human genetic disease - Encyclopedia Britannica

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. 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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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