Final answer:
A genomic library will contain the greatest number of different clones as it includes the entire genome, while a brain or liver cDNA library only includes the active expressions in those tissues and will have a smaller number of clones.
Step-by-step explanation:
Comparing the number of different clones in a genomic library versus a brain or liver cDNA library depends on the source of the DNA and the complexity of the expressed genes in the particular tissue type. A genomic library will contain the greatest number of different clones because it represents all the possible fragments of an organism's genome, including both coding and non-coding regions, regulatory sequences, and introns. On the other hand, cDNA libraries, such as a brain or liver cDNA library, will contain a much smaller number of different clones as they are constructed from mRNA and reflect only the genes that are actively expressed in that particular tissue at the time of RNA extraction. These libraries will lack introns and other non-coding regions present in the genomic DNA.
Briefly, the number of different clones in a brain cDNA library or liver cDNA library will be less because they represent a subset of the entire genome that is actively transcribed in those tissues. The genomic library, however, includes every part of the genome. Therefore, for a student trying to understand the differences, it's imperative to appreciate that the complexity and size of the genomic library will surpass that of tissue-specific cDNA libraries.