A second Deryni factor, carried only on the Y chromosome, is the basis for the human assumption of Deryni powers. (The potential for this phenomenon was discovered by Camber of Culdi and Rhys Thuryn in the mid-890s, though they would not have been aware of any genetic basis for it.) This factor, when activated, is fully equal to the X' factor in power capacity, but is, of course, passed on only through the male line. Hence, a male showing the potential for assumption of Deryni power certainly had a father with the same capability—though this factor may be held and passed without the carrier’s knowledge for generations, as may the X' factor. By itself, the Y' factor will not confer Deryni powers on a male child, for the assumption of power is a difficult and tedious process and may be hampered or enhanced by numerous psychological and physiological factors.

As for those rare individuals who seem to display this potential for power assumption without the requisite Deryni parentage to account for it (Sean Lord Derry, for example), we may find that this is due to a long-dormant Y' factor that has been passed on unwittingly for several generations. Unless the carrier of a Y' factor (or the X') is discovered by a true Deryni, and is informed and guided in realizing this potential, he or she will likely never become aware of this capability.

Nor is the potential to assume Deryni power limited to one bearer at a time in any given family, though the Haldanes have always encouraged this belief, probably to lessen the likelihood of arcane dueling among potential heirs when the succession was in question. Nigel Haldane may be somewhat aware of the truth of the matter; he carries the Y' factor, as do his three sons, and both he and Conall had the Haldane potential activated while Kelson was still alive. In earlier generations, however, it is easy to see how, in a collateral branch of a family carrying the power assumption potential, as Nigel’s is destined to become, that the very awareness of this heritage could be lost—and who is to say how many Haldanes might have spread their seed and sired a line of potential Deryni? Derry, descendant of a long and noble line, may well have gotten his potential this way—perhaps as far back as seven or eight generations. And in an individual of peasant origin, like Warin de Grey? The droit du seigneur may account for many anomalies of birth.

The two Deryni factors, X' and Y', are independent, however—which means that both may be present in a given individual—by definition, male, because of the Y' factor. Again, the Deryni factors are not cumulative, so an X'Y' male would have no appreciable advantage over an X'Y male or an XY' male; however, an X'Y' Deryni might be able to use his powers with greater efficiency, since the powers assumed through the Y' factor come upon him fully functional, with no practice necessary. (An X'Y Deryni must learn to use his powers and hence may be at a disadvantage if he has not had the benefit of formal training.) Thus Kelson, who carries the double-prime configuration X'Y', was able to function as a fully trained Deryni from the start, as soon as he had fully assumed his father’s powers—even though he had had no formal schooling in the use of those powers and had not suspected his X' inheritance. His father Brion likewise came to power at full potential, without training, from the power ritual of his father. Jehana, on the other hand, whether an X'X or an X'X' Deryni, had never permitted herself to use her inheritance and hence could be easily defeated by the puissant and practiced Charissa, descendant of a long line of proficient Deryni sorcerers.