An object of class cross. See
read.cross for details.
markers
A vector of character strings of marker or pseudomarker
names. Pseudomarker names may be of the form "5@21.5" (for chr 5
at 21.5 cM), but then all names must be of this form.
method
If method="prob", the genotype probabilities from
calc.genoprob are used; otherwise we use
fill.geno to impute missing data, with this method.