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Chapter 13
Meiosis and Sexual
Life Cycles
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: Hereditary Similarity and Variation
• Living organisms are distinguished by their ability
to reproduce their own kind
• Heredity is the transmission of traits from one
generation to the next
• Variation shows that offspring differ in appearance
from parents and siblings
• Genetics is the scientific study of heredity and
variation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 13.1: Offspring acquire genes from
parents by inheriting chromosomes
• In a literal sense, children do not inherit particular
physical traits from their parents
• It is genes that are actually inherited
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Inheritance of Genes
• Genes are the units of heredity
• Genes are segments of DNA
• Each gene has a specific locus on a certain
chromosome
• One set of chromosomes is inherited from each
parent
• Reproductive cells called gametes (sperm and
eggs) unite, passing genes to the next generation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Comparison of Asexual and Sexual Reproduction
• In asexual reproduction, one parent produces
genetically identical offspring by mitosis
• In sexual reproduction, two parents give rise to
offspring that have unique combinations of genes
inherited from the two parents
Video: Hydra Budding
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 13-2
Parent
Bud
0.5 mm
Concept 13.2: Fertilization and meiosis alternate
in sexual life cycles
• A life cycle is the generation-to-generation
sequence of stages in the reproductive history of
an organism
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sets of Chromosomes in Human Cells
• Each human somatic cell (any cell other than a
gamete) has 46 chromosomes arranged in pairs
• A karyotype is an ordered display of the pairs of
chromosomes from a cell
• The two chromosomes in each pair are called
homologous chromosomes, or homologues
• Both chromosomes in a pair carry genes
controlling the same inherited characteristics
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 13-3
Pair of homologous
chromosomes
Centromere
Sister
chromatids
5 µm
• The sex chromosomes are called X and Y
• Human females have a homologous pair of X
chromosomes (XX)
• Human males have one X and one Y chromosome
• The 22 pairs of chromosomes that do not
determine sex are called autosomes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Each pair of homologous chromosomes includes
one chromosome from each parent
• The 46 chromosomes in a human somatic cell are
two sets of 23: one from the mother and one from
the father
• The number of chromosomes in a single set is
represented by n
• A cell with two sets is called diploid (2n)
• For humans, the diploid number is 46 (2n = 46)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 13-4
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosomes
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
• Gametes are haploid cells, containing only one set
of chromosomes
• For humans, the haploid number is 23 (n = 23)
• Each set of 23 consists of 22 autosomes and a
single sex chromosome
• In an unfertilized egg (ovum), the sex
chromosome is X
• In a sperm cell, the sex chromosome may be
either X or Y
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Behavior of Chromosome Sets in the Human Life Cycle
• At sexual maturity, the ovaries and testes produce
haploid gametes
• Gametes are the only types of human cells
produced by meiosis, rather than mitosis
• Meiosis results in one set of chromosomes in each
gamete
• Fertilization, the fusing of gametes, restores the
diploid condition, forming a zygote
• The diploid zygote develops into an adult
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 13-5
Key
Haploid gametes (n = 23)
Haploid (n)
Ovum (n)
Diploid (2n)
Sperm
cell (n)
MEIOSIS
Ovary
FERTILIZATION
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
The Variety of Sexual Life Cycles
• The alternation of meiosis and fertilization is
common to all organisms that reproduce sexually
• The three main types of sexual life cycles differ in
the timing of meiosis and fertilization
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• In animals, meiosis produces gametes, which
undergo no further cell division before fertilization
• Gametes are the only haploid cells in animals
• Gametes fuse to form a diploid zygote that divides
by mitosis to develop into a multicellular organism
