 Thomas Hunt Morgan expanded the idea of linkage after noticing that in some instances the observed rate of crossing-over events differed from the expected rate of crossing-over events.He attributed the depressed rates of recombination to the smaller spatial separation of genes on a chromosome; Hypothesizing that genes which are more closely positioned on a chromosome will have smaller rates of recombination than those that are spaced farther apart.The unit of measurement describing the distance between two linked genes is the Centimorgan, and is named after Thomas Hunt Morgan.