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Human Genetics
Concepts and Applications
Tenth Edition

RICKI LEWIS

13

Chromosomes
PowerPoint® Lecture Outlines
Prepared by Johnny El-Rady, University of South Florida
Copyright ©The McGraw-Hill Companies, Inc
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Portrait of a Chromosome
Heterochromatin is darkly staining
- Consists mostly of repetitive DNA
Euchromatin is lighter-staining
- Contains most protein-encoding genes
Telomeres are chromosome tips composed
of many repeats of TTAGGG
- Shorten with each cell division
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Centromeres
The largest constriction of the chromosome
and where spindle fibers attach
The bases that form the centromere are
repeats of a 171-base DNA sequence
Replicated at the end of S-phase
- Facilitated by centromere protein A
CENP-A is passed to next generation
- An example of an epigenetic change
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Subtelomeres
The chromosome region between the
centromere and telomeres
Consists of 8,000 to 300,000 bases
Near telomere the repeats are similar to the
telomere sequence
Contains at least 500 protein-encoding
genes
- About 50% are multigene families that
include pseudogenes
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Subtelomeres

Figure 13
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Karyotype
A chromosome chart
Displays chromosomes arranged by size
and structure
Humans have 24 chromosome types
- Autosomes are numbered 1-22 by size
- Sex chromosomes are X and Y
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Karyotype

Figure 13
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Karyotype
Karyotypes are useful at several levels
1) Can confirm a clinical diagnosis
2) Can reveal effects of environmental
toxins
3) Can clarify evolutionary relationships
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Visualizing Chromosomes
Tissue is obtained from person
- Fetal tissue: Amniocentesis
Chorionic villi sampling
Fetal cell sorting
Chromosome microarray analysis
- Adult tissue: White blood cells
Skinlike cells from cheek swab
Chromosomes are extracted
Then stained with a combination of dyes and DNA
probes
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Amniocentesis
Detects about 1,000 of the more than 5,000 known
chromosomal and biochemical problems
Ultrasound is used to follow needle’s movement
Figure 13
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5a

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Chorionic Villi Sampling
Performed during 10-12th week of pregnancy
Provides earlier results than amniocentesis
However, it does not detect metabolic problems
- And has greater risk of spontaneous abortion
Figure 13
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Fetal Cell Sorting
Fetal cells are distinguished from maternal
cells by a fluorescence-activated cell sorter
- Identifies cell-surface markers
A new technique
detects fetal
mRNA in the
bloodstream of
the mother

Figure 13
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Drawing by German biologist
Walther Flemming

Now

Micrograph of actual stained
human chromosomes
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Staining Chromosomes
In the earliest karyotypes, dyes were used to stain
chromosomes a uniform color
Chromosomes were grouped into decreasing size
classes, designated A though G
In the 1970s, improved staining techniques gave
banding patterns unique to each chromosome
Then researchers found that synchronizing the
cell cycle of cultured cells revealed even more
bands per chromosome
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FISH
Fluorescence in situ hybridization
DNA probes labeled with fluorescing dye
bind complementary DNA

Fluorescent dots
correspond to
three copies of
chromosome 21

Figure 13
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Chromosome Abnormalities
A karyotype may be abnormal in two ways:
1) In chromosome number
2) In chromosome structure
Abnormal chromosomes account for at least
50% of spontaneous abortions
Due to improved technology, more people
are being diagnosed with chromosomal
abnormalities
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24

Polyploidy
Cell with extra chromosome sets is polyploid
Triploid (3N) cells have three sets of
chromosomes
- Produced in one of two main ways:
- Fertilization of one egg by two sperm
- Fusion of haploid and diploid gametes
Triploids account for 17% of all spontaneous
abortions and 3% of stillbirths and newborn
deaths
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Triploidy

Figure 13
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Nondisjunction
at Meiosis II

Figure 13
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Varying degrees of developmental disabilities
Individuals more likely to develop leukemia
Link with one form of Alzheimer disease
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Table 13
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Trisomy 18
Edwards syndrome

Figure 13
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14b

Very rare and generally
do not survive 6 months

Serious mental and physical disabilities
A distinctive feature: Eye fusion
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Table 13
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Figure 13
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Translocations
In a translocation, two nonhomologous
chromosomes exchange segments
There are two major types:

1) Robertsonian translocation
2) Reciprocal translocation
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Robertsonian Translocations
Two nonhomologous acrocentric
chromosomes break at the centromere
and their long arms fuse
- The short arms are often lost
Affect 1 in 1,000 people
Translocation carriers have 45
chromosomes
- Produce unbalanced gametes
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Translocation Down Syndrome
About 5% of Down syndrome results from a
Robertsonian translocation between
chromosomes 21 and 14
Tends to recur in families, which also have
more risk of spontaneous abortions
One of the parents is a translocation carrier
- They may have no symptoms
- However, the distribution of the unusual
chromosome leads to various imbalances
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Figure 13
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Reciprocal Translocations
Two nonhomologous chromosomes
exchange parts
About 1 in 500 people are carriers
- Are usually healthy because they have
the normal amount of genetic material
(but it is rearranged)
However, if the translocation breakpoint
interrupts a gene, there may be an
associated phenotype
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Figure 13
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Inversions
An inversion is a chromosome segment
that is flipped in orientation
5-10% cause health problems probably due
to disruption of genes at the breakpoints
Paracentric inversion = Inverted region
does NOT include centromere
Pericentric inversion = Inverted region
includes centromere
Inversions may impact meiotic segregation
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Segregation of a Paracentric Inversion
Figure 13
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Segregation of a Pericentric Inversion
Figure 13
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Isochromosomes
Chromosomes with
identical arms

Figure 13
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Table 13
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Figure 13

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