A simple two step experiment was performed by Lederberg and Tatum, using the above two strains (strain A = met^-bio^-thr⁺leu⁺thi⁺ and strain B = met⁺bio⁺thr^-leu^-thi^-) to test whether or not the genes are organized into linkage group(s). In step one, a mixture of A and B was grown on a minimal medium supplemented with biotin so that both met⁺bio⁺thr⁺leu⁺thi⁺ as well as met⁺bio^-thr⁺leu⁺thi⁺ could grow. They were designed as met⁺bio^?thr⁺leu⁺thi⁺. In step two, the colonies grown on biotin supplemented medium were tested on minimal medium (deficient for biotin) and were classified into two categories (bio⁺ and bio^-). Similar tests were performed with other supplements (threonine, leucine and thiamine) also and the results are presented in Table-12.1, which suggests linkage, because in the absence linkage, due to independent assortment 1 : 1 ratio between + and - strains for each supplement should be obtained.