In addition to his work on monohybrid crosses, Mendel crossed varieties of peas that differed in two characteristics—that is, he performed dihybrid crosses. For example, he used one homozygous variety of pea with seeds that were round and yellow, and another homozygous variety with seeds that were wrinkled and green. When he crossed the two varieties, the seeds of all the F₁ progeny were round and yellow. He then allowed the F₁ to self-fertilize and obtained the following progeny in the F₂: 315 round, yellow seeds; 101 wrinkled, yellow seeds; 108 round, green seeds; and 32 wrinkled, green seeds. Mendel recognized that these traits appeared in an approximate ratio of 9:3:3:1; that is, ⁹/₁₆ of the progeny were round and yellow, ³/₁₆ were wrinkled and yellow, ³/₁₆ were round and green, and ¹/₁₆ were wrinkled and green.