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 13-6
Key
Haploid
Diploid
n
Gametes
n
Mitosis
n
MEIOSIS
Haploid multicellular
organism (gametophyte)
n
FERTILIZATION
Diploid
multicellular
organism
Animals
Zygote 2n
Mitosis
Mitosis
Mitosis
n
n
n
Spores
Gametes
MEIOSIS
2n
n
Haploid multicellular
organism
n
n
n
Gametes
Diploid
multicellular
organism
(sporophyte)
n
FERTILIZATION
MEIOSIS
2n
Mitosis
n
2n
Mitosis
Plants and some algae
Zygote
FERTILIZATION
2n
Zygote
Most fungi and some protists
• Plants and some algae exhibit an alternation of
generations
• This life cycle includes two multicellular
generations or stages: one diploid and one haploid
• The diploid organism, the sporophyte, makes
haploid spores by meiosis
• Each spore grows by mitosis into a haploid
organism called a gametophyte
• A gametophyte makes haploid gametes by mitosis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 13-6b
Key
Haploid
Diploid
Haploid multicellular
organism (gametophyte)
Mitosis
n
n
Mitosis
n
n
n
Spores
Gametes
MEIOSIS
2n
Diploid
multicellular
organism
(sporophyte)
FERTILIZATION
2n
Mitosis
Plants and some algae
Zygote
• In most fungi and some protists, the only diploid
stage is the single-celled zygote; there is no
multicellular diploid stage
• The zygote produces haploid cells by meiosis
• Each haploid cell grows by mitosis into a haploid
multicellular organism
• The haploid adult produces gametes by mitosis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 13-8b
MEIOSIS II: Separates sister chromatids
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
Cleavage
furrow
Two haploid cells
form; chromosomes
are still double
METAPHASE II
ANAPHASE II
Sister chromatids
separate
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing single chromosomes
LE 13-9
MITOSIS
MEIOSIS
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Propase
Prophase I
Chromosome
replication
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
2n = 6
Chromosomes
positioned at the
metaphase plate
Metaphase
Anaphase
Telophase
Sister chromatids
separate during
anaphase
2n
Tetrad formed by
synapsis of homologous
chromosomes
Tetrads
positioned at the
metaphase plate
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Metaphase I
Anaphase I
Telophase I
Haploid
n=3
Daughter
cells of
meiosis I
2n
MEIOSIS II
Daughter cells
of mitosis
n
n
n
Daughter cells of meiosis II
Sister chromatids separate during anaphase II
n
Property
Mitosis
Meiosis
DNA
replication
Divisions
During
interphase
One
During
interphase
Two
Synapsis and
crossing over
Daughter cells,
genetic
composition
Do not occur
Role in animal
body
Produces cells
for growth and
tissue repair
Form tetrads in
prophase I
Four haploid,
different from
parent cell and
each other
Produces
gametes
Two diploid,
identical to
parent cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Origins of Genetic Variation Among Offspring
• The behavior of chromosomes during meiosis and
fertilization is responsible for most of the variation
that arises in each generation
• Three mechanisms contribute to genetic variation:
– Independent assortment of chromosomes
– Crossing over
– Random fertilization
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Crossing Over
• Crossing over produces recombinant
chromosomes, which combine genes inherited
from each parent
• Crossing over begins very early in prophase I, as
homologous chromosomes pair up gene by gene
• In crossing over, homologous portions of two
nonsister chromatids trade places
• Crossing over contributes to genetic variation by
combining DNA from two parents into a single
chromosome
Animation: Genetic Variation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 13-11
Nonsister
chromatids
Prophase I
of meiosis
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Recombinant
chromosomes
Random Fertilization
• Random fertilization adds to genetic variation
because any sperm can fuse with any ovum
(unfertilized egg)
• The fusion of gametes produces a zygote with any
of about 64 trillion diploid combinations
• Crossing over adds even more variation
• Each zygote has a unique genetic identity
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Evolutionary Significance of Genetic Variation
Within Populations
• Natural selection results in accumulation of
genetic variations favored by the environment
• Sexual reproduction contributes to the genetic
variation in a population, which ultimately
results from mutations
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Mendel’s Experimental, Quantitative Approach
• Advantages of pea plants for genetic study:
