If JPMorgan Chase's (JPM) trading blowup raised worries about booby traps at other financial giants, it might be comforting to know that its securities portfolio is more complex — and perhaps trickier to hedge — than those of rival megabanks.

When they're not lending, banks typically subsist on a diet heavy in government-backed mortgage bonds: debt issued or guaranteed by Fannie Mae, Freddie Mac and Ginnie Mae accounted for more than half of bank securities portfolios at yearend, according to data from the Federal Deposit Insurance Corp.

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