 In physical terms, perfect linkage means that the \(A\) and \(B\) loci are located close together on the same chromosome; the alleles at the two loci are thus inherited as a single unit. Unlinked loci are either on different chromosomes, or on the same chromosome but separated by a considerable distance, hence likely to be broken up by recombination. Where the loci are on the same chromosome, perfect linkage and complete lack of linkage are two ends of a continuum. The degree of linkage is measured by the recombination fraction \(r\), where \(0 \le r \le \frac{1}{2}\). The composition of the set of gametes produced by an organism of the \(A_1 B_1 / A_2 B_2\) genotype can be written in terms of \(r\), as follows: