All the gene s on a single chromosome .
They are inherited as a group; during cell division they act and move as a unit rather than independently. Variations in linkage groups can occur if a chromosome breaks, and the sections join with the partner chromosome if it has broken in the same places. This exchange of genes between chromosomes, called crossing-over, usually occurs during meiosis . Sex linkage is the tendency of a characteristic to be linked to one sex; sex-linked traits in humans include red-green colour blindness and hemophilia.