How do Mendel's experiments show that the
(a) Traits may be dominant or recessive
(b) Traits are inherited independently

(a) Monohybrid cross between two pure breeding varieties always obtained hybrid progeny exhibiting one parental trait while the other trait was never expressed in $F_1$ generation. It suggested that two alleles of a gene can be either dominant or recessive and presence of the dominant allele in $F_1$ hybrids masks the expression of recessive one.

(b) A dihybrid cross between two pure breeding varieties obtains dihybrid progeny exhibiting the only dominant trait. However, selfing of $F_1$ hybrids obtains both parental and new combinations of traits in 9:3:3:1 ratio in $F_2$ generation suggesting an independent assortment of characters.