Which of the following characteristics does not occur in pairs?

Given below are two statements:
Statement $I$: Back cross is the crossing of $F_1$ hybrid with one of the two parents from which they were derived.
Statement $II$: Test cross involves the crossing of $F_1$ hybrid with its homozygous recessive parent.
In light of the above statements,choose the correct answer from the options given below.

Out of the $7$ pairs of contrasting traits selected by Mendel,how many traits were related to the pea pod?

Mendel chose pea plants because

Give scientific reasons: The Mendelian experiments cannot be carried out in human beings.

Mendel's findings were first published in the journal titled: