How does curing a disease have anything to do with understanding genetic causes for things? There are plenty of genetic problems which we understand, but cannot cure. Simple.
But there are also genetic problems that we've found, and FIXED, or at least explained through genetic science.

And do you not understand how that happens?
Those are called recessive genes.

Breeding an albino rat, with an agouti rat (assuming they both carry nothing else) will produce 100% agouti rats.
Breeding an agouti rat from that litter, to an albino rat, will produce albinos and agoutis.
Breeding an albino from that litter, to an albino rat, and you will have 100% albinos.

Albino, is recessive.
You cannot breed an albino rat to an albino rat and get a rat that isn't albino.
An albino rat is genetically c/c, meaning that it is heterozygous albino.

Breeding a c/c to a c/c will only produce more c/c.
If you know what a punnet square is (something you learn in middle school biology) you would be able to calculate the probability of getting albino by breeding c/c to c/c, and it's 100%. How much more exact do we need to get?

You can breed two coloured rats, and get albino, IF both rats are carrying the recessive albino gene. Meaning that a rat which is a/a C/c would be black, carrying the albino gene. and if you bred it to another rat that is a/a C/c then you would get 25% albinos, and the rest would be black.