Chapter Three: How Do Environments Impinge Upon Genes?

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chapter three

Transcript of Chapter Three: How Do Environments Impinge Upon Genes?

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HOW DO ENVIRONMENTS IMPINGE UPON GENES? 33

■ ■ ■ Skip, a regretful man When Skip was a boy he often pestered his mother with questions.

“Why can’t I have my own room?” he would ask. “Why can’t I have a bike?

Why do we have to eat casserole every night?” But his mother would only

reply with aphorisms he didn’t understand. “If pigs had wings they would

fly,” she would say. Or sometimes she would say, “If wishes were candy it

would be Christmas every day.”

Those annoying sayings echo in his ears tonight. He surely wishes he

could make like a pig and fly out of the stinking mess of his life. It’s not

his fault, Skip thinks bitterly. He had to drop out of high school to earn

money for the family. But then he thinks how he didn’t have to get his girl-

friend pregnant. Twice. And he could have gone back to school or learned

a trade. He just never seemed to have the time, energy, or cash.

So here he is at thirty-two, divorced and alone, and no further ahead

than when he was eighteen. He’s just an assistant manager at an all-night

diner with no one to talk to but tired-out waitresses, surly cooks, and

grumpy customers. It didn’t have to be this way, he muses despondently.

Hadn’t he been a real charmer, a really handsome kid? Wasn’t he the best

hitter on his Little League team? Didn’t he used to dream of becoming an

astronaut?

Skip thinks about Marlon Brando’s wailing complaint in the classic

movie, On the Waterfront: “I could’a been a contender!” Me too, thinks

Skip. “I really could have been somebody. So why have I turned out to be

such a loser?”

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Environment illustratedThree examples will illustrate how environments impinge upon genes. Theyconcern a plant, a human disease, and ahuman behavior.

In a classic experiment, seven geneti-cally distinct yarrow plants were collectedand three cuttings taken from each plant.One cutting of each genotype was plantedat low, medium, and high elevations,respectively.

When the plants matured, no onegenotype grew best at all altitudes, and ateach altitude the seven genotypes fareddifferently. For example, one genotypegrew the tallest at the medium elevationbut attained only middling height at theother two elevations. The best growers at

low and high elevation grew poorly atmedium elevation. The medium altitudeproduced the worst overall results, butstill yielded one tall and two medium-tallsamples. Altitude had an effect on eachgenotype, but not to the same degree norin the same way.1

The second example illustrating envi-ronmental effects involves the human disease called PKU. This is the commonname for a medical disorder, phenylke-tonuria, which results when the bodydoes not produce enough of a particularliver enzyme. In the absence of thisenzyme, an amino acid known as pheny-lalanine does not get converted into thenext amino acid in a biochemicalpathway, and therefore too much pheny-lalanine passes into the blood and othertissues. This disturbs brain development,leading to mental retardation and otherproblems.

PKU affects approximately 1 out ofevery 15,000 infants in the U.S.However, most affected infants do notgrow up impaired because of a standardscreening program used in the U.S. andother industrialized societies. Newbornsfound to have high levels of phenylala-nine in their blood can be put on a special, phenylalanine-free diet. If theyare put on this diet right away and stayon it, these children avoid the severeeffects of PKU.

PKU is a genetic condition that stemsfrom any of a number of different muta-tions in a gene that codes for amino acids

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Some medical disorders arecaused by faulty instructions froma single gene. PKU is such a dis-order. If babies with this disorderare identified at birth, treatmentcan prevent the tragic health consequences of PKU.

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that combine to form an enzyme thatconverts phenylalanine. (Scientists oftenrefer to alleles that lead to disorders asmutations, though all alleles — boththose with positive and negative effects—emerge at some point in the evolu-tionary history of a species through theprocess of mutation. In this text, we willrefer to such mutations as “disease-related alleles” or “problematic alleles.”)

