The Manx gene is a dominant gene (known as M), this gene shortens the spine of the cat, however not always to the same extent.  The gene will affect each individual kitten differently - creating various types of Manx:-

• The dimple rumpy - here the cat will have a small indentation at the base of the spine
• The rumpy - a completely tailless cat
• The rumpy riser - a cat who has a small rise at the base of the spine caused by a small piece of cartilage
• The stumpy - a cat with a tail between 1-5 inches long
• The longy - a cat with a tail longer than 5 inches, but shorter than a standard tail

The Manx gene is also semi-lethal which means that no kitten who inherits this gene from both parents will ever survive. Normally these kittens are never born - they are broken down inside the womb before birth, however occasionally this does not happen and the kittens are either still born or die within a couple of hours of birth. 

This inevitably leaves any Manx who has inherited one Manx gene from a parent also carrying the recessive gene, so a Manx cats genetic make-up is "Mm" - when a kitten inherits this recessive gene from both parents this kitten is born with a long tail (often longer than a standard tail), these kittens are known as tailies, or full-tailed Manx.