other advantages of using peas
!  Short generation time
!  A heritable feature that varies among individuals
(such as flower color) is called a character
!  Large numbers of offspring
!  Mating could be controlled; plants could be allowed
to self-pollinate or could be cross pollinated
!  Each variant for a character, such as purple or
white color for flowers, is called a trait
!  Peas were available to Mendel in many different
varieties
Mendel (third from right, holding a sprig of fuchsia)
with his fellow monks.
5
Figure 14.2
6
7
Technique
The Law of Segregation
1
2
!  Mendel chose to track only those characters that
occurred in two distinct alternative forms
Stamens
Parental
generation
(P)
!  He also used varieties that were true-breeding
(plants that produce offspring of the same variety
when they self-pollinate)
Carpel
3
4
9
Figure 14.3-2
Purple
flowers
!  The true-breeding parents are the P generation
!  When Mendel crossed the F1 hybrids, many of the
F2 plants had purple flowers, but some had white
10
Experiment
P Generation
(true-breeding
parents)
!  When Mendel crossed contrasting, true-breeding
white- and purple-flowered pea plants, all of the F1
hybrids were purple
!  When F1 individuals self-pollinate or crosspollinate with other F1 hybrids, the F2 generation
is produced
5
First filial
generation
offspring
(F1)
!  In a typical experiment, Mendel mated two
contrasting, true-breeding varieties, a process
called hybridization
!  The hybrid offspring of the P generation are called
the F1 generation
Results
Figure 14.3-1
P Generation
(true-breeding
parents)
White
flowers
Figure 14.3-3
Experiment
Purple
flowers
Experiment
F1 Generation
(hybrids)
All plants had purple flowers
Self- or cross-pollination
Purple
flowers
!  Mendel reasoned that only the purple flower factor
was affecting flower color in the F1 hybrids
White
flowers
!  Mendel called the purple flower color a dominant
trait and the white flower color a recessive trait
All plants had purple flowers
!  The factor for white flowers was not diluted or
destroyed because it reappeared in the F2
generation
Self- or cross-pollination
F2 Generation
13
705 purple-flowered
plants
14
12
P Generation
(true-breeding
parents)
White
flowers
!  Mendel discovered a ratio of about three to one,
purple to white flowers, in the F2 generation
11
F1 Generation
(hybrids)
8
224 white-flowered
plants
15
16
Table 14.1
Table 14.1a
Table 14.1b
!  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
17
18
19
20
Figure 14.4
Mendel s Model
!  Mendel developed a hypothesis to explain the 3:1
inheritance pattern he observed in F2 offspring
!  First: alternative versions of genes account for
variations in inherited characters
!  Four related concepts make up this model
!  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 concepts can be related to what we now
know about genes and chromosomes
Locus for
flower-color gene
!  These alternative versions of a gene are called
alleles
21
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
Absence of enzyme
!  Mendel made this deduction without knowing
about chromosomes
One allele
results in
sufficient
pigment
T A T T T A G C C A
!  Each gene resides at a specific locus on a specific
chromosome
!  Second: for each character, an organism inherits
two alleles, one from each parent
Enzyme
C T A A A T C G G T
Allele for
purple flowers
!  Alternatively, the two alleles at a locus may differ,
as in the F1 hybrids
ATAAATCGGT
22
!  The two alleles at a particular locus may be
identical, as in the true-breeding plants of
Mendel s P generation
23
24
Figure 14.5-1
P Generation
!  Third: 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
!  Fourth (the law of segregation): the two alleles
for a heritable character separate (segregate)
during gamete formation and end up in different
gametes
!  In the flower-color example, the F1 plants had
purple flowers because the allele for that trait
is dominant
!  Thus, an egg or a sperm gets only one of the two
alleles that are present in the organism
!  The model accounts for the 3:1 ratio observed in
