Southwest Airlines is planning to expand into some key smaller U.S. markets. after agreeing to buy AirTran, which is based out of Atlanta, Ga.

Once the deal is complete in 2012, the company will operate from 100 airports, including those in 37 new cities, and will serve more 100 million customers.

The deal combines two of the country's largest low-fare airlines.

Southwest also announced Monday that it will drop AirTran's bag fees of $20 for a first checked bag once the two airlines combine. Southwest said it continuously lures passengers by refusing to charge for checked bags.

But experts warn that travelers should expect fewer ticket sales when the two airlines combine, since one of Southwest's biggest low-fare competitors is being knocked out.

Southwest, which is based out of Dallas already carries more passengers than any other airline in the U.S.