How can possessing a mutation in both alleles of the cftr gene cause disease?

When you observe mutation in one allele you can see that the other one can functionate as usually and transfer the information to a cell showing that cell is able to produce enough CFTR proteine to functionate well. And you have two alleles with mutation the cell is no longer able to make functional CFTR protein which leads to disease.

It is evolution, because we all have some of the same traits like eye color or shape or ears.

It would be the equator