This article is from the Japanese Bobtails Breed FAQ, by Jean Marie Diaz firstname.lastname@example.org.
Like the Japanese Bobtail, the Manx came about as the result of a
natural mutation occurring in a gene pool limited by the borders of an
island. That is where the similarity begins -- and ends. The Manx is a
heavy-bodied and muscular cat, with no tail at all (in the show
specimen). The Japanese Bobtail is a tall, elegant, refined cat in
appearance, with just a "puff" or a "pom" of a tail.
The genetics differ as well. The Manx gene is a dominant, which is
lethal in the homozygous form. Since all living Manx are thus
heterozygous, any Manx litter can produce tailless (rumpy),
partly-tailed (stumpy), or fully-tailed kittens. The Manx gene is also
linked to genetic problems such as spinal bifida, and hip, pelvic, and
anal abnormalities. In contrast, the Japanese Bobtail gene is
recessive -- two Japanese Bobtails, bred together, will always produce
kittens which are more or less bobtailed. The Japanese Bobtail gene is
also not linked to any other form of spinal or bone abnormality.
Less is known about the American Bobtail, as the breed is still in
development, but it is believed to be a variant of the Manx gene, and
no relation to the Japanese Bobtail. It is being developed as a large,
shaggy, semi-longhaired breed with a tail which is about half the
length of a normal tail.