Genetic Disorders
AP Biology
2006-2007
Chromosomal abnormalities
 Incorrect number of chromosomes

nondisjunction
 chromosomes don’t separate properly
during meiosis

breakage of chromosomes
 deletion
 duplication
 inversion
 translocation
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Nondisjunction
 Problems with meiotic spindle cause errors in
daughter cells



2n
homologous chromosomes do not separate
properly during Meiosis 1
sister chromatids fail to separate during Meiosis 2
too many or too few chromosomes
n-1
n
n+1
n
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Alteration of chromosome number
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Nondisjunction
 Baby has wrong chromosome number

trisomy
 cells have 3 copies of a chromosome

n+1
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monosomy
 cells have only 1 copy of a chromosome
n-1
n
n
trisomy
monosomy
2n+1
2n-1
Human chromosome disorders
 High frequency in humans
most embryos are spontaneously aborted
 alterations are too disastrous
 developmental problems result from
biochemical imbalance

 imbalance in regulatory molecules?
 Certain conditions are tolerated
upset the balance less = survivable
 characteristic set of symptoms = syndrome

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Box #1
 What are two causes of
nondisjunction?
 What is the outcome?
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Down syndrome
 Trisomy 21
3 copies of chromosome 21
 1 in 700 children born in U.S.

 Chromosome 21 is the
smallest human chromosome

but still severe effects
 Frequency of Down
syndrome correlates
with the age of the mother
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Down syndrome & age of mother
Mother’s age
Incidence of
Down Syndrome
Under 30
<1 in 1000
30
1 in 900
35
1 in 400
36
1 in 300
37
1 in 230
38
1 in 180
39
1 in 135
40
1 in 105
42
1 in 60
44
1 in 35
46
1 in 20
48
1 in 16
49
1 in 12
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Rate of miscarriage due to
amniocentesis:
 1970s data
0.5%, or 1 in 200 pregnancies
 2006 data
<0.1%, or 1 in 1600 pregnancies
Genetic testing
 Amniocentesis in 2nd trimester
sample of embryo cells
 stain & photograph chromosomes

 Analysis of karyotype
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Sex chromosomes abnormalities
 Human development more tolerant of
wrong numbers in sex chromosome
 But produces a variety of distinct
syndromes in humans




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XXY = Klinefelter’s syndrome male
XXX = Trisomy X female
XYY = Jacob’s syndrome male
XO = Turner syndrome female
Klinefelter’s syndrome
 XXY male
one in every 2000 live births
 have male sex organs, but
are sterile
 feminine characteristics

 some breast development
 lack of facial hair
tall
 normal intelligence

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Klinefelter’s syndrome
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Jacob’s syndrome male
 XYY Males
1 in 1000 live male
births
 extra Y chromosome
 slightly taller than
average
 more active
 normal intelligence, slight learning disabilities
 delayed emotional immaturity
 normal sexual development

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Trisomy X
 XXX
1 in every 2000 live births
 produces healthy females

 Why?
 Barr bodies
 all but one X chromosome is inactivated
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Turner syndrome
 Monosomy X or X0
1 in every 5000 births
 varied degree of effects
 webbed neck
 short stature
 sterile

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Box #2
 Describe two types of sex chromosome
abnormalities
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replication
error of
Changes in chromosome structure
 deletion

