If a heterokaryon is made by crossing two mutants that have mutations in different genes of the adenine synthesis pathway the phenotype of the heterokaryon will be?

Heterozygous.

2 alleles make up a genotype. One from the mother, one from the father. For example, the genotype for tallness may be TT.

hybrid or heterozygous.

nonsense mutation, missense mutation, deletion, point mutation

Deletion (resulting in a frame shift), duplication (also resulting in a frame shift), or a plain old SNP (change of base). You might also be looking for one which changes the amino acid coding...