This is how it's done.  Note that geneSize is not defined here, but I assume you'll take care of that.

MODULE test
USE IFPORT

INTERFACE getRandom
MODULE PROCEDURE getRandomA
MODULE PROCEDURE getRandomB
MODULE PROCEDURE getRandomC
END INTERFACE

CONTAINS
INTEGER FUNCTION getRandomA()
IMPLICIT NONE
getRandomA = MOD( IRAND(), geneSize-1 ) + 1 !        GENERATE RANDOM FROM 1 TO geneSize-1
END FUNCTION

INTEGER FUNCTION getRandomB(max)
IMPLICIT NONE
INTEGER, INTENT(IN) :: max
getRandomB = MOD( IRAND(), geneSize-1 ) + 1 !        GENERATE RANDOM FROM 1 TO geneSize-1
END FUNCTION

INTEGER FUNCTION getRandomC(min, max)
IMPLICIT NONE
INTEGER, INTENT(IN) :: min, max
getRandomC = MOD( IRAND(), geneSize-1 ) + 1 !        GENERATE RANDOM FROM 1 TO geneSize-1
END FUNCTION
END MODULE