Transgenic animals made by pronuclear injection can be limited for studies of the links between genes and disease because

A)the transgene allele must be dominant to the normal, endogenous allele and some diseases are caused by recessive mutations.
B)the transgene allele must be recessive to the normal, endogenous allele and some diseases are caused by dominant mutations.
C)all diseases are caused by mutations in multiple genes.
D)diseases are not caused by specific alleles of a gene, but by posttranslational modifications of proteins.