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Chapter 14: The Human Genome
Section 14-1 Human Chromosomes
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14–1 Human Heredity
Human Chromosomes
Human Chromosomes
Cell biologists analyze chromosomes by looking at
karyotypes.
Cells are photographed during mitosis. Scientists
then cut out the chromosomes from the
photographs and group them together in pairs.
A picture of chromosomes arranged in this way is
known as a karyotype.
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14–1 Human Heredity
Human Chromosomes
A Human Karyotype
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14–1 Human Heredity
Human Chromosomes
Two of the 46 human chromosomes are known as
sex chromosomes, because they determine an
individual's sex.
• Females have two copies of an X chromosome.
• Males have one X chromosome and one Y
chromosome.
The remaining 44 chromosomes are known as
autosomal chromosomes, or autosomes.
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14–1 Human Heredity
Human Chromosomes
How is sex determined?
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Human Chromosomes
Males and females are
born in a roughly 50 : 50
ratio because of the
way in which sex
chromosomes
segregate during
meiosis.
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14–1 Human Heredity
Human Traits
Human Traits
In order to apply Mendelian genetics to humans,
biologists must identify an inherited trait controlled
by a single gene.
They must establish that the trait is inherited and
not the result of environmental influences.
They have to study how the trait is passed from
one generation to the next.
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14–1 Human Heredity
Human Traits
Pedigree Charts
A pedigree chart shows the relationships within a
family.
Genetic counselors analyze pedigree charts to
infer the genotypes of family members.
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14–1 Human Heredity
A horizontal line
connecting a
male and a
female
represents a
marriage.
A circle
represents
a female.
Human Traits
A square
represents
a male.
A vertical line and a
bracket connect the
parents to their
children.
A circle or square
that is not
shaded indicates
that a person
does not express
the trait.
A shaded circle or square
indicates that a person
expresses the trait.
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14–1 Human Heredity
Human Traits
Genes and the Environment
Some obvious human traits are almost impossible
to associate with single genes.
Traits, such as the shape of your eyes or ears,
eye color, height (e), skin color (e), weight (e),
and intelligence (e) are polygenic, meaning they
are controlled by many genes.
Many of your personal traits are only partly
governed by genetics and they do not follow
Mendel’s laws of inheritance.
(e) = also subject to environmental influences
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14–1 Human Heredity
Human Genes
Human Genes
The human genome includes tens of thousands of
genes.
In 2003, the DNA sequence of the human genome
was published. (Took ~ 10 years to complete)
Humans have about 25,000 genes each with
3,000 base pairs. Scientists estimate that there
are about 3 billion base pairs in the (haploid)
human genome.
In a few cases, biologists were able to identify
genes that directly control a single human trait
such as blood type.
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14–1 Human Heredity
Human Genes
Blood Group Genes
Human blood comes in a variety of genetically
determined blood groups.
A number of genes are responsible for human
blood groups.
The best known are the ABO blood groups and the
Rh blood groups.
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14–1 Human Heredity
Human Genes
The Rh (Rhesus factor) blood group is determined by
a single gene with two alleles—positive and negative.
The positive (Rh+) allele is dominant, so individuals
who are Rh+/Rh+ or Rh+/Rh are said to be Rh positive.
About 85% of all people are positive and contain the
protein (or antigen).
Individuals with two Rh- alleles are said to be Rh
negative.
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14–1 Human Heredity
Human Genes
ABO blood group
• There are three alleles for this gene, IA, IB, and i.
• Alleles IA and IB are codominant.
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14–1 Human Heredity
Human Genes
Individuals with alleles IA and IB produce both A and
B antigens, making them blood type AB.
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14–1 Human Heredity
Human Genes
The i allele is recessive.
Individuals with alleles IAIA or IAi produce only the A
antigen, making them blood type A.
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14–1 Human Heredity
Human Genes
Individuals with IBIB or IBi alleles are type B.
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14–1 Human Heredity
Human Genes
Individuals who are homozygous for the i allele (ii)
produce no antigen and are said to have blood
type O.
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14–1 Human Heredity
Human Genes
Recessive Alleles
The presence of a normal, functioning gene is
revealed only when an abnormal or
nonfunctioning allele affects the phenotype.
Some disorders are caused by autosomal
recessive alleles.
