chapter two - AAAS Home · which means they carry two alleles for every gene (one inherited from the father and one from the mother); the two alleles in a given pair may be identical

chapter tw

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HOW DO GENES WORK WITHIN THEIR ENVIRONMENTS? 22

■ ■ ■ Hoda, a perplexed nurse Hoda has been a pediatric nurse for twenty years, and today was one of

her tougher days. It’s August and so a good number of the young patients

had come in for school immunizations. Hoda thinks to herself on the way

home that if she could have a nickel for every child who cries at getting a

shot, she could retire a rich woman.

Her glum perspective eases up a bit as she gets further away from the

clinic. It was really hot in there today, she tells herself, and for a lot of the

children it is a strange place. She considers the fact that almost every

child who gets upset by a shot is easily consoled with a simple sticker.

And while some children are born crybabies, many others never put up a

fuss at vaccination.

Hoda used to think that a child’s attitude towards shots depended on

the parent with him or her at the clinic, but since she became a parent her-

self she is not so sure about that. Her older son has never minded shots,

but her younger son is a big baby.

Hoda knows that her own conduct as nurse has considerable influence

on how her young patients behave. She tries to be gentle and to do her

business as quickly as she can. While that works for most children, it is not

enough for others. And then there’s always the child who is no trouble for

one vaccination, but who makes a big fuss for the next. “Kids!” thinks

Hoda. “Will I ever figure them out?”

The human genomeThe complete set of genetic material forany organism is called its genome. Inrecent decades, information about thegenomes of several organisms has beenpouring out of a massive internationaleffort called the Human Genome Project.This “sea of data,” as it has been called,confirms the view that genes operatewithin really big and complex systems.

The human genome is organized intotwo sets of twenty-three chromosomes,forty-six in all. These chromosomes aremade of a chemical substance called DNA

(deoxyribonucleic acid), and this DNA, inturn, is made of smaller units: nitrogen-

containing molecules called nucleotides

or bases. There are four different bases,called adenine (A), cytosine (C), thymine(T), and guanine (G). The bases are pairedand linked together to form a double-stranded helix. The order of bases strungalong chromosomal DNA is critical, as wewill explain in a moment.

The human genome contains three billion pairs of these bases. Measuredwith a yardstick, this makes six feet ofDNA — not so very long, except that allof it fits, coiled up, inside the nucleus ofa single cell. Indeed, the entire genomein identical form is packed into nearlyevery nucleus of the body’s one hundredtrillion cells.

The human genome is quite large, butit is not all that large compared to thegenomes of other life forms. A tiny germcalled mycoplasma genitalium has one ofthe smallest genomes yet contains morethan 580,000 base pairs. Wheat has tentimes more DNA than humans.

Function of the human genomeSize is not the only dimension that makesthe human genome impressive.Functional operation is another. At seem-ingly random spots along a chromosome’sstrand of DNA, base pairs are organizedinto units that operate together. These arethe genes.

There is an important term for those“seemingly random spots along a chro-

1 0 BEHAVIORAL GENETICS

Human DNA is contained in 46 chromosomes: 22 pairs plustwo X chromosomes in females orone X and one Y chromosome inmales. For reference purposes, scientists have assigned numbersto the chromosomes using anorder based on length.

CHAPTER 2: HOW DO GENES WORK WITHIN THEIR ENVIRONMENTS? 1 1

mosome’s strand of DNA,” and it willappear frequently in this text. That termis locus; the plural is loci. It means “location of the gene on the chromo-some.” The word locus also has come tostand for “the location of a segment ofDNA within a gene.”

Each gene varies in the order of thebases along its length. The averagehuman gene is three thousand base pairslong. The human genome contains anestimated 30,000 or more genes, yetthese genes comprise less than 5 percentof the genomic material.

Put simply, this is what genes do: They provide the template for a series ofintricate steps that cells follow to createproteins. A gene’s string of bases is organ-ized into triplets. The sequence of DNAtriplets that defines a gene is copied intoa string of RNA (ribonucleic acid, a chem-ical similar and complementary to DNA)triplets. In most cases, each RNA tripletcodes for one of twenty different smallmolecules called amino acids (fondlyreferred to by science teachers as “thebuilding blocks of the body”). Most of theamino acids have names ending in “ine”such as alanine, glycine, and isoleucine.The amino acids coded for by a gene linktogether into a polypeptide chain. Thesechains can be hundreds or thousands oflinks long (shorter strands are referred tosimply as “peptide chains”). A protein isformed when (in most cases) two ormore chains link together into a three-dimensional structure.

