"Essentially, it was a very tiny change," Allison told the Reverend. "An adenine here—" she touched another key, and one of the "AGG" codes flashed brilliantly "—mutated to cytosine—" another key, and the flashing red "A" turned into a yellow "C" and the three-letter group to its right grew suddenly into an enormous chain of the same codes, repeating again and again "—which deactivated the lock and allowed unstable expansion of—"

"Excuse me, My Lady," Sullivan interrupted, "but I think we’re drifting into deep water here. What, precisely, does that m— No." He stopped and raised one hand. "I’m certain that if you told me what it meant, I would be no closer to understanding than I am now. What I truly need to know, I suppose, is what the consequences of this... unstable whatever are."

"Um." Allison sipped some more tea, then shrugged.

"DNA is composed of four nucleotides, Your Grace: adenine, cytosine, guanine, and thymine. They link together in thousands of repeats—codes, if you will—which combine to carry the blueprint for our bodies... and transmit it to the next generation. They link in groups of three, hence the term ‘trinucleotide,’ which usually occur in ‘runs’ of thirty or less, but there are several diseases, such as the one we call ‘Fragile X,’ in which the number of repeats expands enormously, often into the thousands, effectively... well, scrambling a portion of the master code, as it were. Are you with me so far?"

"I believe so," he said cautiously.

"All right. This schematic represents a portion of the nucleotides—in this case cytosine, adenine, and guanine—from the Grayson genome. This trinucleotide here—" she touched her controls and the holo reverted to its original form with the "AGG" flashing once more "—is what we call a ‘lock,’ sort of a blocker to prevent the CGG repeats on either side of it from expanding in a way that would scramble the code. What happened, though, was that when the adenine mutated into cytosine, the ‘lock’ disappeared... and that allowed an unstable expansion of the CGG chain ‘downstream’ of it."

"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?"

"Yes, Your Grace," she said positively. "For one thing, it explains the two things about your population which have most puzzled the Star Kingdom’s geneticists from the beginning of the Alliance." Sullivan raised an eyebrow, and she shrugged. "I’ve already mentioned the incredible rapidity with which your ancestors evolved a ‘natural’ defense against heavy metals. That was number one. But a disparity in male-female birth rates on the scale of Grayson’s, while not all that unusual under distressed conditions, seldom lasts as long as yours has."

"I see." He gazed at her meditatively, then drank more tea. "And is there anything which can be done about this, My Lady?"

"It’s really too early for me to say yes or no to that one, at least with any degree of confidence. I’ve isolated two or three possible approaches, but the site of the problem may well make things difficult, because the mutated gene on the X is near the zinc-finger X protein gene. That’s a key gene in sex determination, and it’s at the Xp22.2—" She paused as his expression began to indicate that he was lost once more.

"It’s at a locus where changes can involve literally dozens of disease states, Your Grace," she simplified. "Many of those diseases are lethal, and others can cause disorders of sex determination. We know a lot more about sex differentiation than whoever whipped up your survival modification did, but we still dislike meddling with it, and particularly in this area. There’s a lot of room for small errors to have large consequences, and even if we avoid the more dangerous disease states, the Beowulf Code specifically prohibits genetic manipulation in order to predetermine the sex of a child." She grimaced. "There were some very unpleasant—and shameful—episodes relating to that in the first and second centuries Ante Diaspora, and I’m afraid they’ve been repeated from time to time on some of the more backward colony worlds since. Nonetheless, I think I could probably at least ameliorate the situation. But whatever I do, it will take time to perfect the methodology... and probably result in at least some decreased fertility among your planet’s male population."

"I see," he said again, and switched his eyes to the holo image above the coffee table once more. "Have you spoken to the Sword’s health authorities about this yet, My Lady?" he asked.

"Not yet," Allison admitted. "I wanted to be certain of my data before I did, and then your visit to Harrington gave me the opportunity to speak to you first. Given the role your Church plays in the day-to-day life of Grayson, I thought it might be wiser to speak to you first."