Kinds of Linkage

T.H. Morgan recognised that linkage is not an all-or-nothing property; it comes in two distinguishable grades depending on whether crossing over ever separates the linked genes. In complete linkage, two genes sit so close together on the chromosome that a crossover essentially never occurs between them, so every gamete carries only the original parental combination of alleles; a testcross of such a heterozygote therefore produces offspring in only the two parental classes, typically in a simple 1:1 ratio, with no recombinant offspring at all. This is rare in nature but was documented by C.B. Bridges in 1919 in male Drosophila, where crossing over is entirely absent during spermatogenesis (though it occurs normally in Drosophila females). In incomplete linkage, by contrast, the two genes are far enough apart along the chromosome that crossing over does occur between them at some measurable frequency, so a testcross yields both parental-type offspring, which still predominate, and a smaller number of recombinant-type offspring that carry the new, crossed-over combination. Hutchinson documented exactly this pattern in maize, where a cross for coloured/full versus colourless/shrunken seed traits g …