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Week 11
CHROMOSOMES
Chapter 10 pages 180-186
Genetics I
Chapter 11 pages 189-201
HOMEWORK
Chapter 10 pages 180-186
Chapter 11 pages 190 - 207
CHROMOSOME NUMBER AND
STRUCTURE
Crossing over of chromosomes creates variation within a
population during meiosis
Proper separation of chromosomes during meiosis is critical
for proper growth and function
Improper separation of chromosomes results in abnormal
chromosome numbers in individuals due to:
1. Nondisjunction = When chromosomes fail to properly
separate during meiosis
2. Errors in crossing-over which result in extra or missing
parts of chromosomes
Crossing Over
NonDisjuction
Aneuploidy
Euploidy= Correct number of chromosomes in a
species
Aneuploidy= A change in the number of
chromosomes due to nondisjuction
Monosomy and Trisomy
Monosomy and Trisomy are two different states
of Aneuploidy (change in x-some # due to
nondisjunction)
Monosomy- an individual has only 1 copy of a
chromosome (2n-1)
Trisomy- an individual has 3 copies of a
chromosome (2n+1)
Monosomy Example
Females normally have
two X chromosomes
Lack of one of the X x-somes results
in:
Turner Syndrome (Monosomy X)
Only 1 copy of the X chromosome
Trisomy Example
Down Syndrome
Extra copy of chromosome
at location 21 (Trisomy 21)
Pheontype:
Types of Nondisjuction
Primary Nondisjunction
- Occurs during meiosis I
- Both homologous chromosomes go into the
same daughter cell
Secondary Nondisjunction
- Occurs during meiosis II
- Sister chromatids fail to separate and both
chromosomes go into the same gamete
Page 180 Figure 10.10
Secondary Nondisjunction
Primary Nondisjunction
Results in 2 normal and 2 aneuploid gametes
No normal gametes produced!!
Three non-lethal Trisomic
Conditions
Trisomy 13
Patau’s Syndrome
Trisomy 18
Edwards Syndrome
Trisomy 21 (only type of aneuploidy where able
to survive beyond early childhood)
Autosomes v. Sex Chromosomes
Autosomes
- All non sex chromosomes
Sex Chromosomes
- The X or Y chromosomes
- Females have two copies of the X
chromosome
- Males have one copy of the X chromosome
and one copy of the Y chromosome
Sex Chromosome Aneuploidy
Change in sex chromosome number
Occurs via nondisjunction during spermatogenesis or
oogenesis
Results in an abnormal number of chromosomes in the
gametes
These abnormalities are better tolerated than
autosomal anueploidy conditions like Trisomy
Normal v. Abnormal Sex x-some
Karyotypes
NORMAL Karyotype
ABNORMAL Karyotype
Only 1 X x-some = Female with Turner Syndrome (sterile)
Two X and 1 Y x-somes = Male with Klinefelter Syndrome (sterile)
Swyer Syndrome
Swyer Syndrome (“hermaphrodite”)
- Deletion of the SRY gene on the Y x-some
- Result is an “XY Female”
- Lack hormone called ‘Testis-determining
factor’
- Male genitals are not fully developed 
female designation
Caster Semenya won gold at world
Championships in 800m race
*Almost stripped of medal due to her
XY Female status (she has no womb/ovaries
but has internal testes)
de la Chapelle Syndrome
de la Chapelle Syndrome
- Movement of the SRY gene from the Y x-some
to an X chromosome
- Results in a “XX Male”
- Men have undersized testes, sterility and
slight breast development
- The SRY gene determines “maleness” NOT the
number of X chromosomes b/c without the
SRY gene, a person will be female
Why are extra sex chromosomes
better tolerated than extra
autosomal chromosomes?