Researchers know that other so-calledmodifier genes also play a role in PKU —these are genes that affect another gene,thereby altering the latter gene’s effect onthe phenotype. Thus, there are many different genotypes underlying PKU, andthis certainly is one reason why the disease manifests itself differently in each child.

Environment is another reason. Theform and severity of PKU are profoundlyinfluenced by such factors as when thecondition is diagnosed, how soon thespecial diet is imposed, and how strictly itis followed.

Yet in an interesting twist, environ-mental effects decline over time. A childwho does not receive a modified dietwithin days of birth is at great risk forbrain damage. The same child in adoles-cence can follow a slightly more flexiblediet without ill effect. Some adults withPKU have binged occasionally or movedoff the diet altogether without observablelosses in cognitive function (though stan-dard medical advice is to maintain thediet for life).

The third example for environmentaleffects concerns human intelligence.Scientists theorize that many differentaspects of brain function factor into ourability to reason and to learn, such asenergy metabolism and neuronal trans-mission speed. To the extent that genestrigger the protein activity that con-structs the brain and are essential to itsfunction, they play a role in intelligence.

Tests used to measure individual intel-ligence are called IQ tests (for intelli-gence quotient). Performance on thesetests varies widely among individuals:most tested individuals obtain scoresthat fall into a middle range, while aminority obtain scores that fall fartherout on the high and low ends. All thescores plotted out form a sort of bell-shaped curve.

In 1987, a scientist named JamesFlynn reported that, based on IQ datafrom many countries, the raw test scoreshave been rising rapidly for severaldecades. So while the bell-shaped varia-

In a randomly selected group ofpeople, behavior traits tend tovary along a continuum known as a bell shaped curve. In thisillustration, a trait is indicated asa "property" that can range fromlow to high. An example would be scores on an intelligence test:most people tend to score nearthe middle, but some score loweror higher. The higher or lower thescore, the fewer the number ofpeople achieving those scores.The Bell Shaped Curve

Low HighProperty

Number

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tion remains, performance across theboard has gone up.

This phenomenon has been dubbedthe Flynn effect. Flynn’s research suggeststhat a good deal of this IQ gain can beattributed to improved performance onanalytical and visual/spatial problems.Performance on verbal and mathematicalproblems has increased, but not as rapidly.

The overall rise in IQ scores cannotpossibly be due to new genetic mutationsintroduced and dispersed throughout theworld’s population, since that kind of evolutionary change would take manyhundreds or thousands of years. The cause therefore must be environ-mental — something that supports testperformance of individuals of all geno-types. Several hypotheses have been proposed to explain this. It could be dueto the fact that people today are betternourished and better educated, or thatmodern culture values and supports test-taking skills, or that electronic mediastimulate the brain.

Whatever the cause or causes, theresult is that modern brains overall are

better functioning on some tasks meas-ured by IQ tests. Recall once again thatgenes code for amino acids, the buildingblocks of the proteins that create thephysical structures of cells and triggeractivity inside them. Therefore, while theenvironment may be altering brain devel-opment and causing IQ to rise, it is doingso through the mechanism of the genes.

Gene/environment interactionsBy now the reader should be convincedthat genes and environment are both crit-ical. Without environment, an organismcould not exist because it is from the envi-ronment that it obtains the essentialmaterials enabling it to grow and survive,such as nutrients, oxygen, and water.Without genes, an organism could notexist because it would not have the mech-anism to extract what it needs from theenvironment.

It is not about nature versus nurture, as that old cliché would have it. It isabout nature-on-nurture-on-nature-on-nurture, round and round and round.

The term for this complex exchange ofreciprocating influence is gene/environ-

ment interaction. It is not so simple a concept as milk and eggs poured into onemixing bowl. Rather, the two act uponand with each other. The same genotypein different environments may lead tosimilar or different phenotypes. The sameenvironment operating upon different

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IQ scores are rising, but this doesnot necessarily mean that today’schildren are smarter than theirparents. There are many theoriesthat explain the IQ rise, but theyall come down to the same basicidea: human environments areinteracting with human genes to produce different results than before.