– There are many varieties with distinct heritable
features, or characters (such as color);
character variations are 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-2
Removed stamens
from purple flower
Transferred spermbearing pollen from
stamens of white
flower to eggbearing carpel of
purple flower
Parental
generation
(P)
Carpel
Stamens
Pollinated carpel
matured into pod
Planted seeds
from pod
First
generation
offspring
(F1)
Examined
offspring:
all purple
flowers
• Mendel chose to track only those characters that
varied in an “either-or” manner
• He also used varieties that were “true-breeding”
(plants that produce offspring of the same variety
when they self-pollinate)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• In a typical experiment, Mendel mated two
contrasting, true-breeding varieties, a process
called hybridization
• The true-breeding parents are the P generation
• The hybrid offspring of the P generation are called
the F1 generation
• When F1 individuals self-pollinate, the F2
generation is produced
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Law of Segregation
• When Mendel crossed contrasting, true-breeding
white and purple flowered pea plants, all of the F1
hybrids were purple
• When Mendel crossed the F1 hybrids, many of the
F2 plants had purple flowers, but some had white
• Mendel discovered a ratio of about three to one,
purple to white flowers, in the F2 generation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-3
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
F2 Generation
All plants had
purple flowers
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-4
Allele for purple flowers
Locus for flower-color gene
Allele for white flowers
Homologous
pair of
chromosomes
• 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
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The third concept is that if the two alleles at a
locus differ, then one (the dominant allele)
determines the organism’s appearance, and
the other (the recessive allele) has no
noticeable effect on appearance
• In the flower-color example, the F1 plants had
purple flowers because the allele for that trait is
dominant
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-5_2
P Generation
Appearance:
Genetic makeup:
Purple
flowers
PP
White
flowers
pp
P
p
Gametes
F1 Generation
Appearance:
Genetic makeup:
Purple flowers
Pp
1
Gametes:
2
1
P
p
2
F1 sperm
P
p
PP
Pp
Pp
pp
F2 Generation
P
F1 eggs
p
3
:1
Useful Genetic Vocabulary
• An organism with two identical alleles for a
character is said to be homozygous for the gene
controlling that character
• An organism that has two different alleles for a
gene is said to be heterozygous for the gene
controlling that character
• Unlike homozygotes, heterozygotes are not truebreeding
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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 © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-7
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype:
pp
If Pp,
then 2 offspring purple
and 1 2 offspring white:
If PP,
then all offspring
purple:
p
1
p
P
p
p
Pp
Pp
pp
pp
P
Pp
Pp
P
P
Pp
Pp
LE 14-8
P Generation
YYRR
yyrr
Gametes YR
yr
YyRr
F1 Generation
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
Sperm
1
Sperm
1
2
YR
1
2
yr
1
1
2
2
1
4
Yr
1
4
yR
1
4
yr
YR
4
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
YYRR
1
YR
Eggs
Eggs
F2 Generation
(predicted
offspring)
4
YyRr
1
Yr
4
yr
YyRr
3
4
yyrr
1
1
yR
4
4
1
Phenotypic ratio 3:1
yr
4
9
16
3
16
3
16
3
16
Phenotypic ratio 9:3:3:1
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Multiplication and Addition Rules Applied to
Monohybrid Crosses
• Segregation in a heterozygous plant is like
flipping a coin: Each gamete has a 1/2 chance
of carrying the dominant allele and a 1/2
chance of carrying the recessive allele
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-9
Rr
Rr
Segregation of
alleles into eggs
Segregation of
alleles into sperm
Sperm
1
R
2
R
1
2
1
r
2
R
R
R
1
r
1
4
4
Eggs
r
r
1
2
R
r
1
4
r
1
4
The Spectrum of Dominance
• Complete dominance occurs when phenotypes
of the heterozygote and dominant homozygote are
identical
• In codominance, two dominant alleles affect the
phenotype in separate, distinguishable ways
• In incomplete dominance, the phenotype of F1
hybrids is somewhere between the phenotypes of
the two parental varieties
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-10
P Generation
Red
CRCR
White
CWCW
CR
Gametes
CW
Pink
CRCW
F1 Generation
Gametes
1
1
F2 Generation
2
CR
2
CR
1
2
1
CW
