“I won’t pretend to understand completely, My Lady,” Sullivan said after a moment, “but I believe I understand the process, in general terms at least. And just how serious a problem is this ’unstable expansion’?”

“Well, in Fragile X, the consequence is—or was, before we learned to repair it—moderate mental retardation. But what resulted here was worse—much worse. It destroyed a portion of the chromosome necessary for early embryonic development.”

“Which means, My Lady?” Sullivan asked intently.

“It means that it produced an embryonic lethal mutation in males, Your Grace,” Allison said simply.

This time the Reverend came bolt upright in his chair, and she nodded to the display still glowing above the coffee table.

“Any male embryo with this mutation cannot be carried to term,” she said. “Female embryos each have two X chromosomes, however, which gives them the chance for an extra copy of the destroyed gene. And the lyonization process, which inactivates one X chromosome in a female, almost always inactivates the structurally damaged one in cases like this, which means that, unlike males with the same problem, they survive.”

“But in that case—” Sullivan stared into the holo for several seconds, then looked back at Allison. “If I understand you correctly, My Lady, you’re saying that no male child with this mutation could live?” She nodded. “In that case, how could our ancestors possibly have survived? If everyone who received the benign mutation also received this one, then how were any living male children born at all?”

“The two mutations are linked in that they were both introduced by the same vector, Your Grace, but that’s the only linkage between them. Everyone got the intended mutation—well, that’s probably an overstatement. Let’s say that everyone who survived got the intentional one, but the unintentional one, fortunately, had incomplete penetrance. That means that thirty percent or so of the males didn’t express the mutation and so survived—but even those who survived could be carriers. To use the Fragile X analogy again, the fragile site from that disease is seen in forty percent of the cells of affected males, but carriers may not show the fragile site at all.”

“I… see,” Sullivan said very slowly.

“There was nothing anyone could have done about it, Your Grace. The original modification was essential if your people were to survive at all. It had to be made, and even assuming that any of the original med team were still alive by the time the harmful side effect began to manifest, and even assuming that they still had the technical capability for genetic level examinations, it was too late to do anything about it,” Allison said quietly, and sat back to wait.

“Sweet Tester,” Sullivan murmured at last, his voice so soft Allison hardly heard him. Then he pushed himself all the way back in his chair and inhaled deeply. He gazed at her for endless seconds, then shook himself.

“I feel certain that you must have felt very confident in your findings before you brought them to my attention, My Lady. May I also assume that your documentation of them will be sufficient to convince other experts of them?”