It is the cross between two plants or animals differing in two pairs of contrasting characters. This cross involves two characters separable in inheritance like flower, colour and height of the plant, colour of the hair and colour of the skin etc.
A homozygous pea plant with yellow round seeds is crossed with a green wrinkled seeded pea plant (homozygous). In peas, yellow seed character Y is dominant over green y and round seed character R is dominant over wrinkled r (each allele when studied separately shows 3:1 ratio). All the F1 generation plants are round and yellow seeded. Yellow round seeded parent produces one type of gametes YR and green wrinkle seeded parent also produces one type of gametes yr and gametes from both the parents undergo fertilisation and result in F1 plant Yy Rr.
Because of the dominance, Round and Yellow colour are expressed in the F1 generation. F1 plants are self- pollinated or inter-crossed. They produce four types of gametes YR, Yr, yR and yr. Since all the four genes separate, they combine in all possible combinations.
Four types of gametes of one parent combine with four types mof gametes of the other parent producing 16 possible combinations. These can be easily represented in Roman square or checker board, F, generation plants show 9:3:3:1 ratio. 9 tall red, 3 tall white, 3 dwarf red and 1 dwarf white flowered plants.