The desperate worry over the health of huge financial institutions with country cousin names  Fannie Mae and Freddie Mac  reflects a reality that has reshaped major spheres of American life: the government has in recent months taken on an increasingly dominant role in assuring that Americans can buy a home or attend college.

Much of the private money that once surged into the mortgage industry has fled in a panicked horde, leaving most of the responsibility for financing American homes to the government-sponsored Fannie and Freddie.

Two years ago, when commercial banks were still jostling for fatter slices of the housing market, the share of outstanding mortgages Fannie and Freddie owned and guaranteed dipped below 40 percent, according to an analysis of Federal Reserve data by Moody’s Economy.com. By the first three months of this year, Fannie and Freddie were buying more than two-thirds of all new residential mortgages.

A similar trend is playing out in the realm of student loans. As commercial banks concluded that the business of lending to college students was no longer quite so profitable, the Bush administration promised in May to buy their federally guaranteed student loans, giving the banks capital to continue lending.