imagesourceQueenslandUniversityofTechnologyimagecaptionTheeggisthoughttohavebeenfertilisedsimultaneouslybytwospermbeforeitdividedDoctorshavedocumentedwhattheysayisonlytheworld'ssecondknowncaseof"semi-identical"twins.Theboyandthegirl,nowfour,fromBrisbane,inAustralia,areidenticalontheirmother'sside.Buttheyshareonlyaproportionoftheirfather'sDNA-placingthem,genetically,somewherebetweenfraternalandidenticaltwins.Expertssaythephenomenonisextremelyrare-embryossuchastheseoftendonotsurvive.Thetwinswhohavedifferentdads-BBCNewsSemi-identicaltwinsdiscoveredProfNicholasFisk,wholedtheteamthatcaredforthemotherandtwinsattheRoyalBrisbaneandWomen'sHospitalin2014,saidthediscoverywasmadethrougharoutinepregnancyscan.Itisthefirsttimesemi-identicaltwinshavebeenidentifiedduringpregnancy.Thefirst-timemotherwas28atthetimeandhadconceivednaturally,doctorssaid.ThecaseisbeingreportedoninTheNewEnglandJournalofMedicine."Themother'sultrasoundatsixweeksshowedasingleplacentaandpositioningofamnioticsacsthatindicatedshewasexpectingidenticaltwins,"ProfFisksaid."However,anultrasoundat14weeksshowedthetwinsweremaleandfemale,whichisnotpossibleforidenticaltwins."Howdidithappen?Identical,ormonozygotic,twinsoccurwhenasingleegg,fertilisedbyasinglesperm,dividesandmakestwobabies.Thesetwinswillbethesamesexandsharethesamegenesandphysicalfeatures.Non-identical,ordizygotic,twinsoccurwhentwoseparateeggsarefertilised,eachbyadifferentsperm,anddevelopinthewombatthesametime.Thesetwinscanbethesameordifferentsexesandarenomorealikethananybrotherorsister,despitebeingborntogether.Inthiscaseofsemi-identical,orsesquizygotic,twins,theeggisthoughttohavebeenfertilisedsimultaneouslybytwospermbeforeitdivided.Ifoneeggisfertilisedbytwosperm,itresultsinthreesetsofchromosomes,ratherthanthestandardtwo-onefromthemotherandtwofromthefather.And,accordingtoresearchers,threesetsofchromosomesare"typicallyincompatiblewithlifeandembryosdonotusuallysurvive".Theidentityofthetwinshasnotbeenrevealed.Whatcausestwins?imagesourceGettyImagesNon-identicalt