[Bioperl-l] E-value of a combined alignment?

[Yee Man Chan]

Hi folks,

	I am aligning mRNAs against human genome using ungapped tblastx. I
got a bunch of HSPs with different e-values. I can observed that some of
them should be in the same group because they are exons of a gene. But
then what is the e-value of all these HSPs combined? 

	I know the formulas of e-value and bit score for BLOSUM62:

Let S' be bit score, S be score, e be e-value, m be the length of HSP,
n be length of database.

S' = (0.318 * S - ln(0.135)) / ln(2)

e = m * n / (2^(S'))

	I am guessing the formula for the e-value of
non-overlapping combined e-value to be:

S'' = (0.318 * sum_of_S - ln(0.135)) / ln(2)

e' = sum_of_m * n / (2^(S''))

	Is this correct? Or do you know the right way to calculate it?

Thanks in advance.
Yee Man