In Section 4.4 we calculated that if we made a genomic library containing fairly small (4 kb) fragments of bacterial genomic DNA we would need more than 5000 individual clones in the library to have a 99% chance of a particular gene being present. For a eukaryotic organism such as the yeast Saccharomyces cerevisiae this number increases to about 14,000 clones and for a human being or a higher plant it would be higher still. Using bacteriophage and cosmid vectors, which make it possible to clone larger DNA fragments, the number of clones in a genomic library can be reduced, but

it is still a formidable task to detect the one or two clones containing your gene of interest.