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GENETICSGENETICSDr. K.S.KrishnakumariDr. K.S.Krishnakumari
Medical geneticsMedical genetics Branch of medicine which deals withBranch of medicine which deals with InheritanceInheritance
Diagnosis and management of diseasesDiagnosis and management of diseases
caused by gene mutations,chromosomecaused by gene mutations,chromosome
abnormalities and multifactorial reasonsabnormalities and multifactorial reasons
Screening and counselling for genetic Screening and counselling for genetic disordersdisorders
ChromosomeChromosome
Physical basis of inheritancePhysical basis of inheritance Located in the nucleusLocated in the nucleus Chrome = colour ; soma = bodyChrome = colour ; soma = body E.StrasburgerE.Strasburger coined the name coined the name
in1875in1875 Chromosomes are made up of Chromosomes are made up of
genesgenes Genes are made up of DNAGenes are made up of DNA
DivisionsDivisions
Cyto geneticsCyto genetics
Molecular geneticsMolecular genetics
Other divisionsOther divisions
Biochemical geneticsBiochemical genetics Cancer geneticsCancer genetics ImmunogeneticsImmunogenetics Developmental geneticsDevelopmental genetics Behavioural geneticsBehavioural genetics Population geneticsPopulation genetics
CytogeneticsCytogenetics
Deals with chromosome and cell Deals with chromosome and cell divisiondivision
Metaphase stage of cell divisionMetaphase stage of cell division
23 pairs of chromosomes 23 pairs of chromosomes (22 pairs are autosomes and 2 sex (22 pairs are autosomes and 2 sex
chromsomeschromsomes
Human chromosomesHuman chromosomes
Present in the nucleusPresent in the nucleus
ChromatinChromatin
Chromosome number– diploid set Chromosome number– diploid set – 2n– 2n
Autosomes and Sex Autosomes and Sex chromosomeschromosomes
Has a centromereHas a centromere
Short and long armsShort and long arms
Short arm – p arm –petit armShort arm – p arm –petit arm
Long arm – q arm – grand armLong arm – q arm – grand arm
StructureStructure
ChromatidsChromatids
Primary constriction - centromerePrimary constriction - centromere
Secondary constriction Secondary constriction
SatelliteSatellite
Structure of Structure of chromosomechromosome
GenesGenes
Fundamental units of inheritanceFundamental units of inheritance
which determine the character of which determine the character of anan
individualindividual
Classification of Classification of chromosomeschromosomes StructuralStructural
FunctionalFunctional
Structural Structural classification classification
Based on the position of centromereBased on the position of centromere .Metacentric .Metacentric - central centromere - central centromere SubmetacentricSubmetacentric – centromere intermedi – centromere intermedi
-ate in position-ate in position Acrocentric Acrocentric - close to the tip - close to the tip These chromosomes may have satellitesThese chromosomes may have satellites TelocentricTelocentric – - Terminal –seen in – - Terminal –seen in
lower animals lower animals
DENVER ClassificationDENVER Classification
Based on four parametersBased on four parameters 1. length1. length 2. position of centromere2. position of centromere 3. presence or absence of 3. presence or absence of
satellitessatellites 4. banding4. banding Classified into Classified into seven groups AtoGseven groups AtoG
A 1to3 large metacentricA 1to3 large metacentric
B 4,5 large submetacentricB 4,5 large submetacentric
C 6-12& X Medium size metacentricC 6-12& X Medium size metacentric
D 13- 15 medium sized D 13- 15 medium sized acrocentric acrocentric
with satellitewith satellite
E 16- 18 16 shorter,metacentric or E 16- 18 16 shorter,metacentric or submetacentric (17 ,18)submetacentric (17 ,18)
F 19,20 Shorter metacentricF 19,20 Shorter metacentric G 21,22, Y Short acrocentricG 21,22, Y Short acrocentric 21,22 have satellite21,22 have satellite
Functional Functional classificationclassification AutosomesAutosomes
Sex chromosomes - X and YSex chromosomes - X and Y
Paris NomenclatureParis Nomenclature
More accurate method More accurate method Banding technique usedBanding technique used Arms divided into 1,2,3 regionsArms divided into 1,2,3 regions Further subdivided into bandