What determines "Fluffiness"?

As I view the threads in this forum, something has me a bit puzzled. I notice that some Shibas are rather flully and some have a short coat. So I am curious... What determines which way they turn out? I have heard multiple ideas as to why this may be, mainly that the climate plays a role in determining fluffy or unfluffy. Does any one have any input on this? Being how Bear is only about 5 months in age and we have not had a winter with him yet, I am just wondering what to expect.


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Thanks!


[mod edit: changed category - @BearsDad - PLEASE ensure that you put your new threads in the appropriate category from now on.]