The tailless trait in the Manx cat is determined by the alleles of a single gene

Question

The tailless trait in the Manx cat is determined by the alleles of a single gene. In the cross Manx X Manx, both tailless and tailed progeny are produced in a ratio of 2 tailless: 1 tailed. All tailless progeny from this cross, when mated with tailed, produce a 1:1 ratio of tailless to tailed progeny. A) Is the allele for the tailless trait dominant or recessive? B) What genetic hypothesis can account for the 2:1 ratio of tailless: tailed and the result of the testcrosses with the tailless cats?

Explanation / Answer

A) The allele for the tailless trait must be dominant. If the trait were recessive, the parents in the Manx x Manx cross would have had to homozygous recessive for the trait to display the Manx phenotype, so their genotype would have been mm, for example. If mated, they would produce all phenotypically tailless progeny with the genotype mm. Since they produced some offspring with tails, at least one of the parents must be heterozygous for the Manx allele (Mm). B) Using Mendelian genetics, the expected phenotypic ratio of this cross is 3:1 (3 Manx, 1 normal). When a phenotypic ratio of 2:1 is observed in a cross between two heterozygotes, there is probably a lethal allele present. Cats who are homozygous for the Manx trait (MM) do not survive embryonic development and aren't born. This skews the phenotypic ratio of the offspring, since any cats who are homozygous for the Manx trait are never born.

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