During conception, some of the small sets of genes from the sperm or the egg will be turned off, and the corresponding copy from the other parent will work on its own, and some will manage other genes.
Then, the embryo starts to read the signals from its environment right after its conception to differentiate multiplying cells by switching some genes in them on and off. Some of these changes remain permanent, and some are reversible. And some can result in neurological disorders and noninfectious diseases.
Since genes were defined as the units that transfer heritable traits, new and new levels of genetic complexity are being discovered, after the double-helix model of DNA was published, and especially in recent years. Only about one percent of genes serve as a template for RNA that contracts unique proteins for building the body, and sections of the genome that were viewed as “junk DNA” turned out to contain the mechanism to manipulate the rest of genes. Several types of genetic switches were discovered. For example, micro-RNAs are nucleic-acid strings that can make genes shut down. They were mimicked in the interference technique RNAi.
Another example is methyl groups, which can either influence genes directly, or interact with another type of switches, proteins called histones, which are not completely switched on or off in at least a third of our genome. Such genes can start making their unique proteins but not finish, and can be accidentally turned into something like cancer stem cells. In mice, if a particular gene that makes a protein Agouti is not switched off during pregnancy by folic acid, a methyl group switch that comes with diet as a vitamin, the embryos develop into obese babies prone to diseases, and even having atypical color.
Genome is a complicated system, therefore it has a great potential for errors. It might have been evolutionary built to make individual organisms die eventually.