Step 1: Background.
Mendel proposed the basic principles of inheritance, but the chromosomal basis was established later.
Step 2: Sutton and Boveri’s contribution.
In 1902–1903, Sutton and Boveri independently proposed that chromosomes carry Mendel’s factors (genes) and thus form the basis of inheritance.
Step 3: Conclusion.
Hence, the chromosomal theory of inheritance was propounded by Sutton and Boveri.