TOV stands for Touch of Velvet. It is a term given to chinchillas who carry the gene which causes an extra dark mask and veil to form over the face and back. Any color chinchilla can carry this gene, and it expresses itself in different ways and sometimes not at all in certain colors.
There is a 'Lethal Factor' associated with the TOV gene which means if two TOV's are bred together, there will be 25% less offspring because this gene cannot be Homozygous.
There is much debate about what happens to that 25%. Do they even begin to form? Are they reabsorbed? Are they just not carried to full term and end up being passed as mumified fetuses eventually? Some people believe that if TOV's are bred to each other continually, blockages can occur.
There is NO proof anywhere that I or any breeders I have spoken to, have been able to find regarding the lethal factor in TOV's and Whites. Usually though, there isn't more than one or two kits in each litter when two TOV's are bred together so it has pretty much become an accepted fact in the pet/hobby breeder circles that there is indeed a lethal factor and most of the time you will be told to 'Never breed two TOV's'.
While 'Lethal Factor' may sound deadly, it really is not as serious as everyone makes it out to be. If I had two outstanding TOV's so that if bred, the quality of the offspring could make up for having less kits in the litter, then I would probably breed them.
Now as for what they look like phenotypically.... Every chin that carries the TOV gene, will have a mask and veil over the face/back, and will have paw stripes.. If the TOV does not show, then it is not carried. Whites are the only exception. When a TOV is bred to a non-TOV, approximately 50% of the offspring will be TOV.
A Black Velvet is genetically a TOV Standard. They have black faces and backs, and the black fades to grey on the sides, and then they have white bellies. They really are beautiful animals. However, it takes alot of careful, selective breeding to keep the veiling just right. You don't want too weak of veiling where it breaks in the neck and just looks faded, but at the same time you don't want the veiling to be too heavy, where it doesn't fade on the sides, but goes all the way to meet the white belly. These very heavily veiled BV's are sometimes jokingly called 'upside-down skunks', and are not very desirable because then they lose the very attractive 'contrasty' appearance. The same goes for the weakly veiled ones.
When you mix TOV with White, it can get a bit more complicated. When you get a White from a TOV parent, there really is no way to be certain it is or is not TOV until the animal is bred to a non-TOV chin and does or does not produce TOV offspring. Alot of the time, TOV Whites will have tipping over the face and sometimes back, but very rarely do you see a White with a dark mask and veil. One thing that is characteristic of the TOV gene in whites is that is causes them to darken with age. Often times they are born heavily marked, lighten up, and then gradually get darker tipping again with age.