Proteins make up the structure of cells:hair, cartilage, bone, and the other phys-ical components of the body are builtfrom protein. In the form of hormones,enzymes, and antibodies, proteins directcell activity. Proteins help transport mate-rials between cells and they help cellscommunicate with each other.

Another critical task of proteins exem-plifies that old phrase “turnabout is fairplay.” Proteins are constructed throughgene activity and — in the form of hormones, growth factors, and other regulatory molecules — proteins alsoaffect gene activity.

The adjective that describes this phe-nomenon is epigenetic. Epi is a Greekroot meaning “upon,” and epigenesismeans the process of affecting the actionof a gene without altering the DNA of thegene itself. Epigenetic effects are pro-duced not only by proteins but also byRNA; by certain genes with managerialresponsibilities; and by the imprinting ofgenes (a little-understood phenomenonby which a gene expresses itself differ-ently depending upon whether it wasinherited from the mother or the father).

These epigenetic factors are whatcause some cells to turn into skin whileothers become part of the liver, bone, orbrain, even though all the cells containthe same package of DNA. Epigeneticmechanisms remain important through-out life, selectively triggering the genes invarious tissues in response to environ-mental stimuli.

A protein can be visualized as a long strand of material thatbends and folds into a complexthree-dimensional shape. This long strand typically is created from two or more chainsof amino acids (and sometimesother chemicals) that have linkedtogether. Each separate chain is created according to theinstructions contained in a gene.

This brings us back to what happenswhen genes are triggered: in scientificshorthand, this is described as “genes regulating proteins” or “genes coding forproteins.” (To be technically accurate,some genes “code” for RNA and somegenes do not actually “code for” anything,but rather serve as a catalyst.) In commonjargon, genes also “go into play” or“express themselves” or, more techni-cally, “undergo transcription and transla-tion.” In a nutshell, here’s what theseterms all mean. The body obtains proteinfrom food and digests it, breaking it downinto the twenty different amino acids.Amino acids are stored inside cells.Prompted into action by epigenetic fac-tors that have themselves been prompted

by environmental influences, the genesinside a cell issue the instructions forreassembling amino acids into polypep-tide chains that combine to form new proteins that are then available to performa variety of tasks in the body.

Genes have one more important func-tion. They are the mechanism by whichthe template for making proteins is passeddown from one generation to the next.Sperm cells and egg cells each carry a halfcomplement of chromosomes, and at conception the two half-sets combine toproduce a new organism with a uniquecombination of genes.

As was just mentioned, somewherearound 30,000 genes are contained in thehuman genome. Yet the body produces


The entire DNA for an organism is repeated in nearly every cell ofthe body. Within each cell, how-ever, the genes act distinctively,as prompted by environmentaland epigenetic factors.

far more than 30,000 proteins. This ispossible because the amino acids produced by genes can combine in dif-ferent ways to make different proteins.

Nearly all the cells of the body containthe entire set of genes. But within eachcell no more than about 5 percent of thegenes are ever expressed. The genes inthe cells of one tissue (for example,kidney cells) may — under certain environmental conditions — becomeactivated and express themselves whilethe equivalent genes in the cells ofanother tissue (for example, the brain)remain inactive.

The bottom line is that the 30,000-some human genes are capable of producing a many-times-larger number ofproteins. These proteins work independ-ently and in combinations to create aneven larger number of outcomes in thecells of the body.

Variety within the human genomeIn addition to size and function, thegenome is impressive along anotherdimension, and that is variety. For eachspecies, the genome comes in unlimitedversions — it differs in every individualwithin a species. This difference is smallpercentage-wise, but it has profoundeffects.

The genetic sequence of any human isestimated to be 99.9 percent identical tothat of any other human’s. Expressed as a

percent that is an overwhelming simi-larity. However, one tenth of a percent of three billion base pairs of DNA is 3 million, a very large number.

