Barbara McClintock

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Barbara McClintock

• By Kalim Kassim and Tim Milsom

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Prior Knowledge

• In 1938, Marcus Rhodes provided the first
  description of an unstable mutation, found by
  using maize.
• This resulted in a variegated phenotype which
  produces irregular patterns of pigmentation and
  pigment production.
• 2 genes caused this irregularity the unstable
  allele by one gene and a dominant mutation by the
  second gene called Dotted.

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Barbara Mclintock

• In the 1940s and 1950s, she showed that this
  irregularity was caused by genetic elements that
  moved or transposed from one locus to another.
• She studied the mosaic patterns of maize seeds
  and the inheritance of these patterns.
• She found 2 new dominant genetic loci that she
  names Dissociatior (Ds) and Activator (Ac)

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• Dissociator caused the chromosomes to break and
  effected neighbouring genes when the activator
  was present.
• Later, in 1948, she found that Ds and Ac could
  transpose on the chromosome.
• By changing the colouration of the kernels over
  each generation, by using controlled crosses, she
  concluded that Ac controls the transposition of
  Ds from chromosome 9 and when Ds is moved there
  is a breakage in the chromosome.

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Conclusion

• When Ds moves, the color gene is released which
  causes colour
• Mosaicism is caused by Ds transposing in random
  cells and not every cell.
• Size of the spot is determined by seed
  development
• Transposition of Ds is determined by the number
  of Ac copies
• Ac controls the transposition of Ds from
  chromosome 9 and when Ds is moved there is a
  breakage in the chromosome.

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Works Cited

• http//mcb.berkeley.edu/courses/mcb140/garriga_08/
  McClintock_notes.pdf Accessed December - 20 11
• http//en.wikipedia.org/wiki/Barbara_McClintock
  Accessed December - 20 11