The Law of Independent Assortment
But all of the above only describe what happens when we follow one particular gene through the cross. Mendel also observed the ratios of different characters as they were crossed together—such as following colour and shape, knowing that round peas are dominant (R) and wrinkly peas are recessive (r). He wanted to know whether certain genes stayed together and were more commonly found with each other, generation after generation, or whether they went their own way—whether they’re inherited independently.
To figure this out, Mendel performed a dihybrid cross, crossing YYRR with yyrr. He found that in the F1 generation, all plants are YyRr, so they’re all yellow and round. But in the F2 generation, after self-pollination, you end up with a ratio of 9:3:3:1—9 parts yellow and round, three parts yellow and wrinkled, three parts green and round, one part green and wrinkled.
If the colour and shape were inherited together, you’d end up with a 3:1 ratio, 3 parts yellow and round, one part green and wrinkled. But because Mendel ended up with the other, more complicated ratio, he realised that the alleles couldn’t be inherited together—they were independently passed down, paying no attention to any of the other alleles. This is called Mendel’s Law of Independent Assortment.
Note that this law only applies to genes on different chromosomes—those on the same chromosome tend to be inherited along with nearby genes (remember it’s more probably that far-away genes are crossed over). Their inheritance are more complex than this law can predict.
Further resources: Educationportal Video