Some of those tenth-of-a-percent differ-ences occur in the genes. Every humanhas the same basic package of genes, buteach gene may show up in a different ver-sion, called an allele. A gene’s alleles differslightly from one another in terms of theorder that the four bases — A, T, C, andG — appear along the DNA strand.Scientists believe that there are two ormore variants for most human genes; theaverage number of normal alleles for agene is estimated at 14, but some geneshave 50 or more. Humans are diploid,which means they carry two alleles forevery gene (one inherited from the fatherand one from the mother); the two allelesin a given pair may be identical (in whichcase they are homozygous) or different(heterozygous). Each person is unique in

1 3CHAPTER 2: HOW DO GENES WORK WITHIN THEIR ENVIRONMENTS?

Nitrogen-containing moleculescalled nucleotides or bases arepaired and lined up on a twistingladder-like structure (a doublehelix) to form a chromosome.Sections of these base pairsoperate together as genes. Other sections have other functions. Species differ in their number of chromosomes.

Image: ©U.S. Department of Energy Human Genome Program, http://www.ornl.gov/hgmis.

terms of his or her particular combinationof alleles.

An individual’s unique set of alleles is referred to as his or her genotype. (The word “genotype” is also used torefer to a subset of genes, a small part ofthe whole genome, that is relevant to theparticular trait being studied.) Throughthe actions of the proteins it directs, eachgenotype contributes to a unique set ofobservable human traits, or phenotype.

In some cases, alleles behave in anadditive fashion: each allele contributes tothe variation in a phenotype in a separate,measurable way. One way to think ofadditive genetic variance is with themetaphor of a potluck meal. Each personbrings a dish; the degree to which themeal is a culinary success depends on thetastiness of individual appetizers,casseroles, and desserts.

In other cases, alleles are nonadditive.A nonadditive effect occurs when an indi-vidual’s two alleles for a gene are dissim-ilar and one has dominance over the otherso its genotype more heavily influencesthe phenotype. Nonadditive effects alsooccur when there is epistasis: one allele atone location in the genome affects theexpression of another allele at anotherlocation. Thus additive and nonadditiveeffects characterize the relationship notonly between the two alleles for one gene(in scientific speak we could say “the twoalleles at one locus”), but also betweenalleles of different genes (“alleles at different loci”).

Similarity across genomesThere is yet another admirable feature ofthe genome, and this is its consistencyacross species. Just as with individualgenomes within a species, genomesacross species differ less than you mightexpect. About half of all fruit fly geneshave parallel genes in the human, whilethe mouse genome corresponds to thehuman almost completely.

The gene sequences of human andchimpanzee are 99.4 percent alike. Whatdiffers between the two species are a veryfew genes (not just alleles, which are vari-ations of the same gene) and, more impor-tantly, the activity levels of the genes. For example, certain genes that affecthuman brain function are much moreactive in humans compared to the corre-sponding genes in the chimpanzee. This is enough to account for the majordifferences in appearances and behaviorsbetween human and chimp.

There is an interesting explanation forwhy the genomes of different species haveso much in common. Scientists proposethat all species stem from a single, simpleorganism that existed eons ago. Laterspecies grew out of that original specieslike branches from the trunk of a tree. In every new species, some of the DNA ofits predecessors is conserved.

This process of change is known as evolution, and the mechanisms by whichspecies emerge include mutation, natural

selection, and genetic drift. A mutation isa change in the DNA of a gene that alters


Genes are “conserved” acrossspecies, which means humans areextremely similar genetically tochimpanzees. Shared genes differin their activity level, and thisaccounts for much of the differ-ence between the two species.

the genetic message coded by that gene.Mutations can occur in any part of a geneinside any cell at any point in life. Theycan be triggered for example by radiation,malnutrition, aging, and physical traumato the cell.

Cells in most parts of the body frequently make copies of themselves sothat tissues can grow and old cells can bereplaced, and this is when most muta-tions occur. In this copying process, bil-lions of bases are copied, and with everycopy there are a few errors. One or morebases are put in the wrong place, left out,or changed. Sometimes extra copies aremade of a string of bases or of whole chromosomes. Sometimes a gene movesto a new location or is deleted.

Errors occur about every million cellreplications, perhaps more frequently. Inmost cases, these mutation have little orno effect on a gene’s action. But in rarecases, the mutation has a major effect; the amino acids produced from a genecontaining an error are different or do notappear. This genotypic difference maysometimes lead to a phenotypic differencethat affects, for good or bad, theorganism’s ability to thrive or survive.

