You Discover a New Allele of a Gene Important for Tail

You discover a new allele of a gene important for tail formation in mice. WT mice have long tails, but mice heterozygous for the allele have short tails. When you cross two heterozygous mice together, you obtain a 2:1 ratio of short-tailed mice to long-tailed mice. None of the short-tailed progeny are homozygous. What type of allele results in short tails?

A) dominant negative allele
B) temperature-sensitive allele
C) lethal allele
D) partially dominant allele
E) incompletely penetrant allele