(a) Mendel demonstrated that traits can be either dominant or recessive through his monohybrid cross. He crossed true breeding, tall (TT) and dwarf (tt) pea plants.In this experiment Mendel concluded that the F1 tall plants were not true breeding, they were carrying both short and tall height traits. They appeared tall only because the tall traits were dominant over the dwarf trait. This shows that traits may be dominant or recessive.(b) Mendel demonstrated that traits are inherited independently through his dihybrid cross. He considered two traits at a time, seed colour and seed shape in which yellow colour (YY) and round shape (RR) are dominant over green colour (yy) and wrinkled (rr).Mendel observed that the F2 progeny of dihybrid cross had a phenotypic ratio of 9 : 3:3:1 and produces 9 plants with round yellow seeds, three plants with round green seed, 3 plants with wrinkled yellow and one plant with wrinkled green seeds. In this experiment, he found that round yellow and wrinkled green are parental combination whereas round green and wrinkled yellow are new combination. In a dihybrid cross between two plants having round yellow (RRYY) and wrinkled green seeds (rryy), four types of gametes are produced (RY, Ry, ry, rY). Each of these gametes segregates independently of each other and each has a frequency of 25% of the total gametes produced.
From this experiment, he concluded that when two pairs of traits are combined together in a hybrid, one pair of character segregates independent of the other pairs of character. This is known as the law of independent assortment.