Question: why do some genes in some peoples' bodies stop working in the middle of their lives?

Reka Nagy answered on 11 Nov 2016:

Genes can stop working properly if they acquire mutations. These mutations happen because DNA gets damaged – sometimes this DNA damage happens because of the harmful rays of the sun, sometimes it happens due to radioactive elements – but the average person doesn’t handle radioactive elements very often (and when some scientists do have to work with these, they wear appropriate protection that makes sure they are kept safe).

Other times, mutations happen simply because the copying machinery in our cells that is responsible for copying DNA makes a mistake. Usually our cells are quite good at detecting this, fixing the mistake, but sometimes a mistake may slip past their scans.