Each parent must contribute a recessive allele
for the disorder to present itself in the offspring.
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14–1 Human Heredity
Human Genes
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14–1 Human Heredity
Human Genes
Dominant Alleles
The effects of a dominant allele are expressed
even when the recessive allele is present.
Two examples of genetic disorders caused by
autosomal dominant alleles are Achondroplasia
and Huntington disease.
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14–1 Human Heredity
Human Genes
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14–1 Human Heredity
Human Genes
Codominant Alleles
Sickle cell disease is a serious disorder caused by a
co-dominant allele.
A person who is co-dominant has the normal allele
and the sickle-cell allele (heterozygous) for the
disease and does not present the sickle-cell
symptoms.
A person who is homozygous for the sickle-cell trait
presents the sickle-cell trait.
Sickle cell is found in about 1 out of 500 African
Americans.
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14–1 Human Heredity
Human Genes
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14–1 Human Heredity
From Gene to Molecule
From Gene to Molecule
How do small changes in DNA cause genetic
disorders?
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14–1 Human Heredity
From Gene to Molecule
In both cystic fibrosis and sickle cell
disease, a small change in the DNA of a
single gene affects the structure of a
protein, causing a serious genetic disorder.
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14–1 Human Heredity
From Gene to Molecule
Cystic Fibrosis
Cystic fibrosis is caused by a recessive allele.
Sufferers of cystic fibrosis produce a thick, heavy
mucus that clogs their lungs and breathing
passageways.
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14–1 Human Heredity
From Gene to Molecule
The most common allele
that causes cystic fibrosis
is missing 3 DNA bases.
As a result, the amino
acid phenylalanine is
missing from the CFTR
protein.
(Cystic fibrosis
transmembrane
conductance regulator)
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14–1 Human Heredity
From Gene to Molecule
Normal CFTR is a
chloride ion channel in
cell membranes.
Abnormal CFTR
cannot be transported
to the cell membrane.
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14–1 Human Heredity
From Gene to Molecule
The cells in the
person’s airways are
unable to transport
chloride ions.
As a result, the
airways become
clogged with a thick
mucus.
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14–1 Human Heredity
From Gene to Molecule
Sickle Cell Disease
Sickle cell disease is a
common genetic
disorder found in
African Americans.
It is characterized by
the bent and twisted
shape of the red blood
cells.
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14–1 Human Heredity
From Gene to Molecule
Hemoglobin is the protein in red blood cells that
carries oxygen.
In the sickle cell allele, just one DNA base is
changed.
As a result, the abnormal hemoglobin is less soluble
than normal hemoglobin.
Low oxygen levels cause some red blood cells to
become sickle shaped.
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14–1 Human Heredity
From Gene to Molecule
People who are heterozygous for the sickle cell allele
are generally healthy and they are resistant to
malaria.
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14–1 Human Heredity
From Gene to Molecule
There are three phenotypes associated with the
sickle cell gene.
An individual with both normal and sickle cell alleles
has a different phenotype—resistance to malaria—
from someone with only normal alleles.
Sickle cell alleles are thought to be codominant.
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14–1 Human Heredity
From Gene to Molecule
Malaria and the Sickle Cell Allele
Regions where malaria is
common
Regions where the sickle
cell allele is common
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14–1
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14–1
A chromosome that is not a sex chromosome is
know as a(an)
a. autosome.
b. karyotype.
c. pedigree.
d. chromatid.
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14–1
Whether a human will be a male or a female is
determined by which
a. sex chromosome is in the egg cell.
b. autosomes are in the egg cell.
c. sex chromosome is in the sperm cell.
d. autosomes are in the sperm cell.
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14–1
Mendelian inheritance in humans is typically
studied by
a. making inferences from family pedigrees.
b. carrying out carefully controlled crosses.
c. observing the phenotypes of individual
humans.
d. observing inheritance patterns in other
animals.
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14–1
An individual with a blood type phenotype of O
can receive blood from an individual with the
phenotype
a. O.
b. A.
c. AB.
d. B.
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14–1
The ABO blood group is made up of
a. two alleles.
b. three alleles.
c. identical alleles.
d. dominant alleles.
