Gene expression for mathematicians: where to start?
I suppose you first need to learn the basics of genetics before investigating the stats behind gene expression clustering. I will based my recommendations on Khan Academy, that is generally speaking a good very introductory source of information for biology and other fields.

Basics of genetics

Here are a few questions for you:


What exactly do we call the genetic code?
Are you aware that genes are transcribed into mRNA that are translated into proteins?
What is an allele?
Do you know what is an exon and an intron?
Do you know what is a TATA box?


If you answered no to any of these question I would recommend that you have a look at Khan Academy > Central Dogma.

Gene regulation

Then, you should have a look at Khan Academy > Gene regulation

Other

Don't hesitate to have a look at other sections in the biology series of khan academy if you want to broaden your understanding of e.g. what DNA looks like, what is happening at reproduction, how do genes segregate, or other things such as evolution. Typically, Khan Academy > biotechnology could also be helpful as to give you a rough feeling of the methods used to investigate gene expression

Gene expression clustering

The literature on clustering gene expression is very rich (see google scholar).