Download 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
 Chromosomal Mutations
 deletion
 duplication
 inversion
 Translocation
 Nondisjunction
 chromosomes don’t separate properly
during meiosis
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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 causes 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
Down syndrome
 Trisomy 21
3 copies of chromosome 21
 1 in 700 children born in U.S.

 Frequency 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
 Karyotype of baby
 Chorionic villus sampling (CVS) – placental cells

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Polyploidy
 Having more than 2n chromosomes

Major method of speciation in plants
 As the number of chromosomes increase so
does the possible cell size
 ex: huge fruits!

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Often lethal in complex animals
Gene Linkage
 Genes found on same chromosome are
considered LINKED!
 How close or far away are they?
 Fewer gamete possibilities the closer they
are! (more will have same genotype as parent)
 Why? Less possibility for crossing over to
occur which creates variability.
 Independent assortment does not apply
 The expected ratio for the cross would be
significantly different in the observed.

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Ex: not following 9:3:3:1 or 1:1:1:1
Parental – genes are the same as parent
Recombinant – genes are different from parent
How do we test for linkage?
 Do a test cross!
 Compare the expected phenotypic
frequencies with the observed
phenotypic frequencies
 Use Chi square to compare data
APMader
Biology2007-08
Testcross showing no linkage!
Phenotypic ratio: 1 : 1 : 1 : 1
Frequency: 25% : 25%: 25% : 25%
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EXPECTED FREQUENCIES
Morgan’s Testcross showing linkage:
Parental Genotype: B+b vg+vg
Genes:
Body color
and wing
shape are
linked!
Pay attention
to
recombinant
numbers!
OBSERVED
FREQUENCIES
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What are the phenotypic frequencies now?
42% : 41% : 9% : 8%
Now… compare the offspring
phenotypic frequencies of
expected vs. observed
Expected:
Frequency: 25% : 25%: 25% : 25%
Observed:
Frequency: 42% : 41% : 9% : 8%
They DO NOT MATCH! Gene 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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Mapping Genes
 Start with the largest map distance and
work down from there.
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