Altcoins depend on ethereum and bitcoins mainly as they are the currencies for which most altcoins trade and a slight dump too makes the price drop. See CDT price now which is 0.04 when it was 0.2 few days back and same goes for other altcoins like Mybit, PPT that are suffering now even when the price is above $200 for ETH and above $4000 for bTC. If bitcoin prices reduce to half, many coins would be valued below 1 cent.
This is true and we see this trend yesterday when The People's Bank of China which is the central bank of Chine put the ban on ICO and its funding in China the price of bitcoin drop 5% and ETH price drop by 12% but others altcoin prices drop to nearly 20%.