It is very unlikely that predictable genetic engineering for animals will become possible earlier than twenty years from now and a time between 50 years and 100 years from now seems most likely.
We are still very far from completely understanding how a single nucleated (eukaryotic) cell works. After understanding a cell we should first be able to completely understand simpler multi-cellular living beings like fungi and plants. The animals are the most complex and there is no doubt that they will be the last for which it will become possible to change some genes and then predict with certainty what kind of animal will develop from such a modified cell.
In any case, when that will happen it will be possible to design the body of an animal, within certain constraints, like we design now any complex machine. Bad designs will always remain possible, but good designs should easy avoid mechanical problems in the legs. By that time the simulation possibilities should be much better than today.
We are still very far from completely understanding how a single nucleated (eukaryotic) cell works. After understanding a cell we should first be able to completely understand simpler multi-cellular living beings like fungi and plants. The animals are the most complex and there is no doubt that they will be the last for which it will become possible to change some genes and then predict with certainty what kind of animal will develop from such a modified cell.
In any case, when that will happen it will be possible to design the body of an animal, within certain constraints, like we design now any complex machine. Bad designs will always remain possible, but good designs should easy avoid mechanical problems in the legs. By that time the simulation possibilities should be much better than today.