is a term which describes the tendency of certain loci or alleles to be inherited together. A linkage group are the alleles that are physically joined on the same chromosome.
Genetic loci on the same chromosome are physically close to one another and tend to stay together during meiosis, and are thus genetically linked.
If no crossing over happens the gametes that are produced by meiosis in this case will have these alleles because they are linked together. For these alleles there are only two different types of gametes.
If crossing over happens between the loci of the two alleles then new gametes can be produced that were not possible before. These are known as recombinants.
The chances of crossing over is random and will increase if the distance between the two alleles is greater and decrease if the alleles are very close on the chromosome.
The phenotype outcomes of a cross will give information that does not fit the 9:3:3:1 pattern.