Serious mutations and high death rates.
The trans gene is already a recessive mutation. It being recessive, means two dragons that are both carrying the gene need to mate to pass it off to offspring.
If the dragon is showing the trans gene, that means it's had the trans gene passed on from both of it's parents. Now if that is multiplied again, through into the next offspring, the mutation itself multiplies and causes serious issues.
It is very frowned upon to breed two trans'
-Brandon