Download Barbara McClintock

Barbara McClintock
By Kalim Kassim and Tim Milsom
Prior Knowledge
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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.
Barbara Mclintock
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In the 1940’s and 1950’s, 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.
Conclusion
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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.
Works Cited
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http://mcb.berkeley.edu/courses/mcb140/garri
ga_08/McClintock_notes.pdf Accessed
December - 20 – 11
http://en.wikipedia.org/wiki/Barbara_McClinto
ck Accessed December - 20 – 11