Biology - An Illustrated Guide to Science (794127), страница 15
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So the gene for the major ABOblood groups in humans exists inthree forms or alleles: A, B, and O.● A and B are both dom in an t to O. Ifthe A gene is present, the red bloodcells will have a protein on their outersurface called protein A.
In geneticdiagrams the dominant allele is usuallyshown with a capital letter and thecorresponding recessive factor isshown with a lowercase letter.● If the B gene is present, the proteinson the red blood cells will be type B.● If the O gene is present, the bloodcells will not contain either of theseproteins.I AI AParental genotypeiOiO●Inheritance of bloodgroupsThe blood groups of the ABO systemare determined by three major alleles:A, B, and O. Both A and B aredominant (IA, IB), while O is recessive(iO).● The ABO alleles follow the standardrules for a monohybrid cross—a crossin which only one pair of traits isconsidered. If a parent with AA (bloodgroup A) is crossed a parent with BB(blood group B), the offspring will beAB. This means that they will haveboth protein type A and type B ontheir red blood cells.
They will belongto blood group AB.● If a parent is AO, he or she will beblood group A because A is dominantto O. If this parent is crossed withsomeone with gen otype BO, theoffspring will distribute themselves inthe same ratio as: one AB: one AO:one OB: one OO. This gives theindividual a 25 percent chance ofbeing blood group AB, A, B, or O.Gametes producedby meiosisF1 genotypeIAIAiOiOI Ai OI Ai OI AiOI Ai OF1 phenotypeAAAAI AiO x iOiOParental phenotypeI AiOParental genotypeGametes producedby meiosisF1 genotypeiOiOIAiOiOiOI Ai OI Ai OI OiOI OiOF1 phenotypeAAOO© Diagram Visual Information Ltd.●I BI B x iAiAParental phenotypeI BI BParental genotypeGametes producedby meiosisF1 genotypeI AI AIBIBIAIAI AI BI AI BI AI BI AI BF1 phenotypeABABABABI BiO x I AiOParental phenotypeI BiOParental genotypeGametes producedby meiosisF1 genotypeF1 phenotypeI Ai OIBiOIAiOI AI BI BiOI AiOiOiOABBAO85Chromosome mutation:typesCONTINUITYKey wordschromosomegeneTypes of chromosome mutationInversionnormal chromosomeposition ofbreaks inchromosomenormal chromosomeposition ofbreaks inchromosomeposition ofjoin inchromosomemiddle pieceof chromosomerotates thenrejoinsAAAAAABBBBBBCCCCCCGDHEIFDEDEFFGGHHIImiddle piece ofchromosome lostDEFDGGGHHHIII●AAAABBBBBCCCCCDDEEDEDFEDFFFEGGHHIFWWWIFXXXGYYYHXXXIIn inversion, a portion of thechromosome is reversed.
This affectsthe order of the bases in the geneticcode, usually making it impossible toread successfully.Translocationpositionof join inchromosomepositionof join inchromosomeIn deletion, a portion of achromosome is lost. This affects thecoding of proteins that use the DNAsequence as well as other amino acidsthat are supposed to be coded fromthe sequence.Inversion●Anormal chromosome●normal chromosomenormal chromosomeChrom osom e mutations involvecomplete packages of gen es ratherthan individual genes.
For this reasontheir effects can be very far-reaching.DeletionEDuplicationposition of breakin chromosome●Fposition of joinsin chromosomeTranslocationChromosome mutationsTranslocation involves a piece of DNAwithin a chromosome being moved toa different position or even a differentchromosome. This effectively shufflesthe genes available to an organism—possibly producing improved varieties.Duplication●extra pieceof homologouschromosomeadded onIn duplication, a portion of DNA in achromosome is copied and re-insertedinto the chromosome.
Since there isno increase in the number of genes,the effects of duplication tend to besmall.© Diagram Visual Information Ltd.Deletion86CONTINUITYKey wordschromosomegenotypemutationChromosome mutationsChrom osom e mutations involvecomplete packages of genesrather than individual genes. Forthis reason their effects can bevery far-reaching.● Some m u tation s involve addingor removing a completechromosome. These are usuallylethal, although the sexchromosomes (XY) seem to beable to suffer from these sorts ofmutations and still produceviable, if damaged, offspring.Chromosome mutation:syndromesChromosome mutation syndromesParentalphenotypes●Homologouschromosomes failto separateduring meiosisKlinefelter’s syndrome●Individuals are male but possessan extra X chromosome to beXXY.
