Sexual Reproduction

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Sexual Reproduction

• Meiosis

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The problem with sexual reproduction

• Each human cell has 46 chromosomes.
• If every egg and every sperm had 46 chromosomes,
  how many would be found
• in the fertilized egg?

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The problem with sexual reproduction

• 92!

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The problem with sexual reproduction

• 92!
• But the fertilized egg has 46 chromosomes,
• 23 from each parent.
• So what happened?

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The problem with sexual reproduction

• The number of chromosomes is reduced by half.
• This is a special type of cell division called
  meiosis.

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The problem with sexual reproduction

• Meiosis is the process by which the number of
  chromosomes is reduced by half.
• During this process the homologous chromosomes
  are separated.

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The problem with sexual reproduction

• Meiosis occurs in those tissues that give rise
  to gametes (eggs or sperm).
• Gametogenesis (the formation of gametes) occurs
  only in the gonads (testes or ovaries).

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The problem with sexual reproduction

• The typical human body cell contains 46
  chromosomes.
• This is called the diploid or 2n state.

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The problem with sexual reproduction

• After meiosis, the homologous pairs are
  separated and the chromosome number is now
  haploid or 1n.

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The problem with sexual reproduction

• Homologous chromosomes contain similar types of
  genetic information.

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The problem with sexual reproduction

• Homologous chromosomes contain similar types of
  genetic information.
• Each member of the pair comes from one parent.

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The problem with sexual reproduction

• The human has 23 pairs of homologous
  chromosomes.
• During meiosis the pairs separate forming
  gametes with only one part of the pair.

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Meiosis

• Meiosis looks like two mitotic divisions.
• The major difference is that 4 cells instead of
  two are formed and the chromosome number is
  reduced by half.

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Important Steps

• In the first metaphase in meiosis, the
  homologous chromosomes pair up.

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Important Steps

• The homologous chromosomes pairing is called a
  tetrad.

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Important Steps

• During this phase, crossing over can occur.
• Crossing over is where chromatids can exchange
  pieces of genetic information.

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Important Steps

• This results in more genetic variation.

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Important Steps

• In the second metaphase of meiosis, the paired
  chromatids split.
• This looks very much like metaphase in mitosis.

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Mitosis vs Meiosis
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The problem with sexual reproduction

• Meiosis can go wrong.
• During Metaphase I or 2, the chromosomes may not
  separate and this results in gametes having the
  wrong number of chromosomes.

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The problem with sexual reproduction

• Nondisjunction is when the chromosomes fail to
  separate properly during meiosis.
• In the human this can result in eggs or sperm
  having the incorrect number of chromosomes.

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The problem with sexual reproduction

• Common syndromes caused by nondisjunction
  include
• 1 Downs Syndrome
• 2 Edwards Syndrome
• 3 Kleinfelters Syndrome
• 4 Turners Syndrome

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Diagnosis

• A karyotype is a pictorial presentation of an
  individuals chromosomes, taken from photographs
  taken through a microscope.
• The chromosomes can then be classified and
  paired up to find any mistakes.

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Diagnosis

• Chromosomes are divided into two types
• Autosomes which are in homologous pairs and the
  Sex Chromosomes.
• These can be paired for the female XX or
  hemizygous for the male XY

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Diagnosis

• The chromosomes are arranged in a numeric
  sequence that aligns the chromosomes from largest
  to smallest.

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Diagnosis

• The karyotype shows the matching homologous
  chromosomes.
• There are 22 pairs of autosomes and 1 set of sex
  chromosomes.

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Diagnosis

• For example, Chromosomal pair 1, one is from the
  mother and the other from the father.
• This individual is 46, XX

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Diagnosis

• Karyotypes can also be used to determine the
  sex of the infant.
• The 23rd pair are the sex chromosomes.

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Diagnosis

• If this pair is a XX, the infant is a female.

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Diagnosis

• If this pair is a XY, the infant is a male.
• 46,XY

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Preparation of a Karyotype

• For prenatal diagnosis, an amniocentesis is
  performed.
• This takes a sample of amniotic fluid which
  contains cells from the infant.

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Preparation of a Karyotype

• Ultrasound Image

• Amniocentesis

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Preparation of a Karyotype
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Preparation of a Karyotype

• The karyotype can be analyzed for any
  chromosomal abnormalities.

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Syndrome 1
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Syndrome 1

• What is the sex?
• Is there any abnormality?

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Syndrome 1 Downs Syndrome

• This is a male with trisomy 21 (3 chromosome
  21)
• Downs syndrome is the most common
  nondisjunctional condition.

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Syndrome 1 Downs Syndrome

• Signs and symptoms include
• Flatten face
• Makes the tongue protrude
• Loose joints poor muscle tone
• Upward slant to the eyes
• Short stature
• Mental retardation (varies)

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Syndrome 2
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Syndrome 2

• What is the sex?
• Is there any abnormality?

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Syndrome 2 Kleinfelters Syndrome

• Signs and symptoms
• These individuals are males with a XXY genotype
• Symptoms are not seen until puberty with a
  failure of the secondary sex characteristics to
  develop

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Syndrome 2 Kleinfelters Syndrome

• Signs and symptoms
• The extra X chromosome causes the testicles not
  develop adequately.
• Some have learning disabilities.

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Syndrome 2 Kleinfelters Syndrome

• Signs and symptoms
• Estrogen and growth hormone are given to allow
  normal development.

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Syndrome 3
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Syndrome 3

• What is the sex?
• Is there any abnormality?

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Syndrome 3 Turners Syndrome

• Signs and symptoms
• These individuals are females with a X O
  genotype
• Symptoms are not seen until puberty with a
  failure of the secondary sex characteristics to
  develop.
• Intelligence is normal

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Syndrome 3 Turners Syndrome

• Signs and symptoms
• The missing X chromosome causes the ovaries not
  to develop.
• Some have learning disabilities.

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Other Syndromes

• Patau, an extra chromosome 13
• Edwards, an extra chromosome 18
• Super females extra X chromosome
• Super males extra Y chromosome