Chapter 3 | Hereditary Inuences on Development115Ournalthemeconcernstheholisticnatureof childdevelopment.Perhapsthis theme is the most basic idea from our investigation ofhereditary inuences on develop-ment. We saw in this chapter that heredity and environment inuence all aspects ofchild development: physical, social, cognitive, and behavioral. Clearly heredity is an important buildingblockforunderstandingthechildasanintegratedlabyrinthof inuencesand outcomes in all aspects ofpsychological functioning.Sex-linked characteristics are those caused by re-cessive genes on the X chromosome when there is no correspondinggeneontheYchromosometomask its effects; they are more common in males.Mostcomplexhumanattributes,suchasintelligence andpersonalitytraits,arepolygenic,orinuencedby many genes rather than a single pair.Hereditary DisordersOccasionally, children inherit congenital defects (for ex-ample, Huntingtons disease) that are caused by abnormal genes and chromosomes.Chromosomal abnormalities occur when the individ-ual inherits too many or too few chromosomes.AmajorautosomaldisorderisDownsyndrome,in which the child inherits an extra 21st chromosome.Manygeneticdisorderscanbepassedtochildrenby parents who are not affected but are carriers ofa reces-sive allele for the disorder.Geneticabnormalitiesmayalsoresultfrommuta-tionschangesinthestructureof oneormoregenes thatcanoccurspontaneouslyorresultfromenviron-mental hazards such as radiation or toxic chemicals.Genetic Counseling, Prenatal Detection, and Treatment of Hereditary DisordersGeneticcounselinginformsprospectiveparentsabout the odds ofgiving birth to a child with a hereditary disorder. Familyhistoriesandmedicaltestsareusedtodetermine whether the parents are at risk.Amniocentesis,chorionicvillussampling,andultra-sound are used for prenatal detection ofmany genetic and chromosomal abnormalities.Medical interventions such as special diets, fetal surgery, drugsandhormones,andgenereplacementtherapycan reduce the harmful effects ofmany heredity disorders (for example, phenylketonuria, or PKU).Hereditary Inuences on BehaviorBehavioralgeneticsisthestudyof howgenesand environmentcontributetoindividualvariationsin development.SUMMARYPrinciples of Hereditary TransmissionDevelopmentbeginsatconception,whenaspermcell fromthefatherpenetratesanovumfromthemother, forming a zygote.A normal human zygote contains 46 chromosomes (23 from each parent), each ofwhich consists ofseveral thou-sand strands ofdeoxyribonucleic acid (or DNA) known as genes.Genesarethebiologicalbasisforthedevelopment ofthe zygote into a person.Developmentof thezygoteoccursthroughmitosisnewbodycellsarecreatedasthe23pairedchromosomes ineachcellduplicatethemselvesandseparateintotwo identical new cells.Specialized germ cells divide by meiosis to produce gam-etes (sperm or ova) that each contain 23 unpaired chromo-somes. Crossing-over and the independent assortment ofchromosomesensurethateachgametereceivesaunique set ofgenes from each parent.Monozygotic(oridentical) twinsresultwhenasingle zygotedividestocreatetwocellsthatdevelopindepen-dently into two individuals.Dizygotic (or fraternal) twins result when two different ovaareeachfertilizedbyadifferentspermcellandthen develop independently into two individuals.Gametes contain 22 autosomes and 1 sex chromosome. FemalessexchromosomesarebothXchromosomes; males sex chromosomes are an X and a Y chromosome.OvacontainanXchromosome.Spermcontaineither an X or a Y chromosome. Therefore, fathers determine the sex oftheir children (depending on whether the sperm that fertilizes the ovum contains an X or a Y chromosome).Genes produce enzymes and other proteins that are nec-essaryforthecreationandfunctioningof newcells,and regulate the timing ofdevelopment. Internal and external environments inuence how genes function.There are many ways in which ones genotype may affect phenotypethe way one looks, feels, thinks, or behaves.Somecharacteristicsaredeterminedbyasinglepair