Mendel began working with the pair of characters,those were seed shape and seed color. The cross in which two traits are studied at same time is dihybrid.
As we know the monohybrid cross for seed color (Y/y x Y/y), which gave a progeny ratio of 3 yellow:1 green.
The seed shape phenotypes were round (determined by allele R) and wrinkled (determined by allele r). The monohybrid cross R/r x R/r gave a progeny ratio of 3 round :1 wrinkled as expected.
“When two contrasting pair of traits are followed in same cross, they stay together in F1 and assort independently during gamete formation of F1”
Mendel studied two traits (seed shape and seed colour) at a time.
Seed shape could be round (RR, Rr) or wrinkled (rr) .
Seed colour may be yellow (YY) or green (yy).
Mendel crossed yellow and round (RRYY) seeds to that of green and wrinkled (rryy) seeds. All F1 individuals were round and yellow (RrYy)
In F2 following phenotyptic ratios were observed
9 (round and yellow) (R-Y-) : 3 (round and green R-yy) : 3 (wrinkled and yellow, rrY-) : 1 (wrinkled and green rryy).
Two new phenotypic ratios (round/ green and wrinkled/ yellow) were produced by independent assortment.
From the result Mendel concluded that different gene pairs assort independently in gamete formation. Now known as Mendel’s second law.