 A key concept in two-locus population genetics is that of linkage, or lack of independence between the two loci. To understand linkage, consider the set of gametes produced by an organism of the \(A_1 B_1 / A_2 B_2\) genotype, i.e., a double heterozygote. If the two loci are unlinked, then the composition of this set (on average) will be \(\{ \frac{1}{4} A_1 B_1, \frac{1}{4} A_1 B_2, \frac{1}{4} A_2 B_1, \frac{1}{4} A_2 B_2\}\), i.e., all four gamete types are equally represented. (This assumes that Mendel’s first law holds at both loci.) So unlinked loci are independent—which allele a gamete has at the \(A\) locus tells us nothing about which allele it has at the \(B\) locus. The opposite extreme is perfect linkage. If the two loci are perfectly linked, then the set of gametes produced by the \(A_1 B_1 / A_2 B_2\) double heterozygote has the composition \(\{\frac{1}{2} A_1 B_1, \frac{1}{2} A_2 B_2\}\); this means that if a gamete receives the \(A_1\) allele at the \(A\) locus, it necessarily receives the \(B_1\) allele at the \(B\) locus and vice versa.