Final answer:
The inability of heterozygous offspring to grow on a minimal medium without tryptophan supplementation indicates that they are auxotrophs for tryptophan, as both alleles involved in tryptophan synthesis are mutated and nonfunctional, confirming the one gene-one enzyme hypothesis and the role of selection pressure.
Step-by-step explanation:
In an experiment with Neurospora, the trp-1 and trp-9 mutants represent a test to evaluate the biosynthesis of the amino acid tryptophan. If the heterozygous offspring cannot grow on a minimal medium without tryptophan supplementation, it suggests that both alleles (trp-1 and trp-9) in the offspring are defective and cannot produce tryptophan themselves. This is because in Neurospora, each gene is hypothesized to encode one enzyme as per the one gene-one enzyme hypothesis proposed by Beadle and Tatum. Consequently, the inability to grow indicates that the essential enzymes needed to synthesize tryptophan are not functional, thus classifying these organisms as tryptophan auxotrophs. This condition can be alleviated by supplementing the medium with tryptophan, confirming the auxotrophic nature of the mutants under selection pressure.