MathBench > Probability

BLAST and (Im)probability


dna bridgeThe Law of Combining says:

If you want to count the possible combinations of ONE PICK from set 1 and ONE PICK from set 2, you can just multiply the size of set 1 by the size of set 2.

Or simply,

Ntot = N1 * N2

If you are picking multiple times from the same set, you can use an exponent:

Ntot = (Nset ) # picks

Using the Law of Combining will often get you some very big numbers, which might be better expressed as


Nucleotides form a 4-letter alphabet and the same Law of Combining applies to them as well.

Amino acids form a 20-letter alphabet. Any amino acid sequence can be coded in MANY possible ways.

Although BLAST contains a huge number of nucleotide sequences, it is tiny compared to the number of POSSIBLE nucleotide sequences.

In order to compare sizes, you need to divide. For scientific notation, this means

"divide the numbers and subtract the zeros".

And finally, BLAST is not much harder than google...



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