Major commercial banks continued to increase their retained portfolios of single-family mortgage loans during the first quarter of 2005, although servicing rights tied to loans sold to other investors remain the largest part of the industry servicing portfolio. The 50 largest financial services holding companies reported a combined $1.01 trillion in single-family mortgages as of the end of March, according to call report data analyzed by Inside Mortgage Finance. That was up nearly $31 billion…