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Genetics Brochure Info:
• Due Date March 5th
• Follow the guidelines on the rubric
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March of Dimes Info
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Hello!
Starting on Monday, all Genetics students, (past, present and future) may register for the March for
Babies!
Remember that this year the March for Babies has a focus on helping children who were born
prematurely.
Most of us know and love some who had a premature birth, and we want the best life possible for
them and for all babies. JOIN OUR WALK!
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Go to our team website and sign up.
http://www.marchforbabies.org/team/t1379641
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You may also DONATE ONLINE!
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Please also come to Room 720 and sign our MARCH FOR BABIES WALL.
You can pick up a walker envelope and a coin bucket if you like.
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Plan to arrive at Marietta Middle School at 8:00 AM April 24 for our group photo. The walk begins at
8:30 and will be about 3 miles long through Marietta neighborhoods. March for Babies will provide
some breakfast and lunch treats.
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IT IS STILL NOT TOO LATE TO DESIGN A MARCH FOR BABIES T-SHIRT!
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Come sign up in Room 720!!!
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Chapter 13
Chromosomes
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Cytogenetics
• A subdiscipline within genetics
• Focuses on chromosome variations
• Abnormal number of copies of genes or
chromosomes can lead to genetic
abnormalities
• Human genome sequence information is
used to identify genes that contribute to
the chromosome-related syndromes
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Portrait of a Chromosome
• Primarily DNA and protein
• Described by size and shape
• Heterochromatin (dark)
• Euchromatin (light)
• Contains:
Figure 13.1
– Telomeres
– Origin of replication sites
– Centromere
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Chromosomes
• Heterochromatin is darkly staining, contains
mostly repetitive DNA
• Euchromatin contains more protein encoding
genes
• Telomeres are chromosome tips composed of
many repeats of TTAGGG and shorten with each
cell division
• Centromere is the largest constriction of the
chromosome and where spindle fibers attach
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Centromeres
• Alpha satellite  the bases that form the
centromere are repeats of a 171-base DNA
sequence
• Centromere  associated proteins form the
kinetochore that attaches to spindle fibers
• Centromere protein A (CENP-A)  involved
in centromere replication is passed to next
generation and nearly identical in all species
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Subtelomeres
• The chromosome region between the centromere and
telomeres
•Consists of 8,000  300,000 bases
•Includes at least 500 protein-encoding genes
•Near telomere the repeats similar to the telomere sequence
•Multigene families that include pseudogenes
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Figure 13.2
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Karyotypes
• Chromosomal chart
• Chromosomes arranged by size and
and structure
• 24 types
• Arranged by largest to smallest
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Centromere Position
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At tip  telocentric
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Close to end  acrocentric
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Displaced from center  submetacentric
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At midpoint  metacentric
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Long arm q
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Short arm p
Figure 13.4
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Chromosomes Differ in Size
Table 13.1
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Visualizing Chromosomes
Obtain tissue from person
Fetal tissue: amniocentesis
chorionic villi sampling
fetal cell sorting
Adult tissue: blood (white blood cells)
cheek swab (buccal cells)
skin cells
tissue biopsy
Prepare cells on slide to remove cell matter
Stain DNA with dyes or DNA probes to visualize DNA
Evaluate chromosomes
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Amniocentesis
Figure 13.5a
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Chorionic Villi Sampling
Figure 13.5b
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Fetal Cell Sorting
Figure 13.5c
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Karyotype
• Metaphase chromosomes are squashed
• Stained with DNA-binding dyes
• Banding patterns help identify chromosomes
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Table 13.2
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FISH
• Fluorescence in situ hybridization
• DNA probes labeled with fluorescing dye bind
complementary DNA
Figure 13.9
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Chromosomal Shorthand
Table 13.3
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Chromosomal Shorthand
• An ideogram represents a
chromosome schematically.
•The major banding regions are
indicated with numbers.
•Example
– Sucrose intolerance is
located at 3q.26
– or chromosome 3, long
arm, major band 26)
Figure 13.10
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The Normal Karyotype
• Human somatic cells contain 46 chromosomes
• 23 diploid chromosomes
 paired homologs of chromosomes 1 to 22
 sex chromosomes (XX or XY)
• Diploid  two sets of each chromosome.
