Errors in Meiosis
Karyotypes & Chromosomal Abnormalities
Karyotype
• A photograph of an
organism’s
chromosomes
• Homologous
chromosomes are
grouped together
• Based on size,
banding pattern, and
location of
centromere
Arranged from largest  smallest,
sex chromosomes at the end
Karyotype
• Helpful in finding
chromosomal
abnormalities
• Will NOT show genetic
traits or disorders
• Can NOT tell if baby has
blue eyes
• Can NOT tell if baby has
cystic fibrosis
• Can tell if baby has too
many chromosomes or
pieces missing
Chromosomal Abnormalities
• Nondisjunction
• Chromosome (or
chromosome pair)
fails to separate
during meiosis
• Results in an incorrect
number of
chromosomes for
daughter cells
Chromosomal Abnormalities
• Problems with meiotic spindle causes errors
• Homologous chromosomes do not separate properly during
Meiosis I
• Sister chromatids do not separate properly during Meiosis II
• Results in too many or too few chromosomes
2n
nn-1
nn+1
error in Meiosis 1
error in Meiosis 2
all with incorrect number
1/2 with incorrect number
Nondisjunction
• Trisomy
• Cells have three copies of a chromosome
• Monosomy
n-1
n
• Cells have one copy of a chromosome
n+1
n
trisomy
2n+1
monosomy
2n-1
Nondisjunction in Humans
• High frequency in humans
Polyploidy = nondisjuction
of ALL chromsome pairs
-occurs in plants
-fatal in animals
• Most embryos spontaneously abort
• Alterations are too disasterous
• Developmental problems result from biochemical imbalance
• Regulatory molecules?
• Hormones?
• Transcription factors?
• Certain conditions are tolerated
• Smaller chromosomes or sex chromosomes = survivable
• Have a characteristic set of symptoms = syndrome
Down Syndrome
• Trisomy 21
• 3 copies of chromosome 21
• 1 in 700 children born in the U.S.
• Chromosome 21 is the smallest human
chromosome
• But still has severe effects
• Frequency of Down’s Syndrome
correlates with mother’s age
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
Rate of miscarriage due to
amniocentesis:
 1970s data
0.5%, or 1 in 200 pregnancies
 2006 data
<0.1%, or 1 in 1600
pregnancies
Genetic Testing
• Amniocentesis in 2nd
trimester
• Sample of embryo cells
• Stain & photograph
chromosomes
• Usually in metaphase
• Analysis of karyotype
Sex Chromosome Abnormalities
• Human development more tolerant of wrong numbers in
sex chromosomes
• Still produces a variety of distinct
syndromes in humans
• XXY = Klinefelter’s Syndrome (male)
• XXX = Trisomy X Syndrome (female)
• XYY = Jacob’s Syndrome (male)
• XO = Turner’s Syndrome (female)
Klinefelter’s Syndrome
• XXY male
• 1 in every 2000 live births
• Have male sex organs, but are sterile
• Feminine characteristics
• Some breast development
• Lack of facial hair
• Tall
• Normal intelligence
Jacob’s Syndrome
• XYY male
• 1 in every 1000 live male births
• Slightly taller than average
• More active
• Normal intelligence, possible
reading disabilities
• Delayed emotional maturity
• Normal sexual development
Trisomy X
• XXX female
• 1 in every 2000 live births
• Produces healthy females
• Why?
• Barr bodies
• All but 1 X chromosome
becomes inactivated
Turner’s Syndrome
• Monosomy X or XO female
• 1 in every 5000 live births
• Varied degree of effect
• Webbed neck
• Short
• Sterile
error of
replication
Changes in Chromosome Structure
• deletion
• loss of a chromosomal
segment
• duplication
• repeat a segment
error of
crossing over
• inversion
• reverses a segment
• translocation
• move segment from one
chromosome to another
Any Questions??