the F2 generation of Mendel’s crosses
Gametes:
P
p
!  Possible combinations of sperm and egg can be
shown using a Punnett square
!  A capital letter represents a dominant allele, and a
lowercase letter represents a recessive allele
!  This segregation of alleles corresponds to the
distribution of homologous chromosomes to
different gametes in meiosis
25
26
Figure 14.5-2
27
28
Figure 14.5-3
P Generation
Gametes:
Appearance:
Purple flowers White flowers
Genetic makeup:
PP
pp
p
P
Gametes:
F1 Generation
p
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Useful Genetic Vocabulary
P Generation
Appearance:
Genetic makeup:
PP
pp
Appearance:
Genetic makeup:
Purple flowers
Pp
1
2
P
1
2
p
Purple flowers
Pp
Gametes:
1
2
P
1
2
p
Sperm from F1 (Pp) plant
F2 Generation
Eggs from
F1 (Pp) plant
P
p
P
p
PP
Pp
!  An organism with two identical alleles for a
character is homozygous for the gene controlling
that character
!  Because of the different effects of dominant and
recessive alleles, an organism s traits do not
always reveal its genetic composition
!  An organism that has two different alleles for a
gene is heterozygous for the gene controlling that
character
!  Therefore, we distinguish between an organism s
phenotype, or physical appearance, and its
genotype, or genetic makeup
!  Unlike homozygotes, heterozygotes are not truebreeding
!  In the example of flower color in pea plants, PP
and Pp plants have the same phenotype (purple)
but different genotypes
pp
Pp
3
29
Appearance:
Genetic makeup:
PP
pp
:1
30
31
32
Figure 14.6
3
Phenotype
Genotype
Purple
PP
(homozygous)
Purple
Figure 14.7
The Testcross
1
Pp
(heterozygous)
1
White
Ratio 3:1
Predictions
If purple-flowered or
parent is PP
Sperm
p
p
!  To determine the genotype we can carry out a
testcross: breeding the mystery individual with a
homozygous recessive individual
Pp
(heterozygous)
pp
(homozygous)
P
!  If any offspring display the recessive phenotype,
the mystery parent must be heterozygous
Eggs
Pp
Pp
Pp
Pp
!  Mendel derived the law of segregation by following
a single character
!  The F1 offspring produced in this cross were
monohybrids, heterozygous for one character
If purple-flowered
parent is Pp
Sperm
p
p
!  A cross between such heterozygotes is called
a monohybrid cross
P
Eggs
P
p
Pp
Pp
pp
pp
Results
1
Ratio 1:2:1
The Law of Independent Assortment
Recessive phenotype,
known genotype:
pp
Dominant phenotype,
unknown genotype:
PP or Pp?
!  An individual with the dominant phenotype could
be either homozygous dominant or heterozygous
2
Purple
Technique
or
33
All offspring purple
34
Figure 14.8
offspring purple and
offspring white
1 2
35
YYRR
P Generation
Figure 14.8b
Hypothesis of
dependent assortment
yyrr
Gametes YR
Experiment
Predictions
Hypothesis of
dependent assortment
2
YR
1
2
1
2
YR
2
yr
YYRR
YyRr
YyRr
yyrr
1
YR
4
4
Yr
4
yR
4
yr
Eggs
3
4
1
YR
1
4
Yr
1
4
P Generation
yR
1
4
1
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
1
YYRR
yyrr
1
2
YR
Gametes YR
yr
1
2
YyRR
1
yr
2
YyRr
1
yyrr
YyRr
1
4
1
Phenotypic ratio 3:1
9
16
YR
4
Yr
4
yR
1
3
16
Yyrr
yyRr
3
16
1
4
Yr
1
4
yR
1
4
yr
YYRR YYRr YyRR YyRr
YYRr
yr
4
9
YyRr
YR
YYrr
YyRr
Yyrr
YyRR YyRr
yyRR
yyRr
4
Phenotypic ratio 3:1
4
1
4
4
YyRr
Yyrr
yyRr
yyrr
yyRr
yyRR
YyRr
YR
Eggs
yr
3
F1 Generation
2
YYRR YyRr
Eggs
yr
yr
1
1
4
1
Sperm
1
Sperm
1
Sperm
Eggs
!  A dihybrid cross, a cross between F1 dihybrids,
can determine whether two characters are
transmitted to offspring as a package or
independently
Hypothesis of
independent assortment
or
1
Sperm
Predicted
offspring of
F2 generation
YyRr
Predicted
offspring of
F2 generation
Hypothesis of
independent assortment
yr
F1 Generation
!  Crossing two true-breeding parents differing in two
characters produces dihybrids in the F1
generation, heterozygous for both characters
36
Figure 14.8a
Experiment
!  Mendel identified his second law of inheritance by
following two characters at the same time
1 2
3
16
yyrr
1
16
3
16
1
16
Phenotypic ratio 9:3:3:1
16
Phenotypic ratio 9:3:3:1
Results
Results
37
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
38
39
Concept 14.2: Probability laws govern