• Haploid gametes  one set of each chromosome
• Euploid cells have a normal chromosome
constitution
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Chromosome Abnormalities
Table 13.4
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Polyploidy
• 17% of all spontaneous abortions and 3% of
stillbirths/newborn deaths show polyploidy
Figure 13.11
• Triploid - three copies of each chromosome
• Produced by
Two sperm fertilize one egg
 Haploid sperm fertilizes diploid egg
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Aneuploidy
Cells with extra or missing chromosomes
• Nondisjunction is a common cause
Gametes produced with one extra chromosome
and another with one missing chromosome
• Nondisjunction during Meiosis I results in copies
of both homologs in one gamete
• Nondisjunction during Meiosis II results in both
sister chromatids in one gamete
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Nondisjunction at Meiosis I
Figure 13.12a
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Nondisjunction at Meiosis II
Figure 13.12b
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Trisomies and Monosomies
• Results in extra or missing copies of all of the
genes on the chromosome
• Most cease developing as embryos
• Some fetuses with trisomy of smaller autosomes
survive to birth
Table 13.5
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Trisomy 21
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Down syndrome
Most common of trisomy
Distinctive facial and physical problems
Many medical problems are treatable
Varying degrees of developmental disabilities
Link with one form of Alzheimer disease
May also be produced by a translocation
Maternal age is a risk factor for having a child
with Down syndrome
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Table 13.6
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Figure 13.7
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Trisomy 18
• Edward syndrome
• Most due to nondisjunction in meiosis II in
oocyte and do not survive
• Serious medical and physical disabilities
Figure 13.14a
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Trisomy 13
• Patau syndrome
• Very rare and generally do not survive
6 months
• Medical and physical abnormalities
• Facial malformation and eye fusion
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Sex Chromosome Aneuploidy
Table 13.7
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Turner Syndrome (45,X)
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Only one copy of X chromosome
1 in 2,500 female births
99% of affected fetuses die in utero
Absence of Y leads to development as a
female
• Phenotypes include short stature, webbing
at back of neck, incomplete sexual
development (infertile), hearing impairment
• Mosaics
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Triplo-X Aneuploidy (47, XXX)
• 1 in 1,000 female births
• Extra copy of every X-linked gene
• Few modest effects on phenotype include
tallness, menstrual irregularities, and slight
impact on intelligence
• X-inactivation of two X chromosomes
occurs and cells have 2 Barr bodies
– May compensate for presence of extra X
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Klinefelter Syndrome (47, XXY)
• 1 in 1,000 male births
• Extra copy of each X-linked gene
• Phenotypes include
– incomplete sexual development
– rudimentary testes and prostate
– long limbs, large hands and feet
– some breast tissue development
• Some cases are not diagnosed until fertility
problems arise or remain undiagnosed
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XXYY Syndrome
• Likely arises to unusual oocyte and
sperm
• AAD, obsessive compulsive disorder,
learning disabilities, infertile
• Treated with testosterone
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XYY Syndrome (47, XYY)
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1 in 1,000 male births
Extra Y chromosome
96% phenotypically normal
Modest phenotypes may include
– great height
– acne and minor speech
– reading disabilities
• Studies suggesting increase in aggressive
behaviors are not supported
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Chromosome Structural
Abnormalities
Figure 13.16
• Chromosomal deletions or duplications result in
extra or missing copies of genes
• Inversions alter the sequence of genes
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A Duplication
Figure 13.18
• Larger regions of deletion or duplication
increase the likelihood that there will be an
associated phenotype
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Translocation
• Nonhomologous chromosome exchange segments
Two major types:
• Robertsonian translocation
• Two nonhomologous acrocentric chromosomes break
at the centromere and long arms fuse. The short arms
are often lost.
• 5% of Down syndrome results from a Robertsonian
translocation between chr 21 and chr 14.
• Reciprocal translocation
• Two nonhomologous chromosomes exchange a
portion of their chromosome arms.
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Segregation of a Robertsonian
Translocation
Figure 13.19
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Reciprocal Translocation
• Exchange of material from one chromosome
arm to another
• Some individuals carry a translocation but are
not missing any genetic material unless a
translocation breakpoint interrupts a gene
Figure 13.20a
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Inversions
• Inverted chromosomes have a region flipped in
orientation
• 5-10% cause health problems probably due to
disruption of genes at the breakpoints
• Inversions may impact meiotic segregation
• Two types of inversions occur:
• Paracentric
– inverted region does NOT include centromere
• Pericentric
– inverted region includes centromere
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Segregation of a Paracentric
Inversion
Figure 13.21
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Segregation of a Pericentric
Inversion
Figure 13.22
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Isochromosomes
• Chromosomes with
identical arms
• Form when
centromeres divide
along the incorrect
plane during meiosis
Figure 13.23
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Ring Chromosomes
• Chromosomes shaped like a ring
• Occur in 1 in 25,000 conceptions
• May arise when telomeres are lost and
sticky chromosome end fuse
• Ring chromosomes have phenotypes
associated with the loss or addition of
genetic material
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Causes of Chromosomal
Abnormalities
Table 13.8
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Uniparental
Disomy
Figure 13.24
• Inheritance of two
chromosomes from one
parent
• May occur
– When nondisjunction
occurs in both parents
(a disomic gamete and
one without homolog)
– Loss or nondisjunction
of one homolog in early
embryo followed by
reduplication of
remaining homolog
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