The previous answer is excellent, and refers to substances a person ingests or is exposed to, which covers, I think, most of the ways in which a person's genes might be damaged.
Another way is radiation exposure. If you research and look at what happened to the generations following the survivors of Hiroshima and Nagasaki, or those of the resident of Chernobyl, Ukraine after the nuclear reactor accident there, you see a long term pattern of genetic mutation and birth defects. Workers in American nuclear plants, or those who handle nuclear waste, have to have high levels of training and a lot of protective gear and procedures to lessen the risk of their exposure and possible genetic damage.
In the twentieth and twenty-first centuries, scientists have determined that there are many things that can damage the genetic code of a person, his/her DNA. Such things as UV light, cigarette smoke, toxic fumes, asbestos, drugs, alcohol, excessive radiation are among them.
Put rather simply, what happens when the above-mentioned things are introduced into a person's system is that proteins in the body can be adversely affected; then, the repair genes of a person's DNA do not work effectively, and such diseases as cancer can affect a person.
Alcohol has very detrimental affects upon unborn children
- when consumed during pregnancy, for example, can greatly affect the fetus as it has been shown that alcohol enters the amniotic fluid. The fetus actually swallows some of its amniotic fluid; since the liver is still underveloped, it cannot process this alcohol and excrete it fast enough to prevent damage to cells. Babies subjected to alcohol in the womb run the risk of being born with congenital heart defect, club foot, kidney defects, mental and physical disabilities.
Drugs affect mental abilities.
- Marijuana, for instance, affects memory, judgment, and the emotions. It has also been shown to weaken problem solving abilities.
- Cocaine can cause loss of interest in life and paranoia. It also reduces the ability to feel physical pleasure.
- Inhalants cause diminished mental powers to learn, remember, and solve problems as they have an immediate effect upon the brain. Fatty tissues protecting the nerve cells are destroyed by inhaling such things as glue, gasoline, paint thinner, and hair spray.
- Steroids can impair memory and ability to learn
- LSD and Ecstacy cause paranoia, confusion, anxiety, and panic attacks.
- Methamphetamine (crystal meth) is extremely damaging, causing reduced motor skills and verbal learning, severe dental problems, violent behavior, visual and auditory hallucinations and delusions. Transmission of HIV and hepatitis B and C can also come as a result of using Meth if people share needles with which to inject the meth.