Week 11

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Week 11

• CHROMOSOMES
• Chapter 10 pages 180-186
• Genetics I
• Chapter 11 pages 189-201

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HOMEWORK

• Chapter 10 pages 180-186
• Chapter 11 pages 190 - 207

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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
• Nondisjunction When chromosomes fail to
  properly separate during meiosis
• Errors in crossing-over which result in extra or
  missing parts of chromosomes

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Crossing Over NonDisjuction
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Aneuploidy

• Euploidy Correct number of chromosomes in a
  species
• Aneuploidy A change in the number of chromosomes
  due to nondisjuction

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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 (2n1)

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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
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Trisomy Example

• Down Syndrome
• Extra copy of chromosome at location 21 (Trisomy
  21)
• Pheontype

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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

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Page 180 Figure 10.10
Secondary Nondisjunction Results in 2 normal and
2 aneuploid gametes
Primary Nondisjunction No normal gametes
produced!!
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Three non-lethal Trisomic Conditions

• Trisomy 13
• Pataus Syndrome
• Trisomy 18
• Edwards Syndrome
• Trisomy 21 (only type of aneuploidy where able to
  survive beyond early childhood)

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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

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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

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Normal v. Abnormal Sex x-some Karyotypes
NORMAL Karyotype
ABNORMAL Karyotype
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Only 1 X x-some Female with Turner Syndrome
(sterile) Two X and 1 Y x-somes Male with
Klinefelter Syndrome (sterile)
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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)
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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

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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 gt 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

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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
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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
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Poly-X FEMALES

• Often called, Superfemales
• Have gt 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

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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

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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

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Deletions

• Occur when
• 1. The end of a x-some breaks off
• When two simultaneous breaks lead to the loss of
  an
• Internal segment
• Deletions can lead to abnormalities

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Duplications

• Occur when
• 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)

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Inversions

• Occur when
• 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

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Translocations

• Occur when
• A chromosome segment moves from one chromosome to
  
• a non-homologous chromosome
• Translocations can be
• 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

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Human Syndromes

• Are discovered when observing karyotypes of
  various syndromes
• Can be understood by looking at patterns of
  inheritance in families

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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
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Deletion Syndromes

• Cri du chat (Cats 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 ? infants cry is reminiscent of a cat
A 8 mo. B 2 yrs. C 4 yrs. D 9 yrs.
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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)

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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)

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Summary

• X-some NUMBER changes
• Autosomal
• Monosomy
• Trisomy
• Sex x-some
• XO XY Female
• XXY XX Male

X-some STRUCTURE changes Deletions Duplications I
nversions Translocations
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QUESTIONS??