Gregor Mendel, the father of modern genetics, knew nothing about genes or chromosomes or DNA or any of that.  Nevertheless, he worked out how genes like G1, G2 and G3 must be transmitted to the next generation

He did this by making crosses between parent plants (he used peas) with different characters and counting the numbers of offspring showing one or other of the parental traits in subsequent generations.  Knowing about genes and chromosomes enables us to visualise how Mendel got his results and came up with his laws.

Put simply, with reference to our imaginary plant with two imaginary chromosomes and three imaginary genes, chromosome 1 will pass to the offspring independently of chromosome 2.

Consequently gene G3 will be inherited independently of G1 or G2.  But G1 and G2 will move together through the generations and so their inheritance will be linked.  In our imaginary example, G1 and G2 red move together, as do G1 and G2 blue.

Independent assortment and linkage - without Mendel we would not understand that this is how inheritance works.