The lethal factor only applies to white chinchillas and animals demonstrating the velvet gene (yes even TOV chins). This means that you should never breed two whites or two velvets together, as babies that end up homozygous for the trait will never develop and be reabsorbed by the females uterus. The dangers of this are not quite known, but its believed to put mom at a higher chance for infection. Whether or not that is true, it would also be unwise to attempt because it lowers the amount of baby production.