Lecture: Sex determination

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Lecture Sex determination
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Outlines

• Sexuality in general
• Sex determination in drosophila
• Sex determination in human
• Comparison of two systems

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Sexuality in general (terminology)

• Gonochorism separate sexes in different
  individual organisms (human, and other animals)
• Hermaphroditism separate sexes in same
  individuals (plants)
• Mechanisms of gonochorisms
• 1) environmental factors (e.g. temperature)
• 2) genetic determination
• X Y chromosomes --- flies and humans
• XX (homogametic - female)
• XY (heterogametic - male)
• Z W chromosomes ---birds
• ZZ (homogametic --- male)
• ZW (heterogametic --- female)

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Sex determination in flies

• X Y genetic system
• XX -- female
• XY -- male
• No of X chr. determines maleness
• (X A 0.5 1 -- male)
• (X A 1 1 -- female)
• mutant
• XO -------gt male
• XXY------gt female

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Sex determination in flies

• No of X chr. determines maleness
• (X A 0.5 1 -- male)
• (X A 1 1 -- female)
• Sxl (sex lethal)
• detect the X-to-A ratio
• tra (transformer)
• dsx (double sex)

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Sex determination in flies

• Sxl (sex lethal)
• detect the X-to-A ratio
• tra (transformer)
• dsx (double sex)
• No of X chr. determines maleness
• (X A 0.5 1 -- male)
• (X A 1 1 -- female)
• mutant
• XO -------gt
• XXY------gt

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Sex determination in human
sry

• X Y genetic system
• Presence of Y decides maleness
• master switch gene -----gt
• Sry (sex-determining region of the Y
  chromosome)
• DSS (dosage-sensitive sex )
• located in X chromosome
• mutations cause sex-reversal
• sex-reversal syndrome
• Abnormality
• XO -------gt
• XXY -----gt

sry present (XY)
sry absent (XX)
DSS is off
DSS is on
Male pathway
Female pathway
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Sex determination in human
sry

• No of expressed copies Sry
• vs
• No of expressed copies of DSS
• sry DSS ------gt male
• sry lt DSS -------gt female
• How mechanistically possible?
• (sry protein bind to DSS protein)
• sry may bind to DSS, and block
• the action of DSS (Male)
• vs
• no sry, thus DSS starts female
• pathway

sry present (XY)
sry absent (XX)
DSS is off (XY)
DSS is on (XX)
Male pathway
Female pathway
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Sex determination in human

• No of expressed copies Sry vs No of expressed
  copies of DSS
• sry DSS ------gt male
• sry lt DSS -------gt female

Sex chr. No. of sry No. of DSS No of
DSS Sex Normal XY 1 1 1 M
XX 0 2 1 F Mutant XdupY 1 2 2 F XYmut 0 1
1 F XXY 1 2 1 M Klinefelter
In mammals, only one X is active and the other
Xs become inactivated. X chromosome inactivation
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Comparison of two systems
Flies X-to-A ratio vs Human Y chr.