The garden pea, Pisum sativum was the model system used by Gregor Mendel to formulate his famous Mendel's laws. But the characters used by Mendel have been since shown to be controlled by genes belonging to different linkage groups or to distant parts of the same linkage group. Thus, Mendel failed to observe the phenomenon of linkage. Instead, linkage was first described and interpreted by Thomas Morgan in his works with Drosophila. Linkage in pea was first described in 1912 by Vilmorin and Bateson. Thus if the history of pea genetics begins with the Gregor Mendel, the history of the pea linkage map begins much later. There are seven linkage groups in pea; present on chromosomes 1, 6, 5, 7, 3, 2 and 4 respectively. So, the correct answer is '7'.