Yin and yang is a philosophical idea in which polar opposites interact with each other. An example is how a stone is dropped into a still pond. The stone causes high wives and low troughs, one cannot exist without the other. On Wikipedia they gave the example of having the back of a hand with no front, it is impossible. This is the idea of yin and yang.
This relates to the joy luck club in the relationships between mother and daughter. The Mother is elderly and full of wisdom but the child is young and foolish. You could not have a world full of wise mothers because they would no longer be wise, they would be average. Same with the daughters, with out the wise mothers they just have average youth and average intelligence. This only one of the examples I found.
