http://www.jw.org/en/publications/magazi...
According to this article, it is not known how sin and death were passed on:
"Humans inherited sin and death, two enemies they cannot escape. Though we do not know exactly how sin and death were physically passed on to Adam’s children, we can see the results."
A careful reading of Genesis makes it abundantly clear what happened, so I'm not sure how the WTS has determined that death and sin are something physical. I do not see any scriptural indication that these things are physical in nature, apart from Romans 5:12 which says, "...[T]hrough one man sin entered into the world and death through sin..."
It should be noted that Paul's commentary, here, doesn't cancel out the actual account found in Genesis, chapter 3. One would think, after reading Romans 5;12, that sin and death mysteriously entered into the human condition by an unknown process. However, going back to Genesis, we see that death was NOT a physical condition, but - rather - a geographical one. If you are in the Garden of Eden, it is possible to live forever by eating from the Tree of Life. If you are NOT in the Garden of Eden, you will NOT be able to eat from the Tree of Life and, therefore, you will die ... eventually. For some reason, people are always forgetting the part about the cherubs who were assigned to block access to the Tree of Life. It is obvious that death entered into the human condition due to a lack of access to a life-sustaining "substance" or component. The notion that we have "bad genes" or that our bodies are imperfect cannot be successfully demonstrated scripturally.
Additionally, perfect people or spirit creatures can sin, so a "sinful nature" is not derived from imperfection. For example, Satan wouldn't have tempted Jesus if there was no chance that he might sin. Satan, himself, had a sinful nature even though has was an angel. The only thing that changed in Adam upon eating the forbidden fruit was his perception. Can the knowledge of good and evil be considered something sinful? If it makes us more like Jesus and Jehovah, then how can it be bad?
Jehovah made Eve's pregnancies more painful (which is something we wouldn't expect from a loving God), and he cursed the ground, caused farming to be more difficult for Adam. Eve was physically altered where reproduction is concerned, but nothing happened to Adam, apparently. One might say that Eve passed on her "bad genes" that made the act of giving birth more difficult for her offspring, but that is a mechanical problem which would not affect her decision-making at all.
Jehovah's Witnesses like to think that we will be restored to our original "perfect" state when we get to Paradise. I contend that we are not necessarily any less perfect now than when Adam and Eve were ejected from the Garden. Any subsequent deterioration would have to be attributed to our being deprived of access to the Tree of Life. Keep in mind that Genesis 3 suggests that Adam and Eve had not yet eaten fruit from the Tree of Life. One would expect it to have a permanent effect (like the Tree of Knowledge), which would mean that banishment from the Garden wouldn't affect their lifespan. Either way, death was IMPOSED geographically and did not result from physical imperfection. Any claims to the contrary are based on the Writing Committee's creative use of their imagination.