===== Fitting X-linked genetic effects =====


<WRAP  caution 30% right :en>
<file C xlink_example.par>
   PED pedigree.dat  +SEX
   DAT xldata.dat
     fixeff 2
     animal
     xlink 
     record
   end
   ANAL UNI
   MODEL
    RAN animal NRM
    RAN xlink SEX
    FIX fixeff 
    TR record
   END
   VAR animal 1
   16
   VAR xlink 1
   4
   VAR error 1
   20
</file>
</WRAP>

WOMBAT has an in-built facility to allow for X-linked genetic effects. In brief,
this is fitted as an additional random effect and WOMBAT can calculate the appropriate inbreeding coefficients and set up
the inverse of the corresponding relationship matrix directly from the list of pedigree information.

The box on the right shows an example for the parameter file for a univariate analysis fitting X-linked genetic effects. The specific requirements are: 
  - The pedigree file needs to have a fourth column with a code of 1 or 2, specifying the number of X chromosomes for this individual (Usually 1=males, 2=females).\\ The code **''+SEX''** after the name of the pedigree file tells WOMBAT to look for this additional column.
  - The data file needs to have a column, here named ''xlink'', which contains the identity of the individual, i.e. which is a duplicate of the column for ''animal''.
  - In the model part of the parameter file ''xlink'' needs to be specified as an additional random effect with covariance option **''SEX''**.\\ The latter instructs WOMBAT to set up the inverse of the X-linked relationship matrix from the list of pedigree information.
  - Finally, the parameter file needs to contain a ''VAR'' statement for the ''xlink'' effect, together with a starting value for the pertaining variance component.

<code | bib>
@ARTICLE{fern90,
    author = {Fernando, R. L. and Grossman, M.},
     title = {Genetic evaluation with autosomal and X-chromosomal inheritance},
   journal = {Theor. Appl. Genet.},
    volume = {80},
      year = {1990},
     pages = {75--80},
       url = {http://dx.doi.org/10.1007/bf00224018}
}
</code>