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Errors of Meiosis
Chromosomal Abnormalities
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Chromosomal abnormalities
Incorrect number of chromosomes nondisjunction
chromosomes dont separate properly
during meiosis
breakage of chromosomes
deletion
duplication
inversion translocation
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Nondisjunction
Problems with meiotic spindle cause errors indaughter cells
homologous chromosomesdo not separateproperly during Meiosis 1
sister chromatidsfail to separate during Meiosis 2 too many or too few chromosomes
2n n
n
n-1
n+1
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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
monosomy cells have only 1 copy of a chromosome
n+1 n
monosomy2n-1
n-1 n
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Human chromosome disorders
High frequency in humans most embryos are spontaneously aborted
alterations are too disastrous
developmental problems result frombiochemical imbalance
imbalance in regulatory molecules?
Certain conditions are tolerated
upset the balance less = survivable
characteristic set of symptoms = syndrome
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Down syndrome
Trisomy 21 3 copies of chromosome 21
1 in 700 children born in U.S.
Chromosome 21 is thesmallest human chromosome
but still severe effects
Frequency of Downsyndrome correlateswith the age of the mother
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Down syndrome & age of mother
Mothers age
Incidence of
Down Syndrome
Under 30
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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 ofwrong numbers in sex chromosome
But produces a variety of distinct
syndromes in humans XXY= Klinefelters syndrome male
XXX= Trisomy X female
XYY= Jacobs syndrome male
XO = Turner syndrome female
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XXYmale 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
Klinefelters syndrome
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Klinefelters syndrome
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Jacobs syndrome male
XYYMales 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
M onosomy X or X0 1 in every 5000 births
varied degree of effects
webbed neck short stature
sterile
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Changes in chromosome structure
deletion loss of a chromosomal segment
duplication
repeat a segment
inversion
reverses a segment
translocation
move segment from one chromosome
to another
errorof
r
eplication
erro
rof
crossin
go
ver
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