time enough for love-时间足够你爱(英文版)-第30部分

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ic　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　description…good…dominant；　bad…recessive…was　in　essence　correct；　it　synopsized　the　mechanisms　by　which　a　race　conserved　its　favorable　mutations　and　destroyed　（eventually）　its　unfavorable　mutations。　〃Bad…dominant〃　was　almost　a　contradiction　in　terms；　as　a　thoroughly　bad　mutation　which　was　dominant　killed　itself　off　（along　with　the　unfortunate　zygote　inheriting　it）　in　one　generation；　either　lethal　in　womb　or　so　damaging　to　the　zygote　that　it　failed　to　reproduce。
　　
　　But　the　usual　weeding　process　involved　bad…recessives。　These　could　remain　in　the　gene　pool　until　one　of　two　events　happened；　each　controlled　by　the　blind　laws　of　chance：　such　a　gene　could　pair　with？a　gene　like　it　when　sperm　fertilized　ovum　and　thereby　eliminate　itself　by　eliminating　the　zygote…hopefully　before　birth；　or…tragically………after　birth。　Or　this　badrecessive　might　be　eliminated　by　chromosome　reduction　at　meiosis　and　the　result　would　be　a　healthy　baby　who　did　not　carry　this　bad　gene　in　its　gonads…a　happy　oute。
　　
　　Both　these　statistical　processes　slowly　weeded　out　bad　genes　from　the　race's　gene　pool。
　　
　　Unfortunately　the　first　of　these　processes　often　produced　babies　viable　but　too　handicapped；　they　needed　help　to　stay　alive…sometimes　needing　economic　help；　born　losers；　who　never　managed　to　support　themselves；　sometimes　needing　plastic　surgery　or　endocrine　therapy　or　other　interventions　or　supports。　When　Captain　Aaron　Sheffield　had　been　practicing　medicine　（on　Ormuzd　and　under　another　name）；　he　had　gone　through　stages　of　increasing　frustration　over　these　poor　unfortunates。
　　
　　At　first　he　had　tried　to　practice　therapy　by　the　Hippocratic　Oath…or…close　to　it；　he　was　by　temperament　unable　to　follow　any　man…made　rule　blindly。　Then　he　had　had　a　period　of　temporary　mental　aberrance　during　which　he　had　sought　a　political　solution　to　what　he　saw　as　a　great　danger：　reproduction　by　defectives。　He　tried　to　persuade　his　colleagues　to　refuse　therapy　to　hereditary　defectives　unless　they　were　sterile　or　sterilized　or…　willing　to　accept　being　sterilized　as　a　precondition　for　receiving　therapy。　Worse　yet；　he　had　attempted　to　include　in　the　definition　of　〃hereditary　defective〃　those　who　displayed　no　stigmata　save　that　they　had　never　managed　to　be　self…supporting……on　a　pla　not　overcrowded　and　which　he　himself　had　selected　centuries　earlier　as　nearly　ideal　for　human　beings。
　　
　　He　got　nowhere；　he　encountered　nothing　but　fury　and　contempt…save　for　a　few　colleagues　who　agreed　with　him　privately　and　denounced　him　publicly。　As　for　laymen；　tar…andfeathers　was　the　mildest　medicine　they　prescribed　for　Dr。　〃Genocide。〃
　　
　　When　his　license　to　practice　was　lifted；　…Lazarus　regained　his　normal　emotional　detachment。　He　shut　up；　realizing　that　grim　old　Mother　Nature；　red　of　tooth　and　claw；　invariably　punished　damnfools　who　tried　to　ignore　Her　or　to　repeal　Her　ordinances；　he　need　not　interfere。
　　
　　So　he　moved　and　changed　his　name　again　and　started　to　get　ready　to　go　off…pla…when　a　plague　hit　Ormuzd。　He　had　shrugged　and　gone　back　to　work；　an　unfrocked　physician　whose　services　were　temporarily　wele。　Two　years　and　a　quarter　of　a　billion　deaths　later　he　was　offered　his　license　back…subject　to　good　behavior。
　　
　　He　told　them　what　to　do　with　that　license　and　left　Ormuzd　as　quickly　as　possible；　eleven　year　later。　He　was　a　professional　gambler　during　that　wait；　that　being　the　handiest　way　he　could　see　at　the　time　for　saving　up　the　necessary。
　　
　　Sorry；　Minerva；　I　was　talking　about　those　mirror　twins。　So　the　silly　little　wench　was　knocked　up；　which　caused　me　to　slip　back　into　my　baby…cotching；　country…doctor　persona；　and　I　stayed　up　all　night　worrying　about　her　and　her　brother　and　the　baby　they　were　going　to　have…unless　I　did　something　about　it。　To　find　out　what　I　should　do；　I　had　to　reconstruct　what　had'happened　and　from　that　what　could　happen。　Having　no　certain　data；　I　had　to　follow　that　old　rule　for　finding　a　lost　mule。
　　
　　First　I　had　to　think　like　that　slave　factor。　A　man　who　auctions　slaves　is　a　scoundrel　but　too　smart　to　risk　a　caper　in　which　he　might　wind　up　a　slave　himself；　or　dead　if　he　was　lucky…which　is　what　would　happen　to　one　who　played　fast　and　loose　on　Blessed　with　the　authority　of　a　bishop。　Ergo；　the　scoundrel　had　believed　what　he　had　said。
　　
