e 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。
Captain Sheffield investigated next a lower incidence of a bad gene： Assume a lethal gene in the gene pool from whi h Joe and Llita's parents。 were derived。 Being lethal； it。 could exist in an adult zygote only if it was masked in gene…pair by its benign twin。 Assume a 5 percent masked incidence in zygotes…still too high to be realistic for a lethal gene…but check it anyhow。 What trend would show？
Parent zygote generation： 100 females； 100 males； each a possible parent for Llita and for Joe…and 5 of the females and 5 of the males carry the lethal gene； masked。
Parent haploid stage： 200 ova； 5 of which carry the lethal gene； 200 spermatozoa； 5 of which carry the lethal gene。
Son…and…daughter zygote generation （possible 〃Joes〃 and possible 〃Uitas〃）： 25 dead through reinforcement of lethal gene； 1；950 carrying the lethal gene masked； 38；025 〃clean〃 at that site。
Sheffield noted that a hypothetical hermaphrodite had crept in through not doubling his sample size in order to avoid anomaly through odd numbers； Oh； the hell with！…it did not change the statistical oute。 No； do it！…start with a sample of 200 males and 200 females with the same lethal…gene incidence for that site。 This gave him：
400 ova； 10 with the lethal gene；
400 spermatozoa； 10 with that lethal gene…10'）
…which gave in the next zygote generation （possible 〃Joes〃 and 〃Llitas〃）： 100 dead； 7；800 carriers； 152；100 〃clean〃… which changed no percentages but got rid of that imaginary hermaphrodite。 Sheffield considered briefly the love life of an hermaphrodite； then got back to work。 The numbers became very cumbersome； jumping to the billions in the next zygote generation （i。e。； Little Nameless； now just started in Llita's belly ）…l 5；210；000 culled by reinforcement； 1。216。800；000 carriers； 24；336；000；000 〃clean〃…and again he wished for a clinic puter and tediously converted the unhandy numbers into percentages： 0。059509 percent； 4。759 percent； 95。18 percent plus。
This showed a decided improvement： approximately 1 defect out of 1；680 （instead of 1 out of 1；600）； the percentage of carriers decrease to below 5 percent and the number of 〃clean〃 increased to above 95 percent in one generation。
Sheffield worked several such problems to confirm what he had seen by inspection： A child from plementary diploid （〃mirror twins〃） had at least as much chance of being healthy as did the offspring of unrelated strangers…plus the happy fact that such a baby's chances were improved by culling at one or more stages by the priest…scientist who had initiated the experimention … almost certain assumption and one that made Joe the best possible mate for his 〃sister〃 rather than the worst。
Llita could have her baby。
VARIATIONS ON A THEME
VII
Valhalla to Landfall
…the best I could for them； Minerva。 Every so often some idiot tries to abolish marriage。 Such attempts work as well as repealing the law of gravity； making pi equal to three point zero； or moving mountains by prayer。 Marriage is not something thought up by priests and inflicted on mankind； marriage is as much a part of mankind's evolutionary equipment as his eyes； and as useful to the race as eyes are to an individual。
Surely； marriage is an economic contract to provide for children and to take care of mothers while they bear kids and bring them up…but it is much more than that。 it is the means this animal； Homo sap。； has evolved…quite unconsciously…for performing this indispensable function…and be happy while doing so。
Why do bees split up into queens； drones； and workers； then live as one big family？ Because； for them； it works。 How is it that fish do okay with hardly a nodding acquaintance between mama fish and papa fish？ Because the blind forces of evolution made that way work for them。 Why is it that 〃marriage〃…by whatever name…is a？universal institution among human beings everywhere？ Don't ask a theologian； don't ask a lawyer； this institution existed long before it was codified by church or state。 It works； that's all； for all its faults it works far better by the only universal test…survival…than any of the endless inventions that shallow…pates over the millennia have tried to substitute for it。
I am not speaking monogamy； I mean all forms of marriage…monogamy； polyandry； polygyny； plural and extended marriages with various thrills。 〃Marriage〃 has endless customs； rules； arrangements。 But it is 〃marriage〃 if…and…only…if the arrangement both provides for children and pensates the adults。 For human beings； the only acceptable pensation for the drawbacks of marriage lies in what men and women can give each other。
I don't meast 〃Eros；~' Minerva。 Sex baits the trap； but sex is not marriage； nor is it reason enough to stay married。 Why bteed a cow when milk is cheap？
panionship； partnership； mutual reassurance； someone to laugh with and grieve with； loyalty that accepts foibles； someone to touch； someone to hold your hand…