In a diploid plant species, an F₁ with the genotype Gg Ll Tt is test-crossed to a pure-breeding recessive plant with the genotype gg ll tt. The offspring genotypes are as follows: Genotype Number Gg Ll Tt   621 Gg Ll tt        3 Gg ll Tt     64 Gg ll tt   109 gg Ll Tt   103 gg Ll tt     67 gg ll Tt       7 gg ll tt   626 1600 Why is the recombination frequency for the outside pair of genes not equal to the sum of recombination frequencies between the adjacent gene pairs?