ofalleles, one ofwhich is inherited from each parent.Insimpledominant/recessivetraits,theindivid-ual displays the phenotype ofthe dominant allele.If agenepairiscodominant,theindividualdis-plays a phenotype in between those produced by the dominant and the recessive alleles.116Part Two | Biological Foundations of DevelopmentThe range-of-reaction principle states that heredity sets arangeof developmentalpotentialsandtheenvironment inuences where in that range the individual will fall.A more recent theory proposes three avenues by which genesinuencetheenvironmentswearelikelytoexperi-ence:throughpassivegenotype/environmentcorrela-tions,evocativegenotype/environmentcorrelations,and active genotype/environment correlations.Therelativeinuenceof thedifferentgenotype/envi-ronmentcorrelationschangesacrossdevelopment,with passive effects predominating in early life, evocative effects operatingthroughoutlife,andactiveeffectsnotplayinga role until later childhood and adolescence.Contributions and Criticisms of the Behavioral Genetics ApproachBehavioralgeneticshashadastronginuenceonour outlookonhumandevelopmentbyshowingthatmany attributes previously thought to be environmentally deter-mined are inuenced, in part, by genes.It has also helped to defuse the nature versus nurture de-bate by illustrating that these two sources ofinuence are complexly intertwined.Behavioral genetics has been criticized as an incomplete theoryof developmentthatdescribes,butfailstoexplain, howeithergenesorenvironmentinuenceourabilities, conduct, and character.Althoughanimalscanbestudiedinselectivebreedingexperiments,humanbehavioralgeneticistsmustconduct familystudies(oftentwindesignsoradoptiondesigns),estimating the heritability ofvarious attributes from simi-laritiesanddifferencesamongfamilymemberswhodiffer in kinship.Hereditarycontributionstovariousattributesare estimatedusingconcordanceratesandheritability coefcients.Behavioralgeneticistscanalsodeterminetheamount of variabilityinatraitthatisattributabletononshared environmentalinuencesandsharedenvironmental inuences.Familystudiesrevealthatheritabilityinuencesintel-lectualperformance,introversion/extroversionand empathicconcern,andpredispositionstodisplaysuch disordersasschizophrenia,bipolardisorder,neurotic disorders, alcoholism, and criminality.Familystudieshaveledsocial-developmentalpsycholo-giststoabandonboththeparenteffectsmodelandthe child effects model and instead to adopt the transactional model,amodelof familyinuencesinwhichparentand child are believed to inuence each other reciprocally.Theories of Hereditary and Environment Interactions in DevelopmentThecanalizationprincipleimpliesthatgeneslimitde-velopment to certain outcomes that are difcult for the en-vironment to alter.CHAPTER 3 PRACTICE QUIZMultipleChoice:Checkyourunderstandingof heredity by selecting the best choice for each question. Answers ap-pear in the Appendix.1. Theprocessbywhichagermcelldivides,producing two gametes (sperm or ovum), is calleda.the double helixb.crossing-overc.meiosisd.mitosis2.Each gamete (sperm or ovum) contains ____________ chromosomes.a.23b.46c.23 pairs ofd.46 pairs of3.Whichof thefollowingisanexampleof asex-linked characteristic?a.dizygotic twinningb.red/green color blindnessc.sickle-cell anemiad.polygenic inheritance4.Congenitaldefectsthatareinheritedincludeallof the following EXCEPTa.prenatal exposure to damaging effectsb.recessive genes for a disorderc.dominant genes for a disorderd.too many or too few chromosomes5.Thediscoverythatmaleswith__________________ werecompetingasfemalesintheOlympicsledtothe current practice ofadministering genetic sex tests to all females in the Olympics.a.Turners syndromeb.Klinefelters syndromec.Down syndromed.Huntingtons disease6. Which method ofdetecting heredity disorders is safest (leastriskof miscarriage)fordetectingmultiplepreg-nancies and gross physical abnormalities?a.chorionic villus samplingb.amniocentesisc.genetic counselingd.ultrasound