Perosis or slipped hocks or chondrodystrophy can also cause leg problems including crooked toes and is encountered in young birds whose diet is deficient in manganese (Mn) or some of the following vitamins: choline, nicotic acid, pyridoxine, biotin or folic acid. This is an anatomic deformation of leg bones in young chickens. Can manifest itself in chicks anytime between hatching and 6-8 weeks, and occasionally older chicks.
Have a google search to see if your chicks match symptoms.

It is very important that breeding birds are getting vitamin and mineral support long before you start incubating eggs. If you can eliminate any probable environmental/deficiency problems first, then you can start to look at genetic problems if the incidence of the paralysis continues