If the mutation occurs in a gamete ofan individual (the egg or sperm cellinvolved in reproduction), then the newmutation appears in the cells of that indi-vidual’s offspring. Such mutations lead tovariety within a species. If the mutationoffers an advantage for survival, then indi-viduals with the mutation will leave more

descendents in the next generation: this ispart of natural selection. Over many generations the mutation may becomepredominant in the species, and over agreat many generations, a collection ofadvantageous mutations in an isolatedpopulation can lead to the developmentof a new species.

The third evolutionary mechanismmentioned above, genetic drift, occurswhen members of a species are separatedinto distinct populations; for example, ifone group of humans migrates a long dis-tance away from another group. Withineach population, certain alleles are passedonto the next generation by chance,increasing or decreasing their likelihoodof being passed on again to the third gen-eration. Over time, the number of allelesthat remain in the population arereduced. The smaller the population, thegreater the reductions in allelic varietyover time. The separated populationsdiverge—drift apart—in terms of thealleles they carry.


A change to the DNA, called amutation, can be triggered by any of a number of causes. Mostmutations result from simpleerrors introduced during replica-tion of the cell. However, someresult from physical damage tothe cell. In this illustration, aninsect is exposed to radiation,possibly leading to one or moremutations, such as the transposi-tion of a base pair; the insertionof extra base pairs; or the duplica-tion of one extra base in a pair.Such mutations sometimes alterthe actions of a gene.

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Imagining the genomeTo summarize, the genome has anextremely large number of componentparts yet is infinitesimal in size. It appearsnearly identically in billions of cells, butoperates dissimilarly inside different cells.One species’ genome translates into anincredibly broad range of genotypes and afar broader range of phenotypes. At thesame time, genomes across speciesresemble each other remarkably.

The genome is so curious that we usemetaphors to help us get a handle on it. Ithas been described as a codebook, a bookof life, or an encyclopedia. It has beenlikened to an instruction book, with eachgene being one instruction in this book.

But these metaphors are not holding upunder the weight of our expandingknowledge. Today, scientists realize thatthe genome is much more dynamic thanany kind of book. They know the samegenes in the cells of different tissues maylead to different effects in different parts

of the body (this is called pleiotropy) and that, by the same token, the sameeffects can result from different genes orsets of genes (this is called genetic

heterogeneity). And scientists know thatwhich instructions genes issue, andwhether or not they do, depends entirelyon the environmental context at anygiven moment in time.

Furthermore, they have learned thatthe genome contains many other impor-tant elements besides instruction-givinggenes. Mixed among and within thecoding genes, which occupy just a smallfraction of the total DNA, are non-codingsequences and other features that havebeen given names based on what isknown about their structure or function:promoters, enhancers, pseudogenes,tandem repeats, telomeres, centromeres,and the like. Some of these elements playcritical epigenetic roles, managing thegenes that do code for protein expression.Others have minor roles or no


An orchestra is one possiblemetaphor for the genome. Both orchestra and genome produce magnificent effectsthrough the collaboration ofmany individual and criticallyimportant participants or parts.

discernable role — we might more wiselysay that their role still awaits discovery.

So a new metaphor is replacing themetaphor of genome-as-text. This is themetaphor of the genome as a communityor collective, working together for ashared purpose. Just as a cast, crew, direc-tors, and producers work together to puton a play, so the elements of the genomework with each other, and with epige-netic characters, to express the chemicalproducts required by a body in theprocess of life.

Behavior and the genomeAll of this description about the genome issimply background to the question athand, which is how genes, operatingwithin environments, connect tobehavior. Behavior results from thegenetic coding that occurs in cellsthroughout the body, but especially in thenervous system: the brain, spine, and network of nerves through which infor-mation is communicated throughout thebody, electrically and chemically. Putsimply, behavior results from lots and lotsof ongoing activity by many, many genespressed into action by the environmentand through epigenetic factors.

Blood types, some simple metabolicprocesses, and a few physical traits stemfrom the actions of a single gene, irrespective of environment. Some healthdisorders such as cystic fibrosis, early-

onset Alzheimer’s, and Huntington’s

disease have been tracked to one gene.Most physical traits and conditions —such as height, blood pressure, weight,and digestive activity — stem from manygenes that vary in activity depending onenvironmental contexts. The same is truefor all complex behaviors. Each is affectedby multiple genes interacting with mul-tiple environmental influences. For anygiven behavior, relevant genes and envi-ronmental factors number in the dozens,hundreds, or perhaps thousands.

