In Meiosis I, homologous chromosomes (for example, two Chromsome 21's) pair and each pair lines up along the meiotic spindle. The paternal and maternal homologous chromosomes (or homologues, for short) exchange homologous segments via crossing over, creating new chromosomes consisting of parts of maternal and paternal chromosomes. Homologous chromosomes (and homologous DNA segments) normally look identical under the microscope. A genetic marker is any DNA segment whose grandparental origin can be traced -- genetic markers are invaluable for identifying gene inheritance.
If two DNA segments (loci) are on different chromosomes, then they assort independently. This means that they follow simple Mendellian genetics; for example, offspring from a DM father and dm mother will have equal proportions of DM, dm, Dm and dM gametes. Another way of interpreting independent assortment of DM and dm is that half of all gametes will be parental (DM and dm -- both alleles are from the same parent) and the other half will be nonparental (Dm and dM -- one allele from each parent).
Syntenic genes are located on the same chromosomes. Since relatively small DNA segments cross over at a time, two genes which are far apart on the chromosome almost never undergo reassortment together. The gametes are likely to thus be recombinant (nonparental) with respect to those loci, with gametes being either Dm or dM as though the genes were on different chromosomes and assorting independently. However, two loci near each other on the chromosome are likely to cross over together, with gametes being nonrecombinant (parental) with respect to each other (Dm or dM). Two genes which very rarely cross over together are thus far apart, and two genes which frequently cross over together are near each other.