idiochromosome

In the somatic cells of the former class he finds in the male the unequal pair, in the female an equal pair, the smaller chromosome being replaced by an equivalent of the larger "idiochromosome."

Wilson called it the 'X chromosome,' McCluny the 'accessory chromosome,' Montgomery the 'hetero-chromosome,' while the names 'heterotropic chromosome' and idiochromosome have also been used.