Unfortunately, many people have a different impression. They think that agene controls a behavioral trait, period.This is genetic determinism, that is, thebelief that the development of anorganism is determined solely by geneticfactors. Genetic determinism is a falsebelief. It comes from misunderstandingsof scientific research.

A great many studies have exploredpossible connections between genetic fac-tors and specific behaviors, such as theage at which a young person begins tosmoke and drink, the friends one selects,a person’s tendency toward divorce,grooming habits, and one’s willingness totake risks, to name just a few. Some ofthese studies have found that close rela-tives tend to be more alike for the trait inquestion than people who are not asclosely related. But these kinds of studiesonly identify correlations between peoplewith similar genetic profiles and certainbehaviors. Correlations are related ratesof incidence, reflecting how much and in


Scientists have only begun toexplore the complex relationshipbetween genetics, environments,and human habits, tendencies,and addictions.

what way two elements co-occur.It’s important to realize that correlation

is not causation: ice cream consumptionand crime both go up in the summer (a positive correlation), but one does notnecessarily cause the other. Correlationstudies in behavioral genetics do notreveal anything about the specific under-lying genes and they only offer clues as tothe relevant environmental variables.

Sometimes researchers will identify aparticular allele for a gene that is found insome people who have the trait and thattherefore is speculated to potentially havesomething to do with the trait. Onecannot jump to a conclusion from thiskind of correlation study, because addi-tional genes and various environmentalfactors also are potentially involved.

So while behavioral genetic studies

do not provide any justification for simplistic talk about “a gene for starting tosmoke” or “a gene for divorce,” peoplesometimes talk like that anyway. Thereare many explanations for why peoplemake these kinds of false statements.Sometimes a scientist overstates the significance of his or her study, sometimesa reporter misinterprets research, some-times the headline to an article oversim-plifies the story, and sometimes naivemembers of the public jump to the wrongconclusion. Such errors are not mutuallyexclusive. But the fact is that so far, scien-tific research has not confirmed any one-to-one correspondence between agene and a human behavior. Behaviorresults from the activity of multiple genesamidst the influence of multiple environ-mental factors.

Hoda’s perplexityWe know that there is no “gene for takingshots bravely.” In a way that’s too bad,because if there were it would explainwhat Hoda in her career as a pediatricnurse has observed — that some childrenare simply afraid of shots and others arenot. But a single gene hypothesis couldnot explain what Hoda has furtherobserved — that children’s responses tothe needle vary continuously across awide spectrum and that an individualchild’s responses can be inconsistent.

Hoda might be interested in hearingabout a classic animal behavior study con-


Correlation does not mean causation: ice cream sales andcrime both rise with the outsidetemperature, but one does notcause the other. In the same way,a person may have certain allelesfor a gene and behave a certainway, but that does not mean theallele causes the behavior.

ducted several decades ago. In this exper-iment, scientists took a group of ordinaryfield mice and put them, one at a time,into a brightly lit open box. The mice hadnever been in the box before, and theyreacted with a range of behaviors.

Some appeared to be fearful. They huddled motionless along the sides.Others appeared to be more brave. Theyroamed about, though they did not strayfar from the sides. A few mice wanderedfreely, even venturing into the middle.

Using various tools, the scientists pre-cisely measured each mouse’s move-ments. They selected the mostimmobilized and the most active. Thenthey bred these selected mice, fearfulwith fearful and brave with brave.

When the next generation came along,scientists again tested each mouse in thebox. They selected the most timid offspring of the timid parents and thebravest offspring of the brave parents. The selected offspring were again bred,like with like. The scientists kept up suchinbreeding for thirty generations.

With each generation, the mice in thetimid group became, as a whole, moretimid, while the mice in the brave groupbecame, as a whole, more brave. The twolines of mice came from the same originalstock, and they were kept and raisedunder identical conditions. But theirresponses to the box were becoming progressively more different.

To make sure no uncontrolled factorwas affecting the behavior of the mice,

the scientists moved the experiment up anotch. They had mice born to the timidline foster-mothered by a mouse from thebrave line, and vice versa. They alsomixed young from each line into onelitter to be raised together by one femaleadult.