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genotypes may also lead to similar or different phenotypes. Different genotypesin different environments may lead tosimilar or different phenotypes. It alldepends upon interactions.

Here’s an example of how gene/envi-ronment interaction can play out in reallife. There is a gene, ALDH-2, whose pro-tein product helps metabolize alcohol.Some people have an allele for ALDH-2that is less effective: alcohol byproductsremain in the tissues instead of beingmetabolized properly. A person with thisallele is more likely to become flushed,dizzy, and nauseous in response todrinking. It is estimated that 50 percent ofAsian people have an ineffective ALDH-2allele.

Overall, Asian immigrants to Americadrink much less alcohol than their children born and raised in the States.

The two genetically similar generationshave different drinking patterns for a cultural (that is, environmental) reason: the younger generation is brought up in aculture that places greater emphasis onalcohol.

Studies show that Asian Americanswith the less active ALDH-2 allele, bothimmigrant and first-generation, drink lessthan their counterparts who have analternate version of the gene. But there isless of a difference in drinking levelbetween those with and without the spe-cial allele in the immigrant generationcompared to their children’s generation.This is because members of the immi-grant generation tend to drink little in anyevent — whether it makes them sick ornot. Both genes and environment affectalcohol consumption, but at differentrates under different circumstances.2

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People who have a particularallele for the gene called ALDH-2experience a harsh physical reac-tion when they consume alcohol.It is estimated that up to half ofAsian people have this ALDH-2allele. Whether they choose todrink or not is shaped by otherfactors. For example, some peoplemay decide that the social pleasures of drinking override its unpleasant side effects.

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Developmental noiseComplicating the process of gene/environment interaction is somethingcalled developmental noise. By this ismeant the variation introduced byminute, random events that occur duringdevelopment and have significant cumu-lative effects on the phenotype.

In a classic science fiction story, a groupof hunters time-travel back millions ofyears to hunt dinosaurs. Their tour guidepre-selects dinosaurs for the kill who arejust about to die from other causes, so asnot to alter the past. But one of thehunters accidentally steps on a butterflyand kills it. When the group travels for-ward again to their own time, everythingabout their world has changed ever soslightly. Just one butterfly died, but gener-ations of offspring never came into exis-

tence, triggering a chain of events thatshaped a different future.3

In the same way, unique and unpre-dictable events occur inside each cell. An extra dollop of mineral is taken up byone cell, while a molecule of vitamin failsto reach the cell next door: These kinds oftiny, unpredictable variations cause cellsto develop differently though they sharethe same function, genotype, andexternal environment. It all adds up tomake observable differences in the wholeorganism.

Developmental noise affects physicalcharacteristics such as the number ofhairs in an eyebrow or the coloring of apatch of skin. Likewise, developmentalnoise can have subtle but far-reachingeffects on behavior.

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In a Ray Bradbury short story, a time traveler to the age ofdinosaurs accidentally steps on a butterfly, thus altering the course of future events. This story illustrates how smalland unpredictable events can cumulatively have significanteffects. In behavioral genetics,this concept is called “develop-mental noise.”

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Gene/environment correlationsAnother factor shaping behavior isgene/environment correlation. A gene/environment correlation occurs whenindividuals with a genetic propensity for atrait live in environments that supportexpression of the trait. This kind of corre-lation can occur in two ways, passive andactive.

Suppose a young girl who is geneticallygifted for music is born into a talentedfamily of musicians (for sake of argument,let’s gloss over the meaning of “geneti-cally gifted for music”). She is surroundedby family members who practice and per-form. Her home is filled with instru-ments, and music plays on the radio allday long. The girl is raised in a home thatsupports the flourishing of her musicalability. This is an example of a passivegene/environment correlation.

