Nature vs. Nurture is not an absolute, meaning it's not a binary cause, either genetics or environment. Human behaviour is very complex and the behaviour is caused by both genetics and environment. Epigenetics is the influence of the environment on turning on or off genes (methylation of DNA and chromatin remodelling) but if you don't have the genes to be turn off or on or you have a different alleles (versions of a genes) you will respond differently to the same environment. Thus genetics is important you can't argue this. As for genetic diseases: Cystic Fibrosis, Huntington's, Sickle cell anemia are all examples of diseases that we can predict with 100% certainty that the person will acquire that disease. Just like I am 100% certain a fly without the eyeless gene will develop without eyes. Genetic disorders like colour blindness, Down Syndrome, Klinefelter syndrome, Turner syndrome can be predicted on the genotype with ~100% certainty. Are we prisoners of our DNA then? No! we can change the outcome for example Phenylketonuria is a genetic disorder that if left untreated can lead to mentally disorders. The treatment is easy, just lower the Phenylalanine (an amino acid) in their diet. So in this case DNA is not not destiny. You can use the information in your genome to change the outcome.
Genes and Genetics are important to who you are!
Don't be afraid of Genetics- Love Genetics!
Ian