The SPSS code below is for mixed effects modeling for a sib-based 
linkage and association test that does not require any parental information.
It is discussed in the following reference:
 
Allison, D. B., Heo, M., Kaplan, N., & Martin, E. R.  (1999). Development
of Sibling-based Tests of Linkage in the Presence of Association for
Quantitative Traits That Do not Require Parental Information. American
Journal of Human Genetics, 64:1754-1764.
 
The three key parameters for the program are:
 
sibship: family id number -- must be the same for the sibling members in a
         same sibship;
g:       index for the genotype (not for the allele);
y:       continuous phenotype values.
 
This SPSS program can also be be used for multi-allelic markers. 

The SPSS code
===============

GLM
  y  BY g sibship
  /RANDOM = sibship
  /METHOD = SSTYPE(3)
  /INTERCEPT = INCLUDE
  /CRITERIA = ALPHA(.05)
  /DESIGN .

Note; Addtion of covariates (e.g., age) for adjustment can be coded
      as follows:

GLM
  y  BY g sibship with age
  /RANDOM = sibship
  /METHOD = SSTYPE(3)
  /INTERCEPT = INCLUDE
  /CRITERIA = ALPHA(.05)
  /DESIGN .