Question

how do mendel's experiments show that traits may be dominant or recessive

Answer 1

Mendel demonstrated that traitscan be either dominant or recessivethrough his monohybrid cross. He crossed true-breeding tall (TT) and dwarf (tt) pea plants. ... They appeared tall only because the tall trait wasdominant over the dwarf trait. Thisshows that traits may be dominant or recessive.

Answer 2

It is because organisms may have homozygous or heterogyzous genotype.in heterozygous genotype he observed that the two allles of gene pair express in such way that the phenotype is of dominant allele.
e.g in flower colour trait Rr , R is for red flower and r for white but overall appearence of colour is red due to dominant allele