Assumable mortgages allow buyers to take over a seller's original mortgage.Feverpitched/Getty, Tyler Le/BIHomebuyers seeking lower mortgage rates are snapping up a rare type of loan.Assumable mortgages allow buyers to take over a seller's original mortgage.But only government-backed loans are eligible, and they require higher, all-cash down payments.A type of mortgage that can offer ultra-low rates is becoming more popular among homebuyers — but they're rare and difficult to come across, real estate experts say.Buyers have been increasingly turning to assumable mortgages, or home-lending schemes where a buyer takes over a seller's mortgage, at the rate it was originally financed.That's likely to lead to a good deal for buyers sifting through homes on the market today.