Python 3 multidimensional dictionary

I'm learning Python 3.X, starting from the basics. Before that, I wrote in PHP.

Now I decided to implement the plan:

1. read CSV
2. as the file lines are read, process them and write them into an array, creating the desired structure
3. ...

Here is my current code

import csv
def gene_name(gene, polfzm, genotype):
    return '_'.join([gene, polfzm, genotype]).strip().replace(' ', '_')
data = {}
genes = []
with open('example-2.cs