　　That　being　so；　I　could　table　the　question　of　why　this　factor　was　missioned　to　sell　these　two；　while　I　tried　to　think　like　a　priest…scientist　engaged　in　human　biological　experimentation。　Forget　the　chance　that　these　two　were　ordinary　siblings…no　point　in　picking　such　a　pair　even　for　a　swindle。　Forget　the　chance　that　they　were　unrelated　in　any　fashion；　as　in　such　a　case　it　would　simply　be　a　normal　case　of　breeding。　Sure；　sure；　any　woman　can　give　birth　to　a　monster；　as　even　with　the　most　gei？ally　hygienic　of　breeding　a　bad　mutation　can　show　up…and　an　alert　midwife　may　neglect　to　give　that　first　life　giving　spank…and　many　have。
　　
　　So　I　considered　only　the　third　hypothesis：　plementary　diploids　from　the　same　parents。　What　would　this　experimenter　do？　What　would　I　do？
　　
　　I　would　use　as　near　perfect　stock　as　I　could　find　and　not　start　the　experiment　until　I　had　both　a　male　and　a　female　parent　who　tested　〃clean〃　geically　in　the　most　subtle　ways　for　which　I　could　test…which　on　Blessed　meant　quite　sophisticated　ways；　for　that　century。　For　a　selected　gene　site　and　an　assumption　of　50…50　in　the　Mendelian　distribution　of　25…50…25；　this　pre…experiment　testing　would　chop　off　the　25　percent　chance　of　reinforcement　of　a　bad　recessive　and　leave　a　distribution　of　one…third　bad；　two…thirds　good；　at　the　parent　generation…possible　parents　of　possible　Joes　and　Llitas；　that　is。
　　
　　Now　I　start　putting　together　mirror　twins　in　my　persona　as　a　'priest…experimenter。　What　happens？　If　we　consider　the　minimum　number　of　gametes　needed　to　represent　this　ohird　and　two…thirds　distribution；　we　get　eighteen　possible　〃Joes；〃　eighteen　possible　〃Llitas〃…but　in　both　male　and　female　two　of　them　show　up　as　〃bad〃…the　bad　recessive　has　reinforced　and　the　zygote　is　defective；　the　experimenter　eliminates　them　。　。　or　he　may　not　need　to；　the　reinforcement　may　be　lethal。
　　
　　We　wind　up　at　this　point　with　an　8　&　1/3　percent　improvement；　or　a　total　improvement　of　25　percent　in　favorable　chances　for　Llita's　baby。　I　felt　better。　If　you　add　the　fact　that　I　am　the　sort　of　a　midwife　who　is　too　busy　helping　the　mother　to　stop　to　spank　a　monster；　the　favorable　chances　went　way　up。
　　
　　But　all　that　this　shows　is　that　bad　genes　tend　to　be　eliminated　at　each　generation…with　the　tendency　greatest　with　the　worst　genes　and　reaching　100　percent　whenever　reinforcement　produces　a　lethal…in…womb…while　favorable　genes　are　conserved。　But　we　knew　that…and　it　applies　also　to　normal　outbreeding　and　even　more　strongly　to　inbreeding；　although　the　latter　is　not　well　thought　of　for　humans　as　it　hikes　up　the　chances　of　a　defective　by　precisely　the　same　amount　that　it　weeds…that　being　the　hazard　that　I　was　afraid　of　for　Llita。　Everybody　wants　the　human　gene　pool　cleaned　up；　but　nobody　wants　its　tragic　aspects　to　take　place　in　his　own　family。　Minerva；　I　I　was　beginning　to　think　of　these　kids　as　〃my　family。〃
　　
　　I　still　did　not　know　anything　about　〃mirror　twins。〃　I　decided　to　investigate　a　more　probable　incidence　of　bad　recessives　at　a　given　site。　Fifty…fifty　is　far　too　high　for　a　really　bad　gene；　the　weeding　is　drastic；　and　the　incidence　drops　to　a　lower　percentage　each　generation；　until　the　incidence　of　a　particular　bad　gene　is　so　low　that　reinforcement　at　fertilization　is　a　rare　event；　as　reinforcement　is　the　square　of　the　incidence；　e。g。；　if　one…in…a…hundred　haploids　carry　this　bad　gene；　then　it　will　be　reinforced　one…in…ten…thousand　fertilizations。　I　speak　of　the　total　gene　pool；　or　in　this　case　a　minimum　of　two　hundred　adult　zygotes；　female　and　male；　random　breeding　in　such　a　pool　will　bring　together　that　bad　reinforcement　only　by　that　long　chance…a　chance　happy　or　unhappy　depending　on　whether　you　look　at　it　impersonally　in　terms　of　cleaning　the　gene　pool　or　personally　in　terms　of　individual　human　tragedy。
　　
　　I　looked　at　it　very　personally；　I　wanted　Llita　to　have　a　healthy　baby。
　　
　　Minerva。　I'm　sure　you　recognized　that　25…50…25　distribution　as　representing　the　most　drastic　case　of　inbreeding；　one　which　can　happen　only　half　the　time　with　line　breeding；　only　a　quarter　of　the　time　with　full　siblings；　in　both　cases　through　chromosome　reduction　at　meiosis。　A　stockbreeder　uses　this　drastic　measure　regularly…and　culls　the　defectives　and　winds　up　with　a　healthy　stabilized　line。　I　have　a　nasty　suspicion　that　such　culling　after　inbreeding　was　sometimes　used　among　royalty　back　on　old　Earth…but　certainly　such　culling　was　not　used　often　enough　or　drastically　enough。　Royalism　might　work　quite　well　if　kings　and　queens　were　treated　like　racehorses…but　regrettably　they　never　were。　Instead；　they　we？e　propped　up　like　welfare　clients；　and　princelings　who　should　have　been　culled　were　encouraged　to　breed　like　rabbits…bleeders；　feebleminded；　you　name　it。　When　I　was　a　kid；　〃royalty〃
　　
　　was　a　bad　joke　based　on　the　worst　possible　breeding　methods。