What did mendel reliz about principles of probability could be used to the result of genetic crosses

Say he crossed peas. Some peas have wrinkled skin and some peas have smooth skin, while other peas had kind of smooth but kind of wrinkled skin, kind of in the middle. 

The peas with wrinkled skin have the genotype WW, and the peas with smooth skin have the genotype ww, and the peas that are in the middle have the genotype Ww.
He noticed that there was a 1:2:1 ratio of WW:Ww:ww peas in his progeny when crossing WW x ww pea plants for the parent generation. In other words, 25% homozygous dominant, 50% heterozygous, and 25% homozygous recessive.

Punnett square:
       WW / ww
WW WW | Ww
ww   Ww | ww