Suppose a boy who is genetically giftedfor music is born into a nonmusicalfamily. As a youngster, his parents takehim to a parade. He is so excited by themarching band that he persuades his par-ents to let him take drum lessons. In highschool, he joins the orchestra and choosesmusic electives. He applies to andreceives a scholarship to an elite musicschool. The boy seeks out activities thatsupport the flourishing of his musicalability. This is an example of an activegene/environment correlation.

The above examples show positive cor-relations, but negative ones may occur:

the musically gifted girl who resists thepath laid out by her musical family, for example. Positive gene/environmentcorrelations increase the range of pheno-typic variation stemming from a givengenotype, while negative correlationsdecrease the range.

Shared and nonshared environments

Does growing up in the same homewith the same parents, same physical surroundings, and same everyday experi-ences make you turn out like your siblings? Does having different friendsmake you different from your siblings?

Let’s take a closer look at the first ques-tion. Growing up together in the samehome — which falls into a category calledshared environment — does make siblingssimilar in terms of the cultural traditionsthey inherit: similar in terms of language,modes of dress, diets, and so on.

However, many studies suggest thatshared home environment does not dovery much to make siblings resemble eachother in terms of personality and actions.Each child turns into a distinct characterwho behaves in individual fashion, despiteparents’ efforts to raise all their childrenimpartially and despite similarities in geno-types of the siblings (remember that biological siblings are half alike genetically,on average, and twins are fully alike genet-ically except for a few differences causedby mutation and epigenetic factors).

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Some people may be musicallygifted, but we do not yet have sufficient scientific data toexplain any relationship betweengenes and talent. We can say,however, that musical emerges from the interaction of genes and environments.

skill

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Now consider the second question.Your unique set of friends is just oneexample of the many, many experiencesthat you do not share with a sibling.These experiences contribute to what iscalled your non-shared environment.Other examples of unique, idiosyncraticnon-shared experiences include your prenatal life, your birth, your childhoodillnesses and accidents, your particularcombination of teachers, your summercamps, and so forth, on through life.

Say that when you turn seven, youundergo the Roman Catholic ritual ofFirst Communion just as all your siblingsdid when they each turned seven.Although to some degree this is a sharedexperience, it falls into the category ofnon-shared environment to the extentthat your communion takes place in a

unique point in historical time and at aunique moment in your life, with aunique set of other communicants, beforea unique congregation, etc., etc.

Such incongruity has led scientists toredefine shared environment. They say itis one that works to make those whoexperience it similar for a particular trait.By the same token, they have redefinednonshared environment to be one thatworks to make those who experience itdissimilar for that trait. Some scientistsare troubled by the seeming circularity inthese definitions; the definition problemunderscores the difficulties that environ-ment presents for researchers.

In any event, it doesn’t take muchimagination to recognize that the numberof nonshared environmental factors inanyone’s life is so large that together theymust have some impact. And indeed,from behavioral genetic studies it appearsthat the nonshared environment has a sig-nificant effect on behavior; it is possiblymore significant than genes or sharedenvironments.

But since much of the nonshared envi-ronment is random, accidental, andunsystematic, it may well defy study. Thismakes things difficult for researchers.

Some scientists are working on devel-oping a theory of the envirome (the envi-ronment surrounding and affecting agenome) so that it can be studied with thesame precision as the genome.Environmental factors, whether shared ornonshared, do not lie on anything so

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Many children attend summercamp, yet each child uniquelyresponds to and is affected by this episode of early life. Summer camp, therefore, is both a shared and non-sharedexperience.

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tangible as a DNA strand; this makes themhard to discern. They are essentially infi-nite; this makes the job difficult to finish.The big question is, what environmentalfactors are relevant, discrete, and measur-able: socioeconomic status? birth order?number of books in the home? occupationof parents? climate? prevailing attitudes inone’s social milieu? Mapping the envi-rome is such a formidable task that somedismiss it as a hopelessly naïve endeavor.And yet researchers chip away at it.