Nothing changed. Mice from the lineinbred for timidity showed fear whenplaced in the open box. Mice from theline inbred for bravery responded withcourage.1

Scientists point to these experiments asproof that behavior traits can be influ-enced by heredity. These experimentsalso proved something else: that morethan one inherited factor was at work toaffect mice behavior. We know thisbecause the intensity of bravery and fear-fulness in the respective lines of miceoccurred over many generations and con-tinued to evolve. If only one or two geneshad been involved, the extreme form ofthe behavior would have been universalwithin a few generations (because eachparent only passes down one allele for


Many genes, plus many environ-mental factors, shape thebehavior of a mouse placed insidean open box.

any gene from the two that it has itself). Fearfulness, its opposite bravery, and

other behaviors are called quantitative

genetic traits because the phenotypes(the observable behaviors) associatedwith the underlying genotypes vary bymeasurable quantities or degrees. Theoriginal generation of mice includedthose that were not very fearful, moder-ately fearful, and quite fearful, plus othersin between. If scored on a scale andplotted on a graph, the range in behaviorwould look like a bell-shaped curve. Thiskind of trait is also called continuous

since its particular characteristics varycontinuously from one extreme to theother. [The opposite kind of trait is discontinuous — you either have it oryou do not, such as a sixth finger on yourhands. Being able to roll your tongue isan example of a discontinuous behavioraltrait, but it is the exception rather thanthe rule. Almost all behavioral traits arecontinuous.]

Each gene involved in a quantitativetrait is called a quantitative trait locus

(QTL). The term QTL is the technicalway of saying “one location among manyin the genome that affects a continuoustrait.” Each QTL may have a major effector a minor effect, but it does not have anexclusive effect.

In addition, as was emphasized before,there is also the effects of the environ-ment. As an experienced nurse, Hodabelieves that how she behaves has somemoderating effect on her young patients.

Scientific support for her belief has beennicely provided by another animal exper-iment, this time involving rabbits. Whenplaced in an open box, rabbits — like themice — show a range of responses fromwithdrawal to quick acclimation. Butresearchers have learned that rabbit reac-tions can be molded by the amount ofstimulation they receive while veryyoung. Baby rabbits that are handled byhumans and exposed to minor shocks ortemperature changes are more likely tobe more free-ranging in the box latercompared to a control group of bunnies.

The degree of fearlessness in rabbitscorrelates directly with the amount ofstimulation they receive early in life. Butbecause the rabbit reactions are shapedby external stimuli, fearlessness as a traitdoes not pass from one generation to thenext. Rabbits that have been stimulateddo not produce offspring that are morefearless, as a group, than offspring ofunstimulated rabbits.2

Some caveats The mice and rabbit studies justdescribed take us only so far. They showus that genes and environment both haveeffects on behaviors, but they do not tellus how they work together to do so.

Furthermore, these are animal studies.We can put mice and rabbits into boxesand fancifully label their behavior“brave” or “fearful,” but we cannotextrapolate from that to how humans


Holding, prodding, and petting a rabbit can make it bolder, but that sort of shaped behavioris not transmitted from one generation to the next.

might behave in strange situations.We cannot take human adults, test

them on a particular task and then, basedon performance, select some to breedtogether in order to concentrate certaininherited factors in their children.Likewise, we cannot deliberately experi-ment with human babies by handlingthem differently to see how theirbehavior might be affected as theymature.

But we can give these animal studiescredit for helping us understand theessential point that both inherited andenvironmental factors contribute tobehavior. Hoda, in her years of working

with children, intuitively knows this. She is aware that children tend towardcertain temperaments, but that her ownactions, as well as other factors beyondanyone’s control, affect the behaviorpatients manifest when in her clinic.Hoda probably understands childrenbetter than she realizes.

Notes

1 See Clark, W. and M. Grunstein (2000, pgs. 86–88) and in

Plomin et al. (1997, pgs. 62–66) for discussions of the mice

studies. The research discussed is DeFries et al. (1978).

2 See Clark, W. and M. Grunstein (2000, pgs. 90–92) for a

discussion of the rabbit studies.


Scientists believe that eachperson’s package of genes mightpartially explain tendencies, suchas why one child has an outgoingpersonality and another is moreshy. Yet many non-genetic factorsalso affect how a human acts in aparticular circumstance.

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chapter two - AAAS Home · which means they carry two alleles for every gene (one inherited from the father and one from the mother); the two alleles in a given pair may be identical