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.
To perform a dihybrid cross, Mendel started with two pure parental lines. One line had wrinkled, yellow seeds r/r·Y/Y. The other line had round, green seeds, with genotype R/R·y/y. When these two lines were crossed, they must have produced gametes that were r·Y and R·y, respectively. Hence, the F1 seeds had to be dihybrid, of genotype R/r·Y/y. Mendel discovered that the F1 seeds were round and yellow. This result showed
that the dominance of R over r and Y over y was unaffected by the condition of the other gene pair in the R/r·Y/y dihybrid. Next, Mendel selfed the dihybrid F1 to obtain the F2 generation.
The F2 seeds were of four different types in the following proportions:
9/16 round, yellow
3/16 round, green
3/16 wrinkled, yellow
1/16 wrinkled, green
From the result Mendel concluded that different gene pairs assort independently in gamete formation. Now known as Mendel’s second law.
Let us consider the gametes produced by the F1 dihybrid R/r ;Y/y (the semicolon shows that we are now embracing the idea that the genes are on different chromosomes). Again, we
will use the branch diagram to get us started because it illustrates independence visually. Combining Mendel’s laws of equal segregation and independent assortment, we can predict that