Mendel started out studying rats, but the head monk at his home said he had to get rid of the gross things. Then he started using pea plants because they were easy to grow, clean, and the traits very simple and straight forward, no multi gene determined traits, such as if it had x(recessive) and X(dominant) the X gene would be the phenotype. Also they were easy to pollinate and wouldn't be pollinated if he didn't want them to.

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Gregor Johann Mendel used pea plant for research because of the following reason:
i) Pure varieties of peas were available,
ii) Pea plant showed the number of easily detectable contrasting characters,
iii) Large number of seeds are produced per plant
iv) Pea plant normally remains closed and undergoes self-pollination.
v) The plants are grown easily and does not require after-care except at the time of pollination.
a gud answer,,shravani!!
Thanks Di :)