In medels experiment, why did wrinkled seed shown up in the F2 generation, even though they were not present in the F1 generation

The F1 generation was heterozygous and contained the recessive gene for wrinkles. They did not show up in the F1 generation as Mendels law states that the dominant gene will always be expressed. There were two recessive genes passed down from the F1 and therefore the F2 could exhibit the wrinkled seed.