Heritability (and environmentability)In the meantime, the field relies on a lesssophisticated tool, a simple mathematicalformula that produces a heritability

estimate. Heritability is the proportion ofphenotypic variation in a population thatis due to genetic variation. Someresearchers also use the word environ-

mentability to describe heritability’s coun-terpart, that is, the proportion ofphenotypic variation in a population thatis due to environmental variation. Theheritability and environmentability forany given trait are proportions thattogether add up to 100 percent.

Here are three simple examples thatdemonstrate heritability. Example 1: The heritability of having a brain in anypopulation of humans is 0, becauseeveryone has a brain; there is no pheno-typic variation. Example 2: The heri-tability of height in a malnourished

population — where everyone’s growth isstunted — will be lower (closer to zero)than in another population that is wellnourished — where everybody’s geneticpotential can be realized. In bothExample 1 and Example 2, zero and lowheritability occur even though genes playa critical role in development and growth.Example 3: The heritability of blood typein a random human populationapproaches 1. The phenotypic variation ismainly attributable to genetic variation.Note that this does not reveal anythingabout the particular blood type of anyindividual person in the group.

Heritability is a slippery, confusing con-cept. Because “heritability” sounds like“inherited,” heritability figures are oftenmisconstrued as describing an individual’schances for inheriting a trait, even thoughheritability is a measure that applies onlyto groups. Another problem with the con-cept is that sometimes when the word“heritable” is used to describe a trait, it ismisunderstood to mean unchangeable.Yet near-sightedness is both a heritabletrait and fixable through eyeglasses, contact lenses, and laser surgery.

For scientists, heritability estimates areunsatisfactory. They are, after all, simplyestimates. They apply only to the popula-tion being studied in one particular environment and at one point in time.They do not reveal anything about thespecific genetic and environmental factorsunderlying a trait. And yet heritabilityestimates have their value.

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Vision is a heritable trait. It varies phenotypically amonghumans, which means thateveryone does not see equallywell. Instead, human eyesightranges from very poor to excellent. If science advances to the point where all vision deficiencies are corrected bylenses and surgery, then visionwould no longer be a heritabletrait.

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It is possible, through a type of studythat will be described in Chapter 4, tocome up with a heritability estimate for atrait that looks like this: additive geneticinfluence, .xx; nonadditive genetic influ-ence, .xx; shared environment, .xx; andnonshared environment, .xx — with allthe .xx’s adding up to 1.0 or 100 percent.

By calculating such estimates,researchers have learned what wereported in our introduction — thatessentially all behavioral traits have agenetic component — and also what wereported earlier in this chapter – that non-shared environments have significantinfluence on a trait compared to the genesand the shared environment. By repeatedapplication of this tool across similarstudies, researchers also have learned thatheritability of a trait can change (thoughfor many traits it remains stable bothacross populations and over time).Heritability measures also have been usedto direct research toward those traits thatin some contexts are highly heritable —the theory being that genes contributingto such traits may be more susceptible todiscovery.

Skip’s regretsSkip, the despondent assistant restaurantmanager, wonders why he is such afailure. Is it the fault of the qualities hewas born with? Can he blame his mother?Is he a victim of circumstance? Could it be— as harsh as it sounds — his own fault?

A possible clue to this conundrumcomes from honeybees. In any honeybeecolony, there is only one queen. She isvery much larger than all the others, and her function is to lay the eggs. The worker bees tend to the queen, takecare of the young, fetch the nectar, andkeep the hive maintained.

From the queen’s fertilized eggs comethe next generation of workers and thefuture queen. From a cluster of eggs, justone will grow into a mature queen thatlooks and behaves quite differently fromall other bees.

Scientists have wondered how one beebecomes queen and other genetically sim-ilar bees do not — as Skip would put it,how one bee turns into a “somebody”and the others remain “losers.” Using arelatively new technique to study DNA,scientists recently learned what happensinside the cells of bees to create differencein status. They have discovered that dietmakes the difference. Larvae develop intoworkers when they are fed nectar andpollen. Larvae develop into queens whenfed royal jelly, a substance secreted fromthe glands of worker bees.

