Linkage and crossing over of genes are alternatives of each other. Justify with an example

Linkage is the tendency shown by the genes on a single chromosome to remain together and to be passed on together during gamete formation. Linked genes do not exhibit the dihybrid ratio of 9:3:3:1. It produces offspring with parental characters. 
Crossing over is the exchange of genes occurring during meiotic prophase I to break old linkage and establish new ones. It produces recombination resulting in new varieties.
Thus, they are alternatives of one another i.e if linkage is present in between genes, no crossing over occurs between them and if crossing over occurs between two genes, they are not linked.

Mendel’s law of independent assortment is true when two genes (say genes for seed shape and seed color of pea) are on different chromosomes. In such  condition, during gamete formation,  the alleles of one gene (R and r) segregates independently of the alleles of the second gene (Y and y). This results in the formation of equal numbers of four different gametes: RY, Ry, rY, and ry.
On the other hand, if two genes (say genes for seed shape and seed color of pea) are on the same chromosome, they will be linked and will be passed on together resulting in production of only two kinds of gametes: RY and ry.
In the F2 generation, the original parental phenotypes are reflected if the genes are linked. When the genes are present on separate chromosomes, the dihybrid ratio of 9:3:3:1 is exhibited. 

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