“You find surprising linkages that you never would have expected,” said Richard Bookstaber, a former hedge fund manager and author of a new book, “A Demon of Our Own Design: Markets, Hedge Funds and the Perils of Financial Innovation.”

“What matters is who owns what, who is under pressure to sell, and what else do they own,” he said. People with mortgage securities found they could not sell them, and so they sold other things. “If you can’t sell what you want to sell,” he said, “you sell what you can sell.”

He recalled that the crisis that brought down the Long-Term Capital Management hedge fund in 1998 started with Russia’s default on some of its debt. Long-Term Capital had not invested in Russia’s bonds, but some of those who owned such bonds, and needed to raise cash, sold instruments that Long-Term Capital also owned, and on which it had borrowed a lot of money.

It appears that in this case, securities backed by subprime mortgages were owned by people who also owned securities backed by leveraged corporate loans. With the market for mortgage paper drying up, and a need to raise cash, they sold the corporate securities and that market began to suffer.

The Wall Street investment banker who wanted a jumbo mortgage had a good credit score, and is not a subprime borrower. But private mortgage securities are now hard to sell, leading to his problem. In the end, he was able to get a mortgage with a lower interest rate, but it will adjust in five years, possibly to a much higher level.

The size of the rate increase he faced is unusual. But all jumbo lenders have raised rates. Bankrate.com reports that conventional 30-year mortgages cost about 6.23 percent now, less than they did a few weeks ago, due to a decline in Treasury bond rates. But the average jumbo rate is now 6.94 percent. The spread between the two rates rose from less than a quarter of a percentage point to more than two-thirds of a point.

Jumbo mortgages are most important in areas with high home prices, most notably on the East and West coasts. “In California, it has shut down the purchase market,” said Jeff Jaye, a mortgage broker in the Bay area. “It has shut down the refi market.”