Sperm
2
CW
Eggs
1
1
2
2
CR
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
Copyright © 2005 Pearson Education, Inc. publishing as 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
• 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 dominant allele in the
population
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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 © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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 © 2005 Pearson Education, Inc. publishing as 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 pigment color
and c for no pigment color ) determines whether
the pigment will be deposited in the hair
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-11
BbCc
BbCc
Sperm
1
1
1
1
1
4
BC
1
4
bC
1
4
1
Bc
4
bc
4
BC
BBCC
BbCC
BBCc
BbCc
4
bC
BbCC
bbCC
BbCc
bbCc
4
Bc
BBCc
BbCc
BBcc
Bbcc
4
bc
BbCc
bbCc
Bbcc
bbcc
9
16
3
16
4
16
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
• Skin color in humans is an example of polygenic
inheritance
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-12
AaBbCc
aabbcc
20/64
Fraction of progeny
15/64
6/64
1/64
Aabbcc
AaBbCc
AaBbcc AaBbCc AABbCc AABBCc AABBCC
Nature and Nurture: The Environmental Impact
on Phenotype
• Another departure from Mendelian genetics
arises when the phenotype for a character
depends on environment as well as genotype
• The norm of reaction is the phenotypic range of
a genotype influenced by the environment
• For example, hydrangea flowers of the same
genotype range from blue-violet to pink,
depending on soil acidity
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Norms of reaction are generally broadest for
polygenic characters
• Such characters are called multifactorial because
genetic and environmental factors collectively
influence phenotype
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-14a
Ww
ww
ww
Ww ww ww Ww
WW
or
Ww
Ww
Ww
ww
Dominant trait (widow’s peak)
Second generation
(parents plus aunts
and uncles)
Third
generation
(two sisters)
ww
Widow’s peak
First generation
(grandparents)
No widow’s peak
LE 14-14b
First generation
(grandparents)
Second generation
(parents plus aunts
and uncles)
Ff
FF or Ff ff
Third
generation
(two sisters)
Attached earlobe
Recessive trait (attached earlobe)
Ff
ff
ff
Ff
Ff
ff
FF
or
Ff
Ff
ff
Free earlobe
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 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
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Sickle-Cell Disease
• 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
• Symptoms include physical weakness, pain, organ
damage, and even paralysis
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Dominantly Inherited Disorders
• Some human disorders are due to dominant
alleles
• One example is achondroplasia, a form of
dwarfism that is lethal when homozygous for the
dominant allele
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Huntington’s disease is a degenerative disease of
the nervous system
• The disease has no obvious phenotypic effects
until about 35 to 40 years of age
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Multifactorial Disorders
• Many diseases, such as heart disease and
cancer, have both genetic and environment
components
• Little is understood about the genetic contribution
to most multifactorial diseases
Copyright © 2005 Pearson Education, Inc. publishing as 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, ultrasound and fetoscopy, allow
fetal health to be assessed visually in utero
Video: Ultrasound of Human Fetus I
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 14-17a
Amniocentesis
Amniotic
fluid
withdrawn
Fetus
A sample of
amniotic fluid can
be taken starting at
the 14th to 16th
week of pregnancy.
Centrifugation
Placenta
Uterus
Cervix
Fluid
Fetal
cells
Biochemical tests can be
performed immediately on
the amniotic fluid or later
on the cultured cells.
Fetal cells must be cultured
for several weeks to obtain
sufficient numbers for
karyotyping.
Biochemical
tests
Several
weeks
Karyotyping
LE 14-17b
Chorionic villus sampling (CVS)
A sample of chorionic villus
tissue can be taken as early
as the 8th to 10th week of
pregnancy.
Fetus
Suction tube
inserted through
cervix
Placenta Chorionic villi
Fetal
cells
Biochemical
tests
Several
hours
Karyotyping
Karyotyping and biochemical
tests can be performed on
the fetal cells immediately,
providing results within a day
or so.
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
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