plasmid blue or white colony

[ali.ghadersohi at jcu.edu.au]

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Ali Ghadersohi                                  fax:   61-77-791 526
Dept of Biomedical and Tropical Veterinary Sciences  phone: 61-77-81
James Cook University of North Queensland
Townsville, Q, 4811
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 Although there is no difference between plasmid pUC19 and plasmid pUC18 
to produce blue colony, in the same plate containing x-Gal, IPTG and appropriate ampicilin.
Plasmid pUC18 produces white colony?!! Why?!