EXHIBIT 4.1

                                INTERACTIVE DATA
                          FINANCIAL TIMES INFORMATION
                           14 WALL STREET, 11th FLOOR
                            NEW YORK, NEW YORK 10005
                                 (212) 306-6596
                                FAX 212-306-6698

                                                                 August 16, 2000

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Merrill Lynch, Pierce, Fenner & Smith
Incorporated
Defined Asset Funds
P.O. Box 9051
Princeton, New Jersey 08543-9051

The Chase Manhattan Bank
4 New York Plaza, 6th Floor
New York, New York 10004
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RE: GOVERNMENT SECURITIES INCOME FUND,
    GNMA SERIES--1Y, DEFINED ASSET FUNDS
    (A Unit Investment Trust) Units of Fractional Undivided Interest-Registered
Under the Securities Act of 1933, File No. 33-57873)

Gentlemen:

    We have examined the Registration Statement for the above captioned Fund.

    We hereby consent to the reference to Interactive Data Services, Inc. in the
Prospectus contained in the Post-Effective Amendment No. 5 to the Registration
Statement for the above captioned Fund and to the use of the evaluations of the
Obligations prepared by us which are referred to in such Prospectus and
Registration Statement.

    You are authorized to file copies of this letter with the Securities and
Exchange Commission.

                                          Very truly yours,
                                          JAMES PERRY
                                          Vice President



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