Monohybrid Cross Problem Set
Problem 12: 2:1 segregation in Manx cats
Manx cats are heterozygous for a dominant mutation that results in no tails (or very short tails), large hind legs, and a distinctive gait. The mating of two Manx cats yields two Manx kittens for each normal, long-tailed kitten,
rather than three-to-one as would be predicted from Mendelian
genetics. Therefore, the mutation causing the Manx cat phenotype is
likely a(n) __________ allele.
Courtesy of PETNET in Australia
Expected genotype ratio
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.
The Biology Project
Department of Biochemistry and Molecular
University of Arizona
Thursday, October 1, 1998
Revised: November 2004
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