Their sexual development isdefective, and they are oftensterile—unable to producesperm. They may have enlargedbreasts and abnornmal bodyproportions.Triple X syndrome●Individuals with triple Xsyndrome possess an extra Xchromosome to give them agen otype of XXX. The extrachromosome in this instanceseems to have very little effect.Turner’s syndrome© Diagram Visual Information Ltd.●The ovum that produced anindividual with Turner’ssyndrome was formed withoutan X chromosome.
If this ovummeets a sperm containing an Xchromosome, a viable female isformed, although she is almostalways infertile. If the ovummeets a sperm with a Ychromosome, a viable embryocannot be formed.spermwith X and Ychromosomesspermwith no sexchromosomesovumwith two Xchromosomesovumwith no sexchromosomesNormalgametesovumsperm withY chromosomesperm withX chromosomesperm withX chromosomeFertilizationPossiblegenotypesof offspringKlinefelter’ssyndromeTriple XsyndromeTurner’ssyndrome87Gene mutation: typesCONTINUITYKey wordsTypes of genetic mutationNormal DNA andpolypeptidecodongenemutationpolypeptide chainSubstitutionnormalDNADNA withbasesubstitutedGene mutationsGene m u tation s involve changes inthe base sequence (the order ofnucleotide bases) of the DNA of agen e. Almost all of these changes willbe harmful and lead to a malfunctionof the gene.
Sometimes they canproduce a new gene, which producesa characteristic that is better suited tothe environment.● Gene mutations fall into threecategories: substitution, insertion, anddeletion.●C G A A C C C G AALATRPALAC C A A C C C G AGLYTRPALASubstitution●codonGnormalpolypeptidechaincodonInsertionmutantpolypeptidechainDeletionDNAwith baseinsertedDNA withbasedeletedInsertion●C C G A A C C C G AC A A C C C G AIn a substitution mutation a singlebase is changed for another one.
Sincethe single base only figures in onecodon , the damage is limited tochanging one amino acid in apolypetide chain . This could besignificant if the amino acid were in acritical place on the chain or had avery specific function in the molecule.Insertion involves adding a base to asequence of DNA. The extra base willdisrupt all of the subsequent codonsbecause they will now be out ofsequence.Deletion● Deletionremoves a single base. Thisalso disrupts all subsequent codonsbecause the “gaps” between thecodons are now in the wrong place.mutantpolypeptidechainLEUcodonGLYVALGLYcodonmutantpolypeptidechain© Diagram Visual Information Ltd.GLY88CONTINUITYKey wordscodonpolypeptide chainGene mutation:sickle-cell shapeSickle-cell mutationMutant DNANormal DNASickle-cell anemiaSickle-cell anemia is a disease of thered blood cells caused by an error inone triplet of one of the polypeptidechain s in beta (!) hemoglobin.● Sufferers from sickle-cell anemia havered blood cells that are irregularshapes, often crescent moon or sickleshapes, which cannot carry oxygen aswell as normal blood cells.
This leadsto a general lack of energy in thesufferer. Their abnormal shape alsomeans that they tend to get stuck insmall blood vessels, leading to painfulclots.●triplet with basesubstitutionnormaltripletDNA triplet coding for one amino acid in ß hemoglobintriplet withbase substitutionC A CnormaltripletC T CGlutamine to valineThe sickle-cell mutation involves asingle triplet substitution from CTC(Cytosine/Thymine/Cytosine) to CAC(Cytosine/Adenine/Cytosine). Thischange leads to the amino acid valine(VAL) being added to the polypeptidechain in place of glutamine (GLU).This change leads, in turn, to otherproblems with the three-dimensionalshape of the hemoglobinformed, and therefore with itsValfunctionality.● Since the gene mutation is a basesubstitution, only one of the codon s,and so one of the amino acids, isaffected.