Mendelian inheritance
The Multiplication and Addition Rules Applied
to Monohybrid Crosses
315
Figure 14.9
!  Using a dihybrid cross, Mendel developed the law
of independent assortment
!  Mendel’s laws of segregation and independent
assortment reflect the rules of probability
!  It states that each pair of alleles segregates
independently of each other pair of alleles during
gamete formation
!  When tossing a coin, the outcome of one toss has
no impact on the outcome of the next toss
!  Genes located near each other on the same
chromosome tend to be inherited together
41
!  The multiplication rule states that the probability
that two or more independent events will occur
together is the product of their individual
probabilities
42
Phenotypic ratio approximately 9:3:3:1
40
32
Rr
Segregation of
alleles into sperm
Sperm
1
R
2
Eggs
2
r
1
4
r
1
r
2
R
R
1
R
r
1
43
2
1
R
R
1
!  Segregation in a heterozygous plant is like flipping
a coin: Each gamete has a ½ chance of carrying
the dominant allele and a ½ chance of carrying the
recessive allele
101
Rr
Segregation of
alleles into eggs
!  Probability in an F1 monohybrid cross can be
determined using the multiplication rule
!  In the same way, the alleles of one gene
segregate into gametes independently of another
gene’s alleles
!  This law applies only to genes on different,
nonhomologous chromosomes or those far apart
on the same chromosome
108
4
4
r
r
1
4
44
Figure 14.UN01
Figure 14.UN02
Solving Complex Genetics Problems with the
Rules of Probability
!  The addition rule 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
!  We can apply the multiplication and addition rules
to predict the outcome of crosses involving
multiple characters
!  A 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
45
46
47
48
Figure 14.10-1
Concept 14.3: Inheritance patterns are often
more complex than predicted by simple
Mendelian genetics
Extending Mendelian Genetics for a Single Gene
!  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
!  When alleles are not completely dominant or
recessive
!  In incomplete dominance, the phenotype of F1
hybrids is somewhere between the phenotypes of
the two parental varieties
49
White
CWCW
Gametes
CR
CW
!  In codominance, two dominant alleles affect the
phenotype in separate, distinguishable ways
50
Figure 14.10-2
Red
C RC R
!  Complete dominance occurs when phenotypes
of the heterozygote and dominant homozygote are
identical
!  When a gene produces multiple phenotypes
P Generation
!  Inheritance of characters by a single gene may
deviate from simple Mendelian patterns in the
following situations:
!  When a gene has more than two alleles
!  However, the basic principles of segregation and
independent assortment apply even to more
complex patterns of inheritance
Degrees of Dominance
51
52
Figure 14.10-3
P Generation
P Generation
Red
C RC R
CR
Gametes
CW
F1 Generation
Gametes
Red
C RC R
White
CWCW
2
CR
1
2
CR
Gametes
2
1
1
2
CR
Eggs
1
1
CR
1
2
!  Alleles are simply variations in a gene s nucleotide
sequence
CW
!  For any character, dominance/recessiveness
relationships of alleles depend on the level at
which we examine the phenotype
Sperm
F2 Generation
53
!  A dominant allele does not subdue a recessive
allele; alleles don t interact that way
Pink
C RC W
Gametes
CW
The Relation Between Dominance and Phenotype
CW
F1 Generation
Pink
C RC W
1
White
CWCW
2
2
CR
1
2
CW
C RC R
C RC W
C RC W
CWCW
!  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
CW
54
55
56
Figure 14.11
Multiple Alleles
Frequency of Dominant Alleles
!  The allele for this unusual trait is dominant to the
allele for the more common trait of five digits per
appendage
!  Dominant alleles are not necessarily more
common in populations than recessive alleles
!  Most genes exist in populations in more than two
allelic forms
Allele
58
Pleiotropy
Extending Mendelian Genetics for Two or More
Genes
Epistasis
A
Genotype
59
IB
i
none
B
(b) Blood group genotypes and phenotypes
!  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
57
IA
Carbohydrate
!  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.