 duplication
crossing over

error of
loss of a chromosomal segment
repeat a segment
 inversion

reverses a segment
 translocation

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move segment from one chromosome
to another
Certain human genetic disorders can be
attributed to the inheritance of single
gene traits or chromosomal changes.
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Sickle cell anemia is a hereditary
blood disorder caused by a single
nucleotide substitution that results
in abnormally shaped hemoglobin.
•
•
•
•
•
Fatigue
Rapid heart rate
Strokes
Painful joints
Delayed growth
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Sickle cell disease is more common in
those of African descent, due to
heterozygote advantage. (People who
are heterozygous for sickle cell have
less chance of acquiring malaria.)
HAHA Normal blood cells
HAHS Heterozygote; less
likely
to get malaria
HSHS Sickle cell disease
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Box #3
 What are the advantages and
disadvantages of the sickle cell trait
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Huntington’s disease is an autosomal
dominant disorder that causes degeneration
of the nervous system. Affected individuals
usually begin showing symptoms at 35-40.
•
•
•
•
•
•
•
Hallucinations
Psychosis
Dementia
Jerking movements
Confusion
Rigidity
Death
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Color blindness is caused by a sex-linked
recessive mutation. Because of this, males
are twice as likely to inherit this trait as
females.
• Trouble seeing colors
and the brightness of
colors.
• Trouble differentiating
between shades of the
same color or similar
colors.
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Tay-Sachs disease is an autosomal
recessive disorder of the nervous
system, more prevalent in
the Ashkenazi Jewish population.
•
•
•
•
Deafness
Blindness
Dementia
Eventual
paralysis
• Seizures
• Death, usually
before age 4 or 5.
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Cherry-red spot
Many ethical, social and medical issues
surround human genetic disorders.
Genetic screening for Tay-Sachs
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Ethical Considerations of Genetics Testing:
 Possible discrimination by employers




or health insurers
The need for ethical standards for work
with human research subjects or
tissues
Consideration of social, cultural and
religious perspectives on genetics and
health
Marketing of genetics tests
Informed consent
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Box #4
 Describe your stance on genetic
testing.
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The Human Genome Project sequenced
the entire human genome.
 An international collaboration between


hundreds of labs in the United States,
the UK, Japan, France, Germany and
Spain.
Began in 1990; completed in 2000.
Mostly funded by the US Department of
Energy, National Institute of Health and
private interest groups.
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The Human Genome Project can help us
understand diseases by:
Genotyping of specific viruses to direct appropriate
treatment
 Identification of oncogenes and mutations linked
to different forms of cancer
 The design of medication and more accurate
prediction of their effects
 Advancement in forensics
 Biofuels and other energy applications;
 agriculture,
 Comparative sequencing to establish evolutionary
relationships (such as BLAST)
 The commercial development of genomics
research related to DNA based products (a
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multibillion dollar industry).
Box #5
 Describe the purpose of the human
genome project.
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Historical Ethical, Legal, and Social
Issues (ELSI) of Genetics
Eugenics advocates
improving
human hereditary
traits through
selective
reproduction of more
desired people and
traits, and reduced
reproduction of less
desired people and
traits.
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Sterilization Laws were practiced by
many states during the 1900s.
 Compulsory



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sterilization of mental
health patients
Men sterilized to treat
aggression and criminal
behavior
Coerced sterilization of
many ethnic groups
The American eugenics
movement was the
model for Nazi eugenics
policies.
Amniocentesis is a medical procedure used
in prenatal diagnosis of chromosomal abnormalities.
A small amount of amniotic fluid, which contains
fetal tissues, is sampled and the fetal DNA is
examined for genetic abnormalities. The test is
usually performed between the 15th – 20th week of
pregnancy.
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Amniocentesis can lead to
Selective Reduction of
Multiple Embryos
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Nadya Suleman , known as
Octomom in the media, is an
American woman who came to
international attention when
she gave birth to octuplets in
January 2009. Suleman
octuplets are only the second
full set of octuplets to be born
alive in the United States. The
circumstances of their high
order multiple birth have led to
controversy in the field
of assisted reproductive
technology as well as an
investigation by the Medical
Board of California of the
fertility specialist involved.
Actress Angelia Jolie made the headlines when
she underwent a prophylactic mastectomy after
finding out that she was genetically predisposed to
breast cancer.
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The Genetic Information
Nondiscrimination Act (GINA)
The Genetic Information Nondiscrimination Act
of 2008, also referred to as GINA, is a new
federal law that protects Americans from being
treated unfairly because of differences in their
DNA that may affect their health. The new law
prevents discrimination from health insurers
and employers.
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Can a gene be patented? Not according
to the US Supreme Court.
• In Association for Molecular Pathology vs.
Myriad Genetics, Inc., the Supreme Court
ruled that a natural gene cannot be
patented. However, synthetic genes,
such as cDNA are patentable. (June 2013)
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Box # 6
 Do you agree with the supreme courts
decision? Explain
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AP Biology
2006-2007