Final answer:
The nematode Caenorhabditis elegans has made a significant contribution to genetics as a model organism.
Step-by-step explanation:
The main contribution to genetics that the nematode has made is the use of the nematode Caenorhabditis elegans as a model organism for studying genetics. This nematode is transparent, allowing researchers to easily observe and study genetic changes within the animal. With only 1,000 cells and a small genome of 20,000 genes, it is an ideal candidate for studying various genetic phenomena like cell-to-cell interactions, cell division, and intracellular transport.