!  In this example, the recessive allele is far more
prevalent than the population’s dominant allele
!  For example, one baby out of 400 in the United
States is born with extra fingers or toes
(a) The three alleles for the ABO blood groups and their
carbohydrates
IAIA or IAi
IBIB or IBi
IAIB
ii
A
B
AB
O
Red blood cell
appearance
Phenotype
(blood group)
60
Figure 14.12
!  Most genes have multiple phenotypic effects, a
property called pleiotropy
!  Some traits may be determined by two or more
genes
BbEe
!  In epistasis, a gene at one locus alters the
phenotypic expression of a gene at a second locus
!  For example, pleiotropic alleles are responsible for
the multiple symptoms of certain hereditary
diseases, such as cystic fibrosis and sickle-cell
disease
1
4
1
BE
!  For example, in Labrador retrievers and many
other mammals, coat color depends on two genes
1
!  One gene determines the pigment color (with
alleles B for black and b for brown)
1
1
4
BE
62
63
bE
1
Be
4
1
4
4
bE
4
Be
4
be
BBEE
BbEE
BBEe
BbEe
BbEE
bbEE
BbEe
bbEe
BBEe
BbEe
BBee
Bbee
BbEe
bbEe
Bbee
bbee
9
61
4
be
Eggs
1
!  The other gene (with alleles E for color and e for
no color) determines whether the pigment will be
deposited in the hair
BbEe
Sperm
: 3
: 4
64
Figure 14.13
Figure 14.14
Polygenic Inheritance
AaBbCc
Sperm
!  Quantitative characters are those that vary in the
population along a continuum
1
1
1
!  Quantitative variation usually indicates polygenic
inheritance, an additive effect of two or more
genes on a single phenotype
1
1
Eggs
1
!  Skin color in humans is an example of polygenic
inheritance
1
1
1
Phenotypes:
1
8
1
8
8
1
1
8
8
1
1
8
1
8
!  Another departure from Mendelian genetics arises
when the phenotype for a character depends on
environment as well as genotype
8
8
8
!  The phenotypic range is broadest for polygenic
characters
8
8
8
!  Traits that depend on multiple genes combined
with environmental influences are called
multifactorial
8
8
8
1
Number of
dark-skin alleles:
65
Nature and Nurture: The Environmental Impact
on Phenotype
AaBbCc
6
64
0
15
64
1
64
2
20
64
15
3
6
64
4
1
64
5
64
66
6
Figure 14.14a
67
68
A Mendelian View of Heredity and Variation
Concept 14.4: Many human traits follow
Mendelian patterns of inheritance
Figure 14.14b
!  An organism’s phenotype includes its physical
appearance, internal anatomy, physiology, and
behavior
!  Humans are not good subjects for genetic
research
!  Generation time is too long
!  An organism’s phenotype reflects its overall
genotype and unique environmental history
!  Parents produce relatively few offspring
!  Breeding experiments are unacceptable
!  However, basic Mendelian genetics endures as
the foundation of human genetics
69
70
Figure 14.15
Pedigree Analysis
Key
Male
!  A pedigree is a family tree that describes the
interrelationships of parents and children across
generations
Female
Affected
male
Ww
ww
2nd generation
(parents, aunts,
Ww ww ww Ww
and uncles)
ww
Ww
Ww
ww
Ff
Ff
FF or Ff ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
Female
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
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?
