Non-Mendelian Manx?
If you cross two Manx cats, you get two tailless kittens for every one tailed kitten. . .
The reason is that the TT genotype is embryonic lethalóit causes the embryo to fail to develop. TT embryos are never born.
Manx cats must therefore have the Tt genotype.
The Tt x Tt cross gives you one TT (lethal) to two Tt (Manx) to one tt (tailed).