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END OF SECTION
Chapter 14: The Human Genome
Section 14-2 Human Genetic Disorders
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14–2 Human Chromosomes
Sex-Linked Genes
Sex-Linked Genes
The X chromosome and the Y chromosomes
determine sex.
Genes located on these chromosomes are called
sex-linked genes.
More than 100 sex-linked genetic disorders have
now been mapped to the X chromosome.
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14–2 Human Chromosomes
Sex-Linked Genes
X Chromosome
The Y chromosome is
much smaller than the X
chromosome and
appears to contain only a
few genes.
Duchenne muscular
dystrophy
Melanoma
X-inactivation center
X-linked severe combined
immunodeficiency (SCID)
Colorblindness
Hemophilia
Y Chromosome
Testis-determining
factor
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14–2 Human Chromosomes
Sex-Linked Genes
Why are sex-linked disorders more common in
males than in females?
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14–2 Human Chromosomes
Sex-Linked Genes
For a recessive allele to be expressed in females,
there must be two copies of the allele, one on each
of the two X chromosomes.
Males have just one X chromosome. Thus, all
X-linked alleles are expressed in males, even
if they are recessive.
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14–2 Human Chromosomes
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14–2 Human Chromosomes
Sex-Linked Genes
Colorblindness
Three human genes associated with color vision
are located on the X chromosome.
In males, a defective version of any one of these
genes produces colorblindness.
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14–2 Human Chromosomes
Sex-Linked Genes
Possible Inheritance of
Colorblindness Allele
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14–2 Human Chromosomes
Sex-Linked Genes
Hemophilia
The X chromosome also carries genes that help
control blood clotting. A recessive allele in either of
these two genes may produce hemophilia.
In hemophilia, a protein necessary for normal
blood clotting is missing.
Hemophiliacs can bleed to death from cuts and
may suffer internal bleeding if bruised.
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14–2 Human Chromosomes
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14–2 Human Chromosomes
Sex-Linked Genes
Duchenne Muscular Dystrophy
Duchenne muscular dystrophy is a sex-linked
disorder that results in the weakening and loss of
skeletal muscle.
It is caused by a defective version of the gene that
codes for a muscle protein.
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14–2 Human Chromosomes
X-Chromosome Inactivation
X-Chromosome Inactivation
British geneticist Mary Lyon discovered that in
female cells, one X chromosome is randomly
switched off.
This chromosome forms a dense region in the
nucleus known as a Barr body.
Barr bodies are generally not found in males
because their single X chromosome is still active.
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14–2 Human Chromosomes
Chromosomal Disorders
Chromosomal Disorders
What problems does nondisjunction
cause?
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14–2 Human Chromosomes
Chromosomal Disorders
The most common error in meiosis occurs when
homologous chromosomes fail to separate.
This is known as nondisjunction, which means,
“not coming apart.”
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14–2 Human Chromosomes
Chromosomal Disorders
If nondisjunction occurs, abnormal numbers
of chromosomes may find their way into
gametes, and a disorder of chromosome
numbers may result.
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14–2 Human Chromosomes
Nondisjunction
Chromosomal Disorders
Homologous
chromosomes
fail to
separate.
Meiosis I:
Nondisjunction
Meiosis II
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14–2 Human Chromosomes
Chromosomal Disorders
Down Syndrome
If two copies of an autosomal chromosome fail to
separate during meiosis, an individual may be born
with three copies of a chromosome.
Down syndrome involves three copies of
chromosome 21.
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14–2 Human Chromosomes
Down syndrome
produces mild to severe
mental retardation.
Chromosomal Disorders
Down Syndrome Karyotype
It is characterized by:
• increased
susceptibility to
many diseases
• higher frequency of
some birth defects
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14–2 Human Chromosomes
Chromosomal Disorders
Sex Chromosome Disorders
In females, nondisjunction can lead to Turner’s
syndrome.
A female with Turner’s syndrome usually inherits
only one X chromosome (karyotype 45,X).
Women with Turner’s syndrome are sterile.
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14–2 Human Chromosomes
Chromosomal Disorders
In males, nondisjunction causes Klinefelter’s
syndrome (karyotype 47,XXY).
The extra X chromosome interferes with meiosis and
usually prevents these individuals from reproducing.
