In mice the recessive form of a gene causes the death of the fetus if present in the homozygous state. Mice heterozygous for this gene were crossbred. What percentage of the live offspring would be homozygous dominant?

Answer:

5) C

Let the dominant allele be (T )and the recessive allele be (t)

T                                  t

T                      TT (33%)                   Tt (66%)

t                       Tt (66%)                    tt (dead)

There can only be two genotypes present in live offspring. The fetus with genotype (tt) will die.
Possibility of homozygous dominant (i.e. TT) = 1/3 = 33%