I am aware of the fatal gene in Japanese and other short-legged breeds, but how do you tell if your parent stock carries it? I have bred two short leg birds together several times and gotten a normal hatch rate. Is it a dominent or recessive gene? Also should Black -Tailed White only be red to Black-Tail White or can other colors be bred in without messing up the colors?

If your Japanese Bantam (or other short legged breed) has short legs, it has the gene. The normal nominal hatch rate for short/short matings is 75%. If you get better rates than that, it is either a statistical fluke, or both parents have long legs. I have hatched many hundreds of Japanese Bantams, and the rate is quite steady when averaged over the long run. Some times I will get 80-90% of the fertile eggs to hatch, but this is invariably offset in another hatch.
As far as outcrossing your BTW to another variety, I have not seen this done with any reliable success. Keep in mind that BTW is one of the toughest varieties to breed, and it is very easy to undo the hundreds of years of breeding that went into them. Japanese Bantam is the only breed with any quality at all in the Black Tailed While Variety.

Thanks for the response! Is the short leg gene always a phenotypical gene or is it sometimes a genotypical gene? I am not only asking for myself, but also I would like to let my 4-H group know of my findings.

Since phenotype refers to an inheritance factor that may or may not be observed, depending on other genetic interactions, short leggedness is not phenotypical. You either have it or you don't. Since the gene for the short legs has a fatal recessive factor, there should not be any ss (short-short) birds, only ls (long-short) genetic pairings. If you have a ll combination, the legs are long, and they will never produce short offspring. You get 50-50 long-short offspring from a long/short mating,but in breeding it is problematic because proper body carriage (short and low)the single most important factor in determining any breed, is basically unselected in the long-legged parent.

I have a quick question about short legs -
My japanese chicks are a little over a month old. How long should the short-legged chicks' legs be?
Also, I have some buff-colored chicks with black on their hackle feathers. They were bought from a hatchery (I know, not suggested) so I don't think they're pure.