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14–2
The average human gene consists of how many
base pairs of DNA?
a. 3000
b. 300
c. 20
d. 30,000
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14–2
Which of the following genotypes indicates an
individual who is a carrier for colorblindness?
a. XCX
b. XCXc
c. XcY
d. XCY
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14–2
Colorblindness is much more common in males
than in females because
a. the recessive gene on the male’s single X
chromosome is expressed.
b. genes on the Y chromosome make genes on
the X chromosome more active.
c. females cannot be colorblind.
d. colorblindness is dominant in males and
recessive in females.
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14–2
The presence of a dense region in the nucleus
of a cell can be used to determine the
a. sex of an individual.
b. blood type of an individual.
c. chromosome number of an individual.
d. genotype of an individual.
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14–2
Nondisjunction occurs during
a. meiosis I.
b. mitosis.
c. meiosis II.
d. between meiosis I and II.
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END OF SECTION
Chapter 14: The Human Genome
Section 14-3 Studying the Human Genome
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14–3 Human Molecular Genetics
Human DNA Analysis
Human DNA Analysis
There are roughly 3 billion (haploid) base pairs in
your DNA.
Biologists search the human genome using
sequences of DNA bases.
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Human DNA Analysis
There are over 1,000 genetic tests available for
hundreds of disorders.
DNA testing can pinpoint the exact genetic basis of
a disorder.
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14–3 Human Molecular Genetics
Some Currently Available DNA-Based Gene Tests
•Alpha-1-antitrypsin deficiency (AAT; emphysema and liver disease)
•Amyotrophic lateral sclerosis (ALS; Lou Gehrig's Disease; progressive motor function loss leading to paralysis and death)
•Alzheimer's disease* (APOE; late-onset variety of senile dementia)
•Ataxia telangiectasia (AT; progressive brain disorder resulting in loss of muscle control and cancers)
•Gaucher disease (GD; enlarged liver and spleen, bone degeneration)
•Inherited breast and ovarian cancer* (BRCA 1 and 2; early-onset tumors of breasts and ovaries)
•Hereditary nonpolyposis colon cancer* (CA; early-onset tumors of colon and sometimes other organs)
•Central Core Disease (CCD; mild to severe muscle weakness)
•Charcot-Marie-Tooth (CMT; loss of feeling in ends of limbs)
•Congenital adrenal hyperplasia (CAH; hormone deficiency; ambiguous genitalia and male pseudohermaphroditism)
•Cystic fibrosis (CF; disease of lung and pancreas resulting in thick mucous accumulations and chronic infections)
•Duchenne muscular dystrophy/Becker muscular dystrophy (DMD; severe to mild muscle wasting, deterioration, weakness)
•Dystonia (DYT; muscle rigidity, repetitive twisting movements)
•Emanuel Syndrome (severe mental retardation, abnormal development of the head, heart and kidney problems)
•Fanconi anemia, group C (FA; anemia, leukemia, skeletal deformities)
•Factor V-Leiden (FVL; blood-clotting disorder)
•Fragile X syndrome (FRAX; leading cause of inherited mental retardation)
•Galactosemia (GALT; metabolic disorder affects ability to metabolize galactose)
•Hemophilia A and B (HEMA and HEMB; bleeding disorders)
•Hereditary Hemochromatosis (HFE; excess iron storage disorder)
•Huntington's disease (HD; usually midlife onset; progressive, lethal, degenerative neurological disease)
•Marfan Syndrome (FBN1; connective tissue disorder; tissues of ligaments, blood vessel walls, cartilage, heart valves and other
structures abnormally weak)
•Mucopolysaccharidosis (MPS; deficiency of enzymes needed to break down long chain sugars called glycosaminoglycans; corneal
clouding, joint stiffness, heart disease, mental retardation)
•Myotonic dystrophy (MD; progressive muscle weakness; most common form of adult muscular dystrophy)
•Neurofibromatosis type 1 (NF1; multiple benign nervous system tumors that can be disfiguring; cancers)
•Phenylketonuria (PKU; progressive mental retardation due to missing enzyme; correctable by diet)
•Polycystic Kidney Disease (PKD1, PKD2; cysts in the kidneys and other organs)
•Adult Polycystic Kidney Disease (APKD; kidney failure and liver disease)
•Prader Willi/Angelman syndromes (PW/A; decreased motor skills, cognitive impairment, early death)
•Sickle cell disease (SS; blood cell disorder; chronic pain and infections)
•Spinocerebellar ataxia, type 1 (SCA1; involuntary muscle movements, reflex disorders, explosive speech)
•Spinal muscular atrophy (SMA; severe, usually lethal progressive muscle-wasting disorder in children)
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•Tay-Sachs Disease (TS; fatal neurological disease of early childhood; seizures, paralysis)
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•Thalassemias (THAL; anemias - reduced red blood cell levels)
•Timothy Syndrome (CACNA1C; characterized by severe cardiac arrhythmia, webbing of the fingers and toes called syndactyly, autism)
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14–3 Human Molecular Genetics
Human DNA Analysis
DNA Fingerprinting
DNA fingerprinting analyzes sections of DNA that
have little or no known function but vary widely
from one individual to another.
