Answer:
In first generation one allele pair with an offspring is lethal, hence the phenotypic ration is 2:1
Explanation:
Let the allele representing the tailless trait with in mice be "t"
the allele representing the wild-type offspring trait with in mice be "t+"
Generation I
Genotype of tailless mice is "tt+"
Cross between two tailless mice will produce following offspring
tt+ * tt+
tt, tt+, tt+, t+t+
Out of four offspring, the one with genotype "tt" is lethal, thus ration of tailless and wild-type offspring is
2:1
Generation I
Genotype of tailless mice is "tt+"
Genotype of wild-type offspring is "t+t+"
Tailless mice crossed with wild-type mice produce-
tt+ * t+t+
tt+, tt+, t+t+, t+t+
Ratio of tailless to wild-type progeny is
2:2
1:1
