Law of Independent Assortment -

"When Crosses are made between plants with two or more than two Contrasting characters, the inheritance of these characters occurs independently like, the inheritance of one Character does not affect the inheritance of Other Characters".

When dihybrid Cross between plants with to pairs of contrasting characters is made between homozygous round and Yellow seeded plant (Genotype RRYY) and homozygous wrinkled and green seeded plant (genotype rryy) both the plants, being homozygous for both the Characters, at first only same type of gametes are poroduced i.e. 'RY' and 'ry' which produce F1 hybrid (genotypety RrYy ) after fertilization . All plants of F1 generation Round Seeded with green coloured Cotelydon. It is so because the Round shape of Seed is dominant over wrinkled seed and Yellow colour of cotyledon are dominant over green coloured Cotelydon.
When the plants of F1 generation (Hybrid) are self pollinated, then F1 hybrid being heterozygous for the genes of both type of free characters, produce four types of gametes 'RY', 'rY', 'Ry' and 'ry'.
As All the four types of gametes of F1 hybrid fuse with each other by all the possible Combinations, forming 4 types of phenotype plants and 9 types of genotypic plants In F2 generation.
In F2 generation following 4 types of seed of plants are obtained:-

The phenotypic ratio in these plants ds 9:3:3:1 respectively. This ratio is called dihybrid ratio.
It is clear that round character is not always found with yellow character and similarly wrinkled character is not always found with green character as they are present in parents.
The plants with new combinations are also obtained. I.e. round with green and wrinkled with yellow.
This proves that the character of Shape of seed and character of colour of cotyledon are always assorted independently. In this way, this two characters give rise to new combinations in F2 generation.