Here's a breakdown of their famous experiment:

1. Neurospora Crassa as Model Organism: They chose Neurospora crassa, a bread mold, because it's a haploid organism, meaning it has only one copy of each gene. This makes genetic analysis simpler.

2. Mutants: They exposed Neurospora to X-rays to induce mutations. They then screened for mutants that couldn't grow on a minimal media (containing only basic nutrients). These mutants required additional nutrients to survive.

3. Metabolic Pathways: Each mutant was then tested for its ability to grow on different media containing specific nutrients. This allowed them to identify the specific metabolic pathway that was disrupted in each mutant.

4. Gene-Enzyme Link: They found that each mutant was deficient in a specific enzyme required for a particular step in a metabolic pathway. They concluded that a gene controls the production of a specific enzyme.

This experiment was crucial in establishing the "one gene, one enzyme" hypothesis, a fundamental concept in genetics. While bread mold was the experimental subject, bread wasn't directly involved in their experiments.