How are Hercules and Io related?

In the myth about Permethius and io, permethius says she will bear a decendant that will free him, which is hercules. however hercules is the son of zeus and Alcmene. and as far as i no io did not have a daughter named alcmene. so how are they related? please include all of Io's decendents that lead up the birth of hercules if you only have a direct bloodline thats fine but both parents of each would help alot. thanks in advance