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Or they might be （as alleged） zygotes resulting from plementary gametes…all genes conserved at reduction…division but with no duplication。 In this case the chance of unfavorable reinforcement would be…what？
Let that wait。 First assumption； that they were no relation but simply raised together from babyhood…no special hazard； forget it。
Second assumption； that they might be full siblings of the usual sort。 Well； they did not look like it…but； more important； that scoundrel had set up a most elaborate 〃store〃 for such a swindle； and had used publicly the name of a bishop to back him up。 The Bishop might be just as crooked （likely…he knew that priesthood …too well！ ）…but why be so careless when slave babies were so cheap？
No； even if he assumed a swindle； there was no reason to expect an unnecessary risk in a setup so elaborate。 So forget that； too： Llita and Joe were not sister and brother in the ordinary sense…although they might have shared the same host…mother's womb。 The latter； if true； was of no geic significance。
So the remaining worry concerned the chance that the slave factor had told the truth…in which case what were the chances of a bad cross？ How many ways could such artificially produced zygotes rebine unfavorably？
Sheffield tried to set up the problem while cursing the lack of sufficient data； plus the fact that the only real puter in the ship was the piloting puter； which could not be programmed for a geics problem。 He wished Libby were aboard。 Andy would have stared at the bulkhead a few minutes； then e up with answers definite where pomible and expressed in probability percentages where not。
A geics problem； even with all pertinent data （many thousands！）； was too unwieldly to solve without puter assistance。
Well； try some simplified illustrative problems and see what insight could be gained。
Primary assumption： Llita and Joe were 〃mirror twins〃… geically plementary zygotes from the same parent zygotes。
Control assumption： They were unrelated other than being part of the home pla's gene pool。 （An extreme assumption； as slaves from the same area were likely to derive from a much smaller gene pool； which might be still further reduced by inbreeding。 But this 〃most favorable normal breeding pattern〃 was the correct control against which he must measure。）
Simplified example： Test one gene site…call it site 1…87 of the twenty…first chromosome…for reinforcement； masking； or elimination； of an assumed 〃bad〃 gene； under each assumption。
Arbitrary assumption： Since this site might hold an unfavorable gene…or two； or nine…in its gene pair； assume that the chance was exactly the same for both primary and control assumptions； and even…i。e。； 25 percent no bad gene in the pair at the site； 50 percent one bad gene； 25 percent two bad genes…an extreme condition since； over the generations； reinforcement （two bad genes at one site） tended toward nonsurvival； either lethal or reducing a zygote's ability to pete。 Never mind； make it even for both of them…there were no data on which to base any bett；er assumption。
Wups！ If a bad reinforcement was visibly demonstrated； or could be shown by tests； such zygotes would not be used。 A scientist petent to attempt this experiment would use specimens as 〃clean〃 in a geic sense as possible…free of all the hundreds （thousands now？） of identifiable hereditary defects； the primary assumption should include this subsidiary assumption。 …
These young people were free of any defect Sheffield could detect in a shipboard？examination…which enhanced the probability that this horsethief had told the truth and these exhibits were sober records of an exotic and successful experiment in gene manipulation。
Sheffield now tended to believe that the experiment had taken place…and wished that he had the resources of a major Howard Clinic； say the one on Secundus； to give these kids a geic going…over that he was not equipped to do aboard ship and not qualified to do in any case。
One nagging doubt lay in how he had acquired these kids。 Why had that gonif been so anxious to sell？ If they were what the exhibits claimed？ Why sell them when breeding the two created plements back together was the next step of the
experiment？
Well； perhaps the kids knew but he had not asked the right questions。 Certain it was that they had been brought up to believe that such was their proper destiny； whoever planned this had induced in the kids from earliest childhood a pairbond stronger than most marriages； in Sheffield's long experience。 More than any of his own（Except one； except one！）
Sheffield put it out of his mind and concentrated on the theoretical consequences。
At the selected site； each parent zygote had been assumed to have three possible states or gene…pairs in probability 25…Under the control assumption； parents （diploid zygotes） both male and female would show this distribution at the selected site：
〃 25％ good…good（〃clean〃 at that site）
〃 good…bad（bad gene masked but could be transmitted）
〃 good…bad（bad gene masked but could be transmitted）
〃 bad…bad（bad reinforcement…lethal or disabling）
But under his modified primary assumption Sheffield assumed that the priest…scientist' would discard bad stock as displayed in zygotes…which would eliminate the fourth group （〃bad…bad〃） and leave a parent…zygote distribution for this site of：
〃 33…1/3％
good…good
〃good…bad
〃good…bad
Such culling gave marked improvement over the original random…chance situation and meiotic division would produce gametes （both sperm and ova） in this incidence：
Good； four out of six； and
Bad； two out of six…
…but with no way to detect the bad genes without destroying the gametes carrying them。 Or so Sheffield assumed； while stipulating that the assumption might not be true forever。 But to protect Llita （and Joe） it was necessary that his assumptions be pessimistic within the limits of available data and know1edge…i。e。； that a bad gene could be spotted only as reinforcement in a zygote。
Sheffield reminded himself that the situation was never as black…and…white as was implied by 〃good~dominant〃 and 〃badrecessive〃…these descriptions were less plex than the real world they were used to image。 A characteristic exhibited by an adult zygote was prosurvival or contrasurvival only in terms of what and when and where…and also in terms of more than one generation。 An adult who died saving its progeny had to be counted a prosurvival whereas a cat that ate her own young was contrasurvival no matter how long she lived。
In the same vein； a dominant gene sometimes was of no importance one way or the other…e。g。； brown eyes。 Just as its corresponding recessive when paired and thereby reinforced to produce blue eyes gave the zygote exhibiting it no measurable disadvantage。 The same was true of many other inheritable characteristics…hair patterns； skin color； et cetera。
Nevertheless this de