Human Genetics and Linked Genes
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Transcript Human Genetics and Linked Genes
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
Acondroplasia
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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