The tailless trait in Manx cats is caused by a dominant genetic mutation known as the Manx taillessness gene (often referred to as the M gene). This mutation affects the development of the cat's spine, particularly the coccygeal (tail) vertebrae.

The tailless trait in Manx cats is caused by a dominant genetic mutation known as the Manx taillessness gene (often referred to as the M gene). This mutation affects the development of the cat's spine, particularly the coccygeal (tail) vertebrae.

Key genetic factors include:

• The Manx gene is dominant and highly penetrant, meaning if a cat inherits one copy of this gene, it usually results in no tail or a very short tail.

• Cats with two copies of this gene (homozygous) usually die before birth (are non-viable), causing spontaneous miscarriages. This is why breeders avoid mating two tailless Manx cats to prevent this outcome.

• The gene causes shortened or absent tails, but the severity varies — some Manx cats have no tail at all ("rumpy"), some have a small tail stump ("stumpy"), and others have a slightly longer tail ("riser" or "longy").

• The mutation disrupts normal tail bone development but can also affect the overall spinal column, sometimes leading to neurological and physical issues collectively called Manx Syndrome, which can impact bowel, bladder control, and hind limb function.

• Recent studies show that the Manx tailless trait is linked to specific mutations in the T gene. These mutations cause frameshifts and premature termination in the Brachyury protein, which is crucial for tail development.

• Heterozygous cats (with one mutated T gene allele) display the tailless phenotype, while homozygous cats (two mutated alleles) tend to be lethal in the womb.

In summary, the taillessness of Manx cats is due to a genetic mutation in a dominant gene affecting tail and spine development, balanced by the risks of breeding two tailless cats due to embryo lethality from double copies of the gene. This genetic basis explains the breed's distinctive tailless appearance and associated health conditions.