Found this while surfing the world wide web... Hope it helps
Here are those undesirable traits of DT that I select to eliminate them from the line so that these traits will not pass o*n to the next generation if they are used in breeding.
The following will show a series of those undesirable traits of DT.
1. Uneven Lobe
Usually the upper lobe is smaller than the lower lobe.
2. Y-Split DT
The split is of Y-shaped and it is somehow ugly and will not have those desirable overlapping lobes.
3. Crooked Spine
This is rather common in DT as well as ST when DT gene is present. A slight (I said SLIGHT) curved spine is acceptable for DT as itís a common trait of DT, but any curved or crooked spine for ST is definitely unacceptable. Too much crooked spine for DT is highly undesirable for breeding as it will pass o*n to the next generation.
4. Deformed Body
This is also quite common when the DT gene is present causing the body to deformed or sometimes shortened. Never use these for breeding. If you used such deformed or shortened body fish for breeding, the deformed gene will pass o*n to the next generation and you will find that most of the body will become shorter and shorter.
5. Fused Lobes
These is an extreme case of incomplete split of the DT. Both the tails are fused together. o*nly choose those with total split tails.