Question

What happens to organisms that develop useless genes?

Answer 1

They die.

Explanation:

Organisms that develop genes that become useless in its survival, will die out.

Natural selection is a concept that outlines this idea, in which organisms that are better suited/adapted to its environment are more likely to survive and produce offspring.

This ultimately helps eliminate faulty organisms.

This image does a great job providing an example. The tan mice are "unsuited" to survive in that habitat - the grey-ish black surface. Thus, the tanned mice die out and the ones who are suited - the black mice - survive to produce offspring of similar phenotypes.

Hope this helps :)