Attached earlobe
Ww
Mating
Offspring, in
birth order
(first-born on left)
ww
Ww ww ww Ww
WW
or
Ww
Free earlobe
ww
Ww
Ww
ww
Widow’s peak
ww
Widow’s peak
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
74
Figure 14.15ab
Affected
male
Affected
female
3rd generation
(two sisters)
(b) Is an attached earlobe a dominant
or recessive trait?
73
Figure 14.15aa
Key
Male
Offspring, in
birth order
Mating
Affected
female
75
Figure 14.15b
Female
1st generation
(grandparents)
Affected
male
Affected
female
Ff
2nd generation
(parents, aunts,
and uncles)
FF or Ff ff
76
Figure 14.15ba
Key
Male
72
Figure 14.15a
1st generation
(grandparents)
!  Inheritance patterns of particular traits can be
traced and described using pedigrees
71
Figure 14.15bb
Mating
Offspring, in
birth order
Ff
ff
ff
Ff
Ff
ff
FF
or
Ff
Ff
ff
3rd generation
(two sisters)
No widow’s peak
77
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?
Free earlobe
Attached earlobe
78
79
80
Figure 14.16
Recessively Inherited Disorders
The Behavior of Recessive Alleles
!  Pedigrees can also be used to make predictions
about future offspring
!  Many genetic disorders are inherited in a
recessive manner
!  Recessively inherited disorders show up only in
individuals homozygous for the allele
!  We can use the multiplication and addition rules to
predict the probability of specific phenotypes
!  These range from relatively mild to life-threatening
!  Carriers are heterozygous individuals who carry
the recessive allele but are phenotypically normal;
most individuals with recessive disorders are born
to carrier parents
Parents
Normal
Aa
Sperm
81
82
A
a
A
AA
Normal
Aa
Normal
(carrier)
a
Aa
Normal
(carrier)
aa
Albino
Eggs
!  Albinism is a recessive condition characterized by
a lack of pigmentation in skin and hair
Normal
Aa
83
84
Cystic Fibrosis
Sickle-Cell Disease: A Genetic Disorder with
Evolutionary Implications
Figure 14.16a
!  If a recessive allele that causes a disease is rare,
then the chance of two carriers meeting and
mating is low
!  Cystic fibrosis is the most common lethal genetic
disease in the United States, striking one out of
every 2,500 people of European descent
!  Consanguineous matings (i.e., matings between
close relatives) increase the chance of mating
between two carriers of the same rare allele
!  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
!  Most societies and cultures have laws or taboos
against marriages between close relatives
85
86
Dominantly Inherited Disorders
Low O2
!  Heterozygotes are less susceptible to the malaria
parasite, so there is an advantage to being
heterozygous in regions where malaria is common
Sickle-cell
hemoglobin
proteins
89
Part of a fiber of
sickle-cell hemoglobin proteins
Sicklecell
disease
Sickled red
blood cells
Sickle-cell allele
Normal allele
Very low O2
Part of a sickle-cell
fiber and normal
hemoglobin proteins
Parents
!  Some human disorders are caused by dominant
alleles
Dwarf
Dd
Sicklecell
trait
!  Achondroplasia is a form of dwarfism caused by a
rare dominant allele
Sickled and
normal red
blood cells
(b) Heterozygote with sickle-cell trait: Some symptoms when blood oxygen is
very low; reduction of malaria symptoms
90
Normal
dd
Sperm
!  Dominant alleles that cause a lethal disease are
rare and arise by mutation
(a) Homozygote with sickle-cell disease: Weakness, anemia, pain and fever,
organ damage
Sickle-cell and
normal hemoglobin proteins
88
Figure 14.18
Sickle-cell alleles
!  About one out of ten African Americans has sicklecell trait, an unusually high frequency
!  Symptoms include physical weakness, pain, organ
damage, and even paralysis
87
Figure 14.17
!  Heterozygotes (said to have sickle-cell trait) are
usually healthy but may suffer some symptoms
