But our genes are not unchanged by their passage through the generations. Replication of our chromosomes introduces errors called mutations. All children acquire a few mutations on top of those inherited from their parents. Occasionally these will make our children run a little faster or think more quickly than ourselves, but mostly, they will do harm. Our genes have been finely tuned to do a particular job inside our cells.
Mutations are, by and large, random.
Just as random tinkering with your car engine is likely to leave you stranded the next morning, random tinkering with your genes is like to leave your offspring similarly stranded. In our brutal past, defective genes would have been weeded out by natural selection, their owners suffering disease, predators or infertility. Modern medicine has changed all that. In the west at least, many of us survive and lead active lives with gene mutations that would have been fatal to our ancestors. I'm not talking about single-gene defects like cystic fibrosis or muscular dystrophy that remain devastating, but the far more frequent mutations that predispose us to ailments like diabetes, heart disease or cancer.