Not only are accents not genetic but even if you want to argue along lines of culture and so forth the change in voice could still be explained away by injury (Fett has surely been through a lot) or deliberate tinkering with his voice modulator to enhance his fearsome persona (if you want an example of this in Star Wars, we have only to look at Darth Vader... note the radical change in voice without the mask to with it).
Not only are accents not genetic but even if you want to argue along lines of culture and so forth the change in voice could still be explained away by injury (Fett has surely been through a lot) or deliberate tinkering with his voice modulator to enhance his fearsome persona (if you want an example of this in Star Wars, we have only to look at Darth Vader... note the radical change in voice without the mask to with it).