Manx cats are heterozygous for a dominant mutation that results in no tails (or very short tails). The mating of two heterozygous dominant Manx cats yield two-to-one ratio of Manx kittens to normal, long-tailed kittens, rather than three-to-one ratio as would be predicted from Mendelian genetics. How can you explain the results?
The first weblink below says this:
"The expected genotype ratio when two heterozygotes are crossed is 1 (homozygous dominant) : 2 (heterozygous) : 1 (homozygous recessive). When a phenotypic ratio of 2 : 1 is observed, there is probably a lethal allele. Individuals homozygous for the lethal allele (tt in this case) do not survive embryonic development, and are not born. The heterozygotes have a phenotype distinct from normal cats"
This is to say that when two heterozygote Manx cats (Tt, Tt) reproduce together, the kittens can be, by Mendelian genetics, TT (homozygous dominant, 1/4 of the kittens), Tt (heterozygous dominant, 1/2 of the kittens) and tt (homozygous recessive). However, if a ratio of 2:1 Manx to normal long-tail kittens is observed rather than the 3:1 as predicted by Mendelian genetics, this implies that the combination of two of the same type of allele from two mating heterozygous Manx cats is lethal. That is to say, the kittens of either type tt or TT from two breeding heterozygous Manx cats don't make it past the embryonic stage so that that phenotype is a lethal combination and produces cats that cannot survive into being.
In Manx cats, it turns out that the dominant allele is deadly in homozygous individuals. Heterozygotes will live and display the tailless phenotype, while recessive homozygotes will have normal length tails, and dominant homozygotes will not survive to birth.
In a sense, the dominant allele in the heterozygous cats should be denoted differently, eg T_1 for the lethal dominant allele in homozygotes as opposed to T_0 for the nonlethal dominant allele in heterozygotes.
They are called 'Manx' cats since they originated on the Isle of Man, off the coast of the United Kingdom. Because the cats sometimes produce kittens that don't survive past the embryonic stage in the womb, a cat being Manx is seen as a disorder such that the cat is viewed as, in a sense, unhealthy.