The coring of two plants differing in two pairs of contrasting traits is called dihybrid cross. In dihybrid cross, two characters (colour and shape) are considered at a time . Mendel considered the seed shape (round and wrinkled ) and cotyledon colour (yellow & green ) as tow two characters . In seed shape round (R) is dominant over wrinkled (r) : in cotyledon colour yellow (Y) is dominant over green (Y). Hence the pure breeding round yellow parent is represented by the genotype RRYY and the pure breeding green wrinkled parent is represented by the genotype rryy. During gamete formation the paired genes of a character assort out independently of the other pair . During the `F_1 x F_1` fertilization each zygote with an equal probability receives one of the four combinations from each parent . The resultant gametes thus will be geneticall different and they are of the following four types :
1) Yellow round (YR) - 9/16
2) Yellow wrinkled (Yr) -3/16
3) Green round (yR) - 3/16
Green wrinkled (yr) - 1/16
These four types of gametes of `F_1` dihybrids unite randomly in the process of fertilization and produce sixteen types of individuals in `F_2` in the ratio of 9:3:3:1 as shown in the figure. Mendel.s 9:3:3:1 dihybrid ratio is an ideal ratio based on the probability including segregation , independent assortment and random fertilization. In sexually reproducing organism/plants from the garden peas to human beings , Mendel.s findings laid the foundation for understanding inheritance and revolutionized the field of biology . The dihybrid cross and its result led Mendel to propose a second set of generalisations that we called Mendel.s Law of independent assortment.
