Kitten drags legs? Neurological Gene Disorder?

Our Manx cat had two kittens. One had a tail and one had no tail. They are about 5 weeks old and the Manx kitten is walking as normal. The kitten with the tail just drags it's back legs around. Her front legs work as normal and the other kitten is completely normal. They remind me of flippers because she can push them out to the side but can't stand on them. They won't even really go in a normal position. They don't really look or feel malformed, but they won't go in a normal position or stay in a position close to normal. They lay rather flat. When she's crawling around she drags herself around with her front legs and makes an attempt to push herself with her back legs. Her last litter of kittens had a kitten with a similar back legs problem. It was given to a happy, caring home. I believe that both litters had the same father, who has a tail. The crippled kitten looks like the father (Long haired, tail, colored the same) while the Manx kitten looks like the mother(Manx, short haired, tiger colored). Neither parents have anything wrong with them by looking at their Phenotype, not their actual genotype. Any help? Possibly Genetic?