human genetic disease - Encyclopedia Britannica

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human genetic disease, any of the diseases and disorders that are caused by mutations in one or more genes. With the increasing ability to control ... humangeneticdisease TableofContents IntroductionClassesofgeneticdiseaseDiseasescausedbychromosomalaberrationsNumericalabnormalitiesStructuralabnormalitiesAbnormalitiesofthesexchromosomesDiseasesassociatedwithsingle-geneMendelianinheritanceAutosomaldominantinheritanceAutosomalrecessiveinheritanceSex-linkedinheritanceDiseasesassociatedwithsingle-genenon-MendelianinheritanceTripletrepeatexpansionsMitochondrialDNAmutationsImprintedgenemutationsDiseasescausedbymultifactorialinheritanceGeneticsofcancerCognitiveandbehavioralgeneticsGeneticdamagefromenvironmentalagentsVirusesPlants,fungi,andbacteriaIndustrialchemicalsCombustionproductsAlcoholUltravioletradiationIonizingradiationMolecularoxygenManagementofgeneticdiseaseGeneticcounselingCalculatingrisksofknowncarriersEstimatingprobability:Bayes’stheoremDiagnosisPrenataldiagnosisGenetictestingOptionsfortreatmentEthicalissues FastFacts RelatedContent Media Images More MoreArticlesOnThisTopic AdditionalReading Contributors ArticleHistory Home Health&Medicine Conditions&Diseases GeneticDiseases humangeneticdisease Print print Print Pleaseselectwhichsectionsyouwouldliketoprint: TableOfContents Cite verifiedCite Whileeveryefforthasbeenmadetofollowcitationstylerules,theremaybesomediscrepancies. Pleaserefertotheappropriatestylemanualorothersourcesifyouhaveanyquestions. SelectCitationStyle MLA APA ChicagoManualofStyle CopyCitation Share Share Sharetosocialmedia Facebook Twitter URL https://www.britannica.com/science/human-genetic-disease More GiveFeedback ExternalWebsites Feedback Corrections?Updates?Omissions?Letusknowifyouhavesuggestionstoimprovethisarticle(requireslogin). FeedbackType Selectatype(Required) FactualCorrection Spelling/GrammarCorrection LinkCorrection AdditionalInformation Other YourFeedback SubmitFeedback Thankyouforyourfeedback Oureditorswillreviewwhatyou’vesubmittedanddeterminewhethertorevisethearticle. JoinBritannica'sPublishingPartnerProgramandourcommunityofexpertstogainaglobalaudienceforyourwork! ExternalWebsites WorldHealthOrganisation-GenesandHumanDisease BritannicaWebsites ArticlesfromBritannicaEncyclopediasforelementaryandhighschoolstudents. geneticdisorder-StudentEncyclopedia(Ages11andup) By ArthurRobinson |SeeAll • EditHistory humangeneticdisease Seeallmedia RelatedTopics: cysticfibrosis thalassemia sicklecellanemia inbornerrorofmetabolism phenylketonuria ...(Showmore) Seeallrelatedcontent→ humangeneticdisease,anyofthediseasesanddisordersthatarecausedbymutationsinoneormoregenes.Withtheincreasingabilitytocontrolinfectiousandnutritionaldiseasesindevelopedcountries,therehascometherealizationthatgeneticdiseasesareamajorcauseofdisability,death,andhumantragedy.Rare,indeed,isthefamilythatisentirelyfreeofanyknowngeneticdisorder.Manythousandsofdifferentgeneticdisorderswithdefinedclinicalsymptomshavebeenidentified.Ofthe3to6percentofnewbornswitharecognizedbirthdefect,atleasthalfinvolveapredominantlygeneticcontribution.Furthermore,geneticdefectsarethemajorknowncauseofpregnancylossindevelopednations,andalmosthalfofallspontaneousabortions(miscarriages)involveachromosomallyabnormalfetus.About30percentofallpostnatalinfantmortalityindevelopedcountriesisduetogeneticdisease;30percentofpediatricand10percentofadulthospitaladmissionscanbetracedtoapredominantlygeneticcause.Finally,medicalinvestigatorsestimatethatgeneticdefects—howeverminor—arepresentinatleast10percentofalladults.Thus,thesearenotrareevents. BritannicaQuiz 44QuestionsfromBritannica’sMostPopularHealthandMedicineQuizzes Howmuchdoyouknowabouthumananatomy?Howaboutmedicalconditions?Thebrain?You’llneedtoknowalottoanswer44ofthehardestquestionsfromBritannica’smostpopularquizzesabouthealthandmedicine. Acongenitaldefectisanybiochemical,functional,orstructuralabnormalitythatoriginatespriortoorshortlyafterbirth.Itmustbeemphasizedthatbirthdefectsdonotallhavethesamebasis,anditisevenpossibleforapparentlyidenticaldefectsindifferentindividualstoreflectdifferentunderlyingcauses.Thoughthegeneticandbiochemicalbasesformostrecognizeddefectsarestilluncertain,itisevidentthatmanyofthesedisordersresultfromacombinationofgeneticandenvironmentalfactors.Thisarticlesurveysthemaincategoriesofgeneticdisease,focusingonthetypesofgeneticmutationsthatgiverisetothem,therisksassociatedwithexposuretocertainenvironmentalagents,andthecourseofmanaginggeneticdiseasethroughcounseling,diagnosis,andtreatment.ForfullexplanationofMendelianandnon-Mendeliangenetics,geneticmutationandregulation,andotherprinciplesunderlyinggeneticdisease,seethearticleheredity.Thegeneticsoftumourdevelopment,brieflyexplainedinthisarticle,arecoveredatlengthinthearticlecancer.ClassesofgeneticdiseaseMosthumangeneticdefectscanbecategorizedasresultingfromeitherchromosomal,single-geneMendelian,single-genenon-Mendelian,ormultifactorialcauses.Eachofthesecategoriesisdiscussedbrieflybelow.DiseasescausedbychromosomalaberrationsAbout1outof150livenewbornshasadetectablechromosomalabnormality.Yeteventhishighincidencerepresentsonlyasmallfractionofchromosomemutationssincethevastmajorityarelethalandresultinprenataldeathorstillbirth.Indeed,50percentofallfirst-trimestermiscarriagesand20percentofallsecond-trimestermiscarriagesareestimatedtoinvolveachromosomallyabnormalfetus.Chromosomedisorderscanbegroupedintothreeprincipalcategories:(1)thosethatinvolvenumericalabnormalitiesoftheautosomes,(2)thosethatinvolvestructuralabnormalitiesoftheautosomes,and(3)thosethatinvolvethesexchromosomes.Autosomesarethe22setsofchromosomesfoundinallnormalhumancells.Theyarereferredtonumerically(e.g.,chromosome1,chromosome2)accordingtoatraditionalsortorderbasedonsize,shape,andotherproperties.Sexchromosomesmakeupthe23rdpairofchromosomesinallnormalhumancellsandcomeintwoforms,termedXandY.Inhumansandmanyotheranimals,itistheconstitutionofsexchromosomesthatdeterminesthesexoftheindividual,suchthatXXresultsinafemaleandXYresultsinamale.NumericalabnormalitiesNumericalabnormalities,involvingeithertheautosomesorsexchromosomes,arebelievedgenerallytoresultfrommeioticnondisjunction—thatis,theunequaldivisionofchromosomesbetweendaughtercells—thatcanoccurduringeithermaternalorpaternalgameteformation.Meioticnondisjunctionleadstoeggsorspermwithadditionalormissingchromosomes.Althoughthebiochemicalbasisofnumericalchromosomeabnormalitiesremainsunknown,maternalageclearlyhasaneffect,suchthatolderwomenareatsignificantlyincreasedrisktoconceiveandgivebirthtoachromosomallyabnormalchild.Theriskincreaseswithageinanalmostexponentialmanner,especiallyafterage35,sothatapregnantwomanage45orolderhasbetweena1in20and1in50chancethatherchildwillhavetrisomy21(Downsyndrome),whiletheriskisonly1in400fora35-year-oldwomanandlessthan1in1,000forawomanundertheageof30.Thereisnocleareffectofpaternalageonnumericalchromosomeabnormalities.AlthoughDownsyndromeisprobablythebest-knownandmostcommonlyobservedoftheautosomaltrisomies,beingfoundinabout1outof800livebirths,bothtrisomy13andtrisomy18arealsoseeninthepopulation,albeitatgreatlyreducedrates(1outof10,000livebirthsand1outof6,000livebirths,respectively).Thevastmajorityofconceptionsinvolvingtrisomyforanyofthesethreeautosomesarenonethelesslosttomiscarriage,asareallconceptionsinvolvingtrisomyforanyoftheotherautosomes.Similarly,monosomyforanyoftheautosomesislethalinuteroandthereforeisnotseeninthepopulation.Becausenumericalchromosomalabnormalitiesgenerallyresultfromindependentmeioticevents,parentswhohaveonepregnancywithanumericalchromosomalabnormalityaregenerallynotatmarkedlyincreasedriskabovethegeneralpopulationtorepeattheexperience.Nonetheless,asmallincreasedriskisgenerallycitedforthesecouplestoaccountforunusualsituations,suchaschromosomaltranslocationsorgonadalmosaicism,describedbelow.StructuralabnormalitiesStructuralabnormalitiesoftheautosomesareevenmorecommoninthepopulationthanarenumericalabnormalitiesandincludetranslocationsoflargepiecesofchromosomes,aswellassmallerdeletions,insertions,orrearrangements.Indeed,about5percentofallcasesofDownsyndromeresultnotfromclassictrisomy21butfromthepresenceofexcesschromosome21materialattachedtotheendofanotherchromosomeastheresultofatranslocationevent.Ifbalanced,structuralchromosomalabnormalitiesmaybecompatiblewithanormalphenotype,althoughunbalancedchromosomestructuralabnormalitiescanbeeverybitasdevastatingasnumericalabnormalities.Furthermore,becausemanystructuraldefectsareinheritedfromaparentwhoisabalancedcarrier,coupleswhohaveonepregnancywithastructuralchromosomalabnormalitygenerallyareatsignificantlyincreasedriskabovethegeneralpopulationtorepeattheexperience.Clearly,thelikelihoodofarecurrencewoulddependonwhetherabalancedformofthestructuraldefectoccursinoneoftheparents.Evenasmalldeletionoradditionofautosomalmaterial—toosmalltobeseenbynormalkaryotypingmethods—canproduceseriousmalformationsandintellectualdisability.Oneexampleiscriduchat(French:“cryofthecat”)syndrome,whichisassociatedwiththelossofasmallsegmentoftheshortarmofchromosome5.Newbornswiththisdisorderhavea“mewing”crylikethatofacat.Intellectualdisabilityisusuallysevere. 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