H. J. Muller used the ClB technique to identify many radiation-induced recessive lethal mutations on Drosophila’s X chromosome, which is now known to contain more than a thousand genes. These mutations could be propagated in stock cultures by keeping them in heterozygous condition with the ClB chromosome. Would you expect all these lethal mutations to be alleles of one essential X-linked gene, or to be alleles of different essential X-linked genes? Why couldn’t H. J. Muller determine the answer to this question experimentally?