The thing that makes altcoins depends on bitcoin and ethereum is that most altcoins trade with bitcoin, when the price of bitcoin drops then automatically the value of altcoins also drops and vice versa.
Yes it is a pair which I guess most large and become the majority in Exchange. We can see all of the altcoin also have a pair to usd, eth and btc. It makes all the dependent against the movement of the market price which corresponds to the movement of these coins.