Depending on the nutrition each beereceives in the larval stage (an environ-mental input), certain genes are switchedon (through epigenetic effects) that influ-ence development (by coding for partic-ular amino acids). Scientists have foundseven different genes in honeybees thatare activated differentially by nutrition,though they suspect many more are

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involved. The question not answered inthis research is how worker bees choosewhich larvae to supply with royal jellyand which to feed the commonplace dietof nectar and pollen.4

Scientists pursuing another recent lineof research have uncovered a second hon-eybee phenotype switch. Female honey-bees graduate from hive-keepers intoforagers, usually at about two weeks ofage. This job change has been tracked tothe effect of a single gene. The same geneexists in fruit flies, and it determineswhether a fly seeks out food near home orsearches in a wider range. Further studyis needed to discover what triggers thegenes to trigger the change in behavior.

Researchers believe that the same genemay operate in humans, though they canonly speculate as to the behavior it affects.5

The ability of insects with similar geno-types to acquire substantially differentphenotypes under different environ-mental conditions occurs not only in hon-eybees, but also in other social insectssuch as ants and termites. Nutrition, tem-perature, day length, and other environ-mental factors interact with the genes ofthese insects to affect phenotypes. Severalspecies of butterfly change wing colorwith the changing seasons. Dung beetlesgrow horns or not, depending on theirdiet. Many such examples can be foundin nature.

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Scientists believe that a honeybee’s occupation, such ashive-keeper or forager, may bedetermined by epigenetic factorsswitching a gene on and off.Human occupations are notdecided in such a biologicalmanner.

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Developmental pathwaysThe route from egg to larva to worker orqueen is called a developmental pathway.Somehow, Skip’s developmental pathwayhas taken him to a place he does notenjoy. He has not become a leader like aqueen honeybee. He has not evenclimbed up the career ladder like theworker bee promoted from hive-keeper toforager. The bees at least are part of asocial group, but Skip feels lonely andalone.

Alas, Skip cannot know what wentwrong in his life by looking at honeybees.There is no human equivalent to royaljelly that, had his mother fed it to him,would have turned him into a greatachiever. Human occupations are notdecided at the whim of a few genes triggered on or off. Changing seasons andchanges in diet do not determine thehuman ability to make wise relationshipdecisions.

Unlike honeybees, humans — at leastin free societies — have options.Certainly humans have consciousness —a sense of existence within a surrounding,a sense of being able to take action.

Skip remembers that when he was achild his mother used to say, “If pigs hadwings they would fly.” That was her wayof telling him not to pine for what he doesnot have. Science cannot tell Skip pre-cisely which environmental inputs couldhave been put into place to stimulatewhich epigenetic effects to trigger whichgenes for which amino acids that would

combine to make which proteins thatwould have made him more successfuland happier. Even if he knew all that, Skip could not change his past.

However, Skip can make use of the scientific metaphor of complexity. At age32, he still has time to move forward onhis developmental path. Science cannotadvise him on what to do to improve him-self, but he has a thinking mind; he canconsider his assets and his availablechoices.

Skip has now what he did not havebefore: wisdom earned through experi-ence and a desperate resolve to change.What he needs is some self-respect. Heshould remember that all human beingsare genetically similar, just as the honey-bees in a hive are. Skip has always hadthe inborn potential to turn out differently— to become better. He still has it.

Notes

1 Clausen et al. (1948).

2 See Carey (2003, pgs. 78-79) for discusson of ALDH-2

research.

3 Ray Bradbury’s “A Sound of Thunder” (1952).

4 Evans, J. D. and D. E. Wheeler (1999).

5 Ben-Shahar, Y., et al. (2002).

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The developmental pathway ofany living organism continuesthroughout life. Through theirconscious behavior, humans areable to exert at least some control over their destinies.

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