Males and females produce equal amounts of gene product
eventhough females have 2 X chromosomes
Only one of the X x-somes is functional
If a person has > 1 X x-some, the others are inactive and are
known as BARR BODIES
Therefore, gene dosage is the same whereas in autosomes,
extra copies leads to unequal gene dosage
TURNER SYNDROME (XO)
Only 1 Sex Chromosome (X)
Incidence= 1 in 10,000 females (.01%)
Phenotype
Widely spaced nipples
Low posterior hairline
Neck webbing
Ovaries, oviducts and uterus are underdeveloped
Do not undergo puberty or menstruate
Breasts do not develop
Normal intelligence
“Normal” lives if take hormone supplements
KLINEFELTER SYNDROME
Male with Klinefelter has 2 or more X x-somes in
and 1 Y x-some
Extra X x-some becomes a Barr Body
Incidence is 1 in 500 to 1,000 males (.1-.2%)
Phenotypes
Underdeveloped prostate gland and testes
No facial hair
Some breast development
Large hands and feet
Long arms and legs
Sterile
Risk of breast cancer, osteoporosis and lupus
(normally affect females)
Testosterone therapy is a solution
Poly-X FEMALES
Often called, “Superfemales”
Have > 2 X x-somes
Females with 3 X x-somes tend to be tall and thin
Incidence is 1 in 1,500 females (.07%)
XXXX females are very tall and severly mentally
retarded
JACOBS SYNDROME
X Y Y Males “Supermales”
Due to nondisjunction during spermatogenesis
Frequency of karyotype is 1/1,000 (.1%)
Males are taller than average, have persistent
acne, speech and reading issues, are fertile
CHANGES IN CHROMOSOME
STRUCTURE
Changes in chromosome structures are mutations
X-somes can break due to radiation, organic
chemicals and viruses
End of chromosomes break and can go back
together improperly which leads to chromosomal
mutations:
Deletions
Duplications
Translocations
Inversions
Deletions
Occur when:
1. The end of a x-some breaks off
2. When two simultaneous breaks lead to the loss of an
Internal segment
Deletions can lead to abnormalities
Duplications
Occur when:
1. A chromosomal segment is present more than once in
the same chromosome
Duplications may or may not cause visible abnormalities
(depends on size of the duplicated region)
Inversions
Occur when:
1. A chromosomal segment is turned 180 degrees
Inversions usually do not cause visible abnormalities
However, reversed gene sequences can cause
duplications or deletions in offspring
Translocations
Occur when:
1. A chromosome segment moves from one chromosome to
a non-homologous chromosome
Translocations can be:
1. Balanced (as shown on left)- A reciprocal swap
2. Unbalanced- extra material from on x-some and missing
material from another (occurs when people with translocations
have offspring)
Unbalanced translocations can lead to miscarriage or if
the fetus survives, the child will have severe symptoms
Down syndrome can also be caused by a translocation btwn.
X-somes 21 and 14
Human Syndromes
Are discovered when observing karyotypes of various
syndromes
Can be understood by looking at patterns of
inheritance in families
Deletion Syndromes
Williams Syndrome
- Chromosome 7 loses a small piece of the end
Phenotype
Broad forehead
Low nasal bridge
Anteverted (upturned) nostrils
Full cheeks
Wide mouths
Small chin
Large ears
Musical and verbal abilities are unaffected
Deletion Syndromes
Cri du chat (Cat’s Cry) syndrome
-Chromosome 5 is missing a piece of the end
Phenotype
Smaller head
Mentally retarded
Facial abnormalities
Abnormal development of the glottis and larynx
 infant’s cry is reminiscent of a cat
A= 8 mo. B= 2 yrs. C= 4 yrs. D= 9 yrs.
Translocation Syndromes
If the translocation is balanced, no abnormal
phenotypes
If the translocation breaks an allele into two,
health problems arise
Ex. Translocation between x-some 2 and 20
leads to eye and internal organ abnormalities
and severe itching (the translocation disrupts
the alleles on x-some 20)
Cancer and Translocations
Translocations can induce certain cancers
Ex. Translocation of x-some 22 into x-some 9 can cause chronic
myelogenous leukemia = “The Philadelphia Chromosome”
Ex. Burkitt Lymphoma- from x-some 8 to 14, large tumors in
jaws (common in children from equatorial Africa)
Summary
X-some NUMBER
changes
X-some STRUCTURE
changes
Autosomal
Deletions
Duplications
Inversions
Translocations
Monosomy
Trisomy
Sex x-some
XO
XXY
XY Female
XX Male
QUESTIONS??