Human chromosomes

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Human chromosomes

• There are 46 chromosomes (23 homologous pairs)
  in each somatic cell
• 22 pairs of autosomes
• 1 pair of sex chromosomes
• XX Female OR XY Male
• Karyotype - chromosomes are arranged according to
  shape and size

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Karyotype
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Nondisjunction and chromosomal disorders

• Nondisjunction failure of chromosomes to
  separate and segregate into daughter cells
• Nondisjunction may occur during meiosis 1 or
  meiosis 2
• Abnormal number of chromosomes may result

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Abnormal meiosis

• Abnormal meiosis
• Sometimes mistakes occur during meiosis
• Segments of chromosome (block of genes) can be
  lost or added chromosome mutation / aberration
• Cell may be missing a chromosome or have an extra
  chromosome (aneuploidy incorrect number of
  chromosomes)
• Plant cells have more than 2n sets of
  chromosomes in nucleus (3n, 4n called
  polyploidy)
• Examples of aneuploidy
• Downs syndrome (3 x chromosome 21- trisomy)
• Turners syndrome (female has 1 X chromosome
  monosomy)
• Klinefelters syndrome (3 x sex chromosome XXY)
  

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How Downs syndrome happens

• Trisomy chromosome 21
• Chromosomes of homologous pair 21 do not separate
  during meiosis
• One gamete (?) may have two chromosomes for
  chromosome 21 and the other (?) does not have a
  chromosome 21.
• Fertilisation 2 x 21 (?) 1 x 21 (?) 3 x
  chromosomes 21
• Extra chromosome 21 (trisomy) offspring/baby
• This process is called non-disjunction (failure
  to separate)

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The diagram below shows an abnormality during a
certain phase of meiosis with the number 21
chromosome pair (labelled A).

• 1. What is the specific purpose of this phase of
  the cell division?
• 2. What specific genetic condition does this
  abnormality (shown by A) cause?
• Give TWO symptoms of this genetic disorder.
• People with this condition are often sterile.
  Explain why you think this is so.

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Karyotype
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How trisomy and monosomy with pair 23 happen

• Sex chromosomes - Trisomy and Monosomy
• Chromosomes of homologous pair 23 do not separate
  during meiosis
• One gamete (?) may have two chromosomes for
  chromosome 23 and the other (?) does not have a
  chromosome 23.
• Fertilisation 2 chromosomes 23 (?) 1
  chromosome (?) 3 chromosome 23
• Extra chromosome 23 (trisomy - XXY)
• AND
• Fertilisation 0 (?) 1 (?) 1 X chromosome 23
• Only ONE X chromosome (monosomy - X0)

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How Polyploidy happens

• Polyploidy Plant cells with more than the
  diploid set of Chromosomes
• Chromosomes of all the homologous pairs of one
  organism do not separate during meiosis
• One gamete (?) may have two sets of chromosomes.
• Fertilisation 2n (?) n (?) 3n (triploid)
• OR
• Self pollination and Fertilisation 2n 2n 4n
  (tetraploid)

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Normal meiosis
MEIOSIS I
Replicate DNA
Results of crossing-over not shown
MEIOSIS II
Normal monosomic gametes
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Nondisjunction during meiosis I
MEIOSIS I
Replicate DNA
Non-disjunction
MEIOSIS II
Disomic gametes
Nullisomic gametes
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Nondisjunction during meiosis II
MEIOSIS I
Replicate DNA
MEIOSIS II
Non-disjunction
Disomic
Nullisomic
Monosomic
Monosomic
gametes
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EXAMPLES OF NON-DISJUNCTION
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AN EXTRA COPY OF CHROMOSOME 21 CAUSES DOWN
SYNDROME

• This condition is called trisomy 21.
• Person with this condition suffers from
• the Down syndrome.
• Characteristic facial features
• Round face
• Flattened nose bridge
• Small, irregular teeth
• Short stature
• Heart defects

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Klinefelter's syndrome, 47, XXY

• It is the most common sex chromosome disorder and
  the second most common condition caused by the
  presence of extra chromosomes
• Symptoms
• Language impairment
• Lanky, youthful build or rounded body type
• Low levels of Testosterone and small testicles /
  Infertile

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Turners Syndrome (X)

• Common symptoms
• Short stature
• swelling of the hands and feet
• Broad chest and widely spaced nipples
• Low hairline
• Low-set ears
• Reproductive sterility
• Increased weight, obesity
• Small fingernails
• Characteristic facial features
• Webbed neck

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Down Syndrome
1 in 1,250 births 47 chromosomesXY or XX 21
Trisomy Nondisjunction
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Nondisjunction