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Transcript
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

AP Biology
Chromosomal Abnormalities
 Nondisjunction
 chromosomes don’t separate properly
during meiosis
 Chromosomal Mutations
 deletion
 duplication
 inversion
 translocation
AP Biology
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

AP Biology
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
AP Biology
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
AP Biology
Alteration of chromosome number
error in Meiosis 1
error in Meiosis 2
all with incorrect number
AP Biology
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
AP Biology
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
AP Biology
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
AP Biology
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




AP Biology
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
AP Biology
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

AP Biology
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)
AP Biology
AP Biology
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%
AP Biology
Morgan’s Testcross showing linkage:
AP Biology
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
AP Biology
AP Biology