!  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 mucus buildup in some internal
organs and abnormal absorption of nutrients in the
small intestine
!  Sickle-cell disease affects one out of 400 AfricanAmericans
D
d
d
Dd
Dwarf
dd
Normal
d
Dd
Dwarf
dd
Normal
Eggs
91
92
Multifactorial Disorders
Genetic Testing and Counseling
Figure 14.18a
!  The timing of onset of a disease significantly
affects its inheritance
!  Many diseases, such as heart disease, diabetes,
alcoholism, mental illnesses, and cancer have
both genetic and environmental components
!  Huntington’s disease is a degenerative disease
of the nervous system
!  Genetic counselors can provide information to
prospective parents concerned about a family
history for a specific disease
!  No matter what our genotype, our lifestyle has a
tremendous effect on phenotype
!  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
93
94
95
96
Figure 14.19
Counseling Based on Mendelian Genetics and
Probability Rules
Tests for Identifying Carriers
Fetal Testing
!  Using family histories, genetic counselors help
couples determine the odds that their children
will have genetic disorders
!  For a growing number of diseases, tests are
available that identify carriers and help define the
odds more accurately
!  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
!  The tests enable people to make more informed
decisions about having children
(a) Amniocentesis
Ultrasound
monitor
!  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
!  However, they raise other issues, such as whether
affected individuals fully understand their genetic
test results
(b) Chorionic villus sampling (CVS)
1
Amniotic
fluid
withdrawn
Fetus
Placenta
Chorionic
villi
Uterus
Fetus
Placenta
Uterus
Cervix
Fluid
!  Other techniques, such as ultrasound and
fetoscopy, allow fetal health to be assessed
visually in utero
Fetal
cells
Ultrasound
monitor
Centrifugation
Several
hours
Biochemical
2
and genetic
Several
tests
weeks
1
Suction
tube
inserted
through
cervix
Cervix
Several
hours
Fetal cells
2
Several
weeks
Several
hours
3
97
98
Video: Ultrasound of Human Fetus
Newborn Screening
99
100
Karyotyping
Figure 14.UN03a
Figure 14.UN03b
!  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
101
Relationship
among alleles
of a single gene
2
R
1
2
Heterozygous phenotype
same as that of homozygous dominant
Incomplete
dominance
of either allele
Heterozygous phenotype
intermediate between
the two homozygous
phenotypes
Codominance
r
1
15
64
20
2
15
64
3
6
64
4
64
1
5
64
6
Both phenotypes
expressed in
heterozygotes
104
Figure 14.UN07
Epistasis
PP
Description
Example
The phenotypic
expression of one
gene affects the
expression of
another gene
Pp
BbEe
Ww
BbEe
BE bE Be
ww
ww
Ww
be
BE
bE
Be
Ww
ww
ww
Ww
Ww
WW
or
Ww
ww
ww
be
CRCR CRCW CWCW
9
Polygenic
inheritance
IAIB
In the population
some genes have more
than two alleles
ABO blood group alleles
Pleiotropy
One gene affects
multiple phenotypic
characters
Sickle-cell disease
:3
:4
A single phenotypic
character is affected AaBbCc
by two or more
genes
AaBbCc
IA, IB, i
Widow’s peak
106
Figure 14.UN08
64
103
Relationship among
two or more genes
Example
Multiple alleles
105
0
6
Figure 14.UN06
Description
Complete
dominance
of one allele
Sperm
1
64
Figure 14.UN05
Rr
Segregation of
alleles into sperm
Number of
dark-skin alleles:
102
Figure 14.UN04
1
Phenotypes:
107
Figure 14.UN09
No widow’s peak
108
Figure 14.UN10
Figure 14.UN11
George
Arlene
Sickle-cell alleles
Low O2
Sickle-cell
hemoglobin
proteins
Part of a fiber of
sickle-cell hemoglobin proteins
Sicklecell
disease
Sandra
Tom
Sam
Sickled red
blood cells
Wilma
Ann
Michael
Carla
Daniel
Alan
Tina
Christopher
109
110
111
112

Chapter 14 - Mendel and the Gene Idea

Transcription

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