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Chapter 15:
Human Genetics and Disorders
AP Biology
2006-2007
Human Chromosome Disorders
 Some have high frequency in humans
most embryos are spontaneously aborted
 developmental problems result from
biochemical imbalance

 Certain conditions are tolerated
upsets the balance less = survivable
 characteristic set of symptoms =
syndrome

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Chromosomal Abnormalities
 Nondisjunction
 chromosomes don’t separate properly
during meiosis
 Chromosomal Mutations
 deletion
 duplication
 inversion
 translocation
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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
Autosomal Disorders
 Dominant

Achondroplasia
 Recessive

 Dwarfism
 Melanin pathway
 Lethal in
mutated
 photosensitivity
homozygous cond.


Acromegaly

Phenylketoneuria
 Gigantism
 Cannot break down
 Overactive pituitary
phenylalanine
 Lead to mental
retardation
Huntington’s
 Degeneration of
nervous system
 Manifests around 40
years of age
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Albinism

Cystic fibrosis
 Cl- channel problem
 Mucus buildup
 Death around 40
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 - aneuploidy
n-1
n
n+1
n
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Alteration of chromosome number
error in Meiosis 1
error in Meiosis 2
all with incorrect number
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1/2 with incorrect number
Nondisjunction
 Cause of aneuploidy (wrong chromosome #)

trisomy
 cells have 3 copies of a chromosome

monosomy
 cells have only 1 copy of a chromosome
n+1
n
n
n-1
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trisomy
monosomy
2n+1
2n-1
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
Sex Chromosomes Abnormalities
 Human development more tolerant of
wrong numbers in sex chromosome

Cause is nondisjunction
 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
Genetic testing
 Amniocentesis in 2nd trimester
sample of embryo cells from fluid
 stain & photograph chromosomes

 Analysis of karyotype
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Polyploidy
 Occurs when eukaryotes have more
than 2n chromosomes
Major method of speciation in plants
 As the number of chromosomes
increase so does the possible cell size
ex: huge strawberries!
Often lethal in animals

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Gene Linkage
 Genes found on same chromosome are
considered LINKED!
 How close or far away are they?
 Fewer gamete possibilities the closer they
are!
 Why? Less possibility for crossing over to
occur.
 Independent assortment does not apply
 No linkage if genes are on separate
chromosomes (# of recombinants
increases)
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This shows linked genes with no crossing over and
crossing over.
How can we test to see if genes are linked?
Mendelian Testcross no linkage
Phenotypic ratio: 1 : 1 : 1 : 1
Frequency: 25% : 25%: 25% : 25%
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Morgan’s Testcross showing linkage:
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Chromosome Mapping
 Calculate the frequency of recombinant
offspring.
 Recombination frequency = # map units

Ex: 13% recombinant frequency = 13
map units
 Greater % = greater distance
 Lower % = closer distance
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