Barbara McClintock

1. Barbara McClintock By Kalim Kassim and Tim Milsom

2. 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.

3. Barbara Mclintock • 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)

4. 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.

5. 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.

7. 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