Well, I can't say it any better than Twisterdog did. I too believe it's a combination.

Through a bizarre chain of events, I have met someone who saw my dog, Franklin, being born. She worked for the musher who bred Franklin's litter. According to her, from the moment he was born, Franklin has been shy. His mother's shy, his littermates are shy--not the same extent Franklin is. Three of his littermates went home with my friends as soon as they were old enough. They are still a bit timid with new things, but they are generally happy dogs--actually quite competitive on our local sprint mushing circuit. Franklin wasn't so lucky and he ended up in our shelter at a year old and stayed there until he was 2. He's been here almost 2 years now. He's happy and outgoing with me and my husband. He's a confident dog in his harness. Take him somewhere new, introduce someone new, he freaks out. That genetic shyness is still there. Unfortunately, I am seeing more and more of what I believe is genetic shyness in husky lines.

I have another dog, Kayleigh, who arrived here starved, aggressive, incredible prey drive, food protective...you name the bad habit, Kayleigh had it! Almost three years later we've overcome many of Kayleigh's issues. However, she's still dog aggressive at times and she still has a high prey drive. I used to think that with enough time and training, we could overcome those things too. I blamed much of it on Kayleigh's puppyhood, which I know was awful. Then I met a guy who owns her littermate. Tika has always had a great life, spoiled, trained, well treated...a great life! She's dog aggressive and like Kayleigh, anything smaller than her is a snack!

For my own dogs, I have to know what it is that sets of the "nature" things and control their access to those triggers--Franklin doesn't come to town with me unless he has to go to the vet. I lock all the cats up when Kayleigh comes in the house,ect.