In certain areas of the world,sickle-cell anemia offers a degree ofprotection against the malaria parasite.●normalchain withglutamicacidabnormal chainwith valinePart ofß hemoglobinHisLeuThrProValGluLysValHisLeuThrProGlu© Diagram Visual Information Ltd.Redbloodcellssickle cellsnormal cellsGluLys89Gene mutation:sickle-cell anemiaCONTINUITYKey wordshemoglobinheterozygoushomozygouspolypeptide chainrecessiveGenetics of sickle-cell anemiafemale withsickle-cell traitmale withsickle-cell traitSickle-cell anemia andtraitSickle-cell condition is caused by adefective gene for the protein betahem oglobin . The defect is a singlebase, which leads to valine beingsubstituted for glutamine in thepolypeptide chain .● If an individual has two sickle genes(Hb SHb S), they will suffer form the fullcondition and exhibit symptoms.● A heterozygou s individual with onlyone sickle gene (Hb AHb S) will showsickle-cell trait. In sickle-cell trait,symptoms are not visible, and theblood can appear normal except inconditions of low oxygen (e.g.,airplanes, some surgical procedures),when complications can occur.●Parental phenotypeHbA HbSParental genotypeGametesproducedby meiosisHbAspermHbA HbSHbSspermHbAovumHbSovumInheritance of sickle-cellgeneThe sickle-cell gene is a recessive alleleinherited following all the normalrules for monohybrid crosses.
Thegene is not sex-linked and is equallycommon in men and women.● A hom ozygou s normal parent (Hb A,Hb A) crossed with a homozygoussickled parent (Hb SHb S) will produceoffspring that are heterozygous andwill have sickle-cell trait (Hb AHb S).● In order to produce children with thefull sickle-cell condition, both of theparents must possess at least onesickle-cell allele ( e.g., two parentswith sickle-cell trait).● Sickle-cell genes are more common inAfricans and their descendants thanother racial groups.F1 genotypeHbA HbAHbA HbSHbA HbSHbSHbSchild withsickle-cell traitchild withsickle-cell traitchild withsickle-cell anemiaF1 phenotypechild withnormal hemoglobin© Diagram Visual Information Ltd.●90CONTINUITYKey wordsfossil recordspeciesEvidence for evolution:primitive and advancedEvolution of the horseForefoot of Hyracotheriumthird metacarpalsecond digitPrimitive and advanced●Primitive characteristics arecharacteristics that existed prior to amore advanced form that developedfrom it.
Primitive does not alwaysmean simpler, as some forms ofdevelopment occur that involve loss ofstructures and complexity. Primitiveand advanced can only be used whenreferring to particular pairs ofcharacteristics, and a characteristicmay be primitive in one relationshipbut advanced in another.Hyracotherium(Eocene)1.3 feet(0 .4 m)fourth digitthird digitForefoot of Mesohippusfifth digitthird metacarpalMesohippus(Oligocene)The development of the modernhorse is well-documented through thefossil record. A gradual increase inbody size is noticeable over millions ofyears as primitive forms gave rise tolarger, more advanced species.second digit2.0 feet(0 .6 m)fourth digitthird digitIncrease in size●fifth digitForefoot of Merychippusthird metacarpal3.3 feet(1.0 m)Merychippus(Miocene)second digitfourth digitDecrease in complexityAs the body size increased from thesmallest Hyracotheriu m of the Eoceneera (roughly 50 million years beforepresent) to the largest Equ u s (themodern horse), there was a parallelfall in complexity of forefoot bones.● The entire weight of the horse is nowborne on the third digit, with theother digits much reduced in size andimportance.third digit●Forefoot of PliohippusPliohippus(Pliocene)fourth digitsecond digit3.3 feet(1.0 m)third metacarpalthird digitForefoot of Equusfourth digit© Diagram Visual Information Ltd.second digitthird metacarpal5.3 feet(1.6 m)Equus(Pleistoceneto recent)third digit91Evidence for evolution:adaptive radiationCONTINUITYKey wordsspeciesAdaptive radiation of Darwin’s finchesFood sourcesGalapagos finchesAdaptive radiationAdaptive radiation occurs when asingle primitive form of a speciesdevelops into a wider range ofadvanced forms, each of which isadapted to particular environmentalconditions.● One of the best-documented examplesof adaptive radiation is provided byDarwin’s finches from the GalapagosIslands.●large seedslarge ground finchcactus seeds and nectarcactus ground finchGalapagos Islands●Typicalmainlandtype(ancestral)flying insectsThe Galapagos Islands are a remotegroup of islands in the Pacific Ocean600 miles due west of Ecuador.
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