Two genes that occur on the same chromosome are linked and all the genes on a single chromosome form a linkage group.
The remaining 22 chromosomes that aren't sex-linked (carried on sex chromosomes) are called autosomes. Therefore, autosomal linkage is where two or more genes are carried on the same autosome.
Assuming no crossing over, all linked genes remain together during meiosis and so pass into gametes and therefore offspring together. They do not segregate in accordance with Mendel's Law of independent Assortment.
Two homologous alleles are needed if all four alleles are to be present.
If genes are linked, only one homologous pair is needed if all four alleles are to be present.