Stocks opened higher this morning, with the S&P 500 (SNPINDEX:^GSPC) and the narrower, price-weighted Dow Jones Industrial Average (DJINDICES:^DJI) up about 0.4% each as of 9:55 a.m. EST.

The impact of a bruised Apple
Stocks are up this morning, but the S&P 500's six-day rally is at risk, and that comes down to a single stock: Apple (NASDAQ:AAPL).

Apple reported yesterday afternoon that it sold a record 47.8 million iPhones in the quarter ended Dec. 31 -- a 29% year-on-year increase -- but those figures didn't match Wall Street's lofty expectations. Although the company did beat the consensus earnings-per-share forecast, it also disappointed with regard to revenue and forward revenue guidance for the current quarter.

According to Reuters, options activity in the hours leading up to yesterday afternoon's earnings announcement suggests investors were banking on a 7% one-day move in the shares. That looks rosy compared with the 10% decline in the shares as of 10 a.m. EST, which represents a $48 billion loss in market value. The world's most valuable company, Apple represents 3.5% of the S&P 500. In other words, a 10% drop in its shares subtracts 0.35% from the value of the index. That's just the first-order impact, mind you; a marked drop in Apple shares puts pressure on other technology-sector shares.

While Apple isn't a member of the Dow, the index is also vulnerable to its stock tremors via the technology stocks it contains and telecommunication carriers AT&T and Verizon (NYSE:VZ), which are a part of the iPhone ecosystem. On Tuesday, Verizon reported lower-than-expected wireless profit margins due to heavy subsidies to smartphone manufacturers including Apple.

Ten days ago, this column asked whether Apple could carry the S&P 500. The answer, for now, is yes -- it can carry it down.

Fool contributor Alex Dumortier, CFA has no position in any stocks mentioned; you can follow him @longrunreturns. The Motley Fool recommends Apple. The Motley Fool owns shares of Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.