In 1959 Susumu Ohno showed that the two X-chromosomes of mammals were different: one appeared similar to the autosomes; the other was condensed and heterochromatic.
En 1959 Susumu Ohno demostrou que os dous cromosomas X de mamíferos eran diferentes: un deles parecía similar aos autosomas; o outro estaba condensado e en estado heterocromático.