Only identical twins are genetically identical.
DNA samples can be obtained from blood, sperm,
and hair strands with tissue at the base.
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Human DNA Analysis
Chromosomes
contain large
amounts of DNA
called repeats that do
not code for proteins.
This DNA pattern
varies from person to
person.
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14–3 Human Molecular Genetics
Human DNA Analysis
Restriction enzymes are used to cut the DNA into
fragments containing genes and repeats.
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14–3 Human Molecular Genetics
Human DNA Analysis
DNA fragments are
separated using gel
electrophoresis.
Fragments containing
repeats are labeled.
This produces a series
of bands—the DNA
fingerprint.
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14–3 Human Molecular Genetics
Human DNA Analysis
DNA Fingerprint
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14–3 Human Molecular Genetics
The Human Genome Project
The Human Genome Project
What is the goal of the Human Genome
Project?
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14–3 Human Molecular Genetics
The Human Genome Project
In 1990, scientists in the United States and
other countries began the Human Genome
Project.
The Human Genome Project is an ongoing
effort to analyze the human DNA
sequence.
In June 2000, a working copy of the human
genome was essentially complete.
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The Human Genome Project
Research groups are analyzing the DNA sequence,
looking for genes that may provide clues to the basic
properties of life.
Biotechnology companies are looking for information
that may help develop new drugs and treatments for
diseases.
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The Human Genome Project
A Breakthrough for Everyone
Data from publicly supported research on the
human genome have been posted on the Internet
on a daily basis.
You can read and analyze the latest genome data.
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14–3 Human Molecular Genetics
Gene Therapy
Gene Therapy
What is gene therapy?
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14–3 Human Molecular Genetics
Gene Therapy
In gene therapy, an absent or faulty
gene is replaced by a normal, working
gene.
The body can then make the correct
protein or enzyme, eliminating the cause of
the disorder.
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14–3 Human Molecular Genetics
Viruses are often used
because of their ability to
enter a cell’s DNA.
Gene Therapy
Normal hemoglobin gene
Virus particles are
modified so that they
cannot cause disease.
Genetically engineered virus
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Gene Therapy
A DNA fragment containing a replacement gene is
spliced to viral DNA.
Bone marrow cell
Nucleus
Chromosomes
Genetically engineered virus
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Gene Therapy
The patient is then infected with the modified virus
particles, which should carry the gene into cells to
correct genetic defects.
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14–3
DNA fingerprinting analyzes sections of DNA
that have
a. Little or no known function but are identical
from one individual to another.
b. little or no known function but vary widely
from one individual to another.
c. a function and are identical from one
individual to another.
d. a function and are highly variable from one
individual to another.
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14–3
DNA fingerprinting uses the technique of
a. gene therapy.
b. allele analysis.
c. gel electrophoresis.
d. gene recombination.
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14–3
Repeats are areas of DNA that
a. do not code for proteins.
b. code for proteins.
c. are identical from person to person.
d. cause genetic disorders.
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14–3
Data from the human genome project is
available
a. only to those who have sequenced the DNA.
b. to scientists who are able to understand the
data.
c. by permission to anyone who wishes to do
research.
d. to anyone with Internet access.
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14–3
Which statement most accurately describes
gene therapy?
a. It repairs the defective gene in all cells of the
body.
b. It destroys the defective gene in cells where
it exists.
c. It replaces absent or defective genes with a
normal gene.
d. It promotes DNA repair through the use of
enzymes.
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END OF SECTION