Interesting. So, this WC animal of NERD's, did it produce T- when put with other known het T- animals? Or was it when it was put with other T+/het T+ animals?
I can think of a case where a T+ animal could throw T- looking animals when put with another T+ type. That would be that there are multiple genes that can give rise to the T+ phenotype. This would be possible because the T+ phenotype comes from a disruption to (but not a total dysfunction of) the conversion of tyrosine to melanin. It is possible that disrupting two seperate steps in this pathway effectively destabilizes the pathway such that no melanin is produced so the animal would look like a T- animal.
However, this would imply that the hets for each of these two types of T+ would have some type of effect on the system alone because it would be the double het (one T+ type1 allele and one T+ type2 allele) that looks like a T- animal. (Do you know if anyone has taken one of the T- looking animals and bred it back to either a T+ or T- animal and what those offspring might have looked like??)
Having a dominant negative phenotype from separate genes whose alleles are normally recessive is not unheard of but it is more than a little rare.
I would definitely be interested in hearing if anyone pursues investigation of these occurances...