Altering the human genome
Concerning the first question, it is very well possible to alter human genetics by injecting a highly radio-active substance that will eventually cause mutations almost all cells. The lifespan will be dramatically reduced, however and the condition is known as "radiation sickness"(link to Wikipedia).

But the person probably means so-called "directed mutations" (link to Wikipedia), when the changes of genetic information is not random, but rather at a certain place and towards a certain function.

Second point, this is the area of extensive research: if we could specifically cure a gene than many diseases that are incurable now (like cystic fibrosis) could be effectively treated.  The complete nanotechnology (link to Wikipedia) is all about it.

As to the third point, this is also a very old discussion about the role of nature (=genetics) and nurture (environment). Answering this question in the scope of other questions I can suggest that (genetic) knocking-out some inhibiting receptor systems in brain can lead to certain malfunctions and deceases, including the lymbic system that is responsible for emotions and perception.