Linkage Groups

Every gene physically present on a given chromosome forms part of a single group of genes that are inherited together as a unit, unless separated by crossing over; this group is called a linkage group. Because every chromosome an organism possesses defines one such group, the total number of linkage groups an organism has is fixed by its haploid chromosome number -- an organism with a haploid number of 7, for instance, will have exactly 7 linkage groups, one per chromosome type. This gives geneticists a useful cross-check: once enough genes have been mapped by breeding experiments to sort cleanly into a certain number of independent linkage groups (genes within a group show measurable linkage to each other, while genes in different groups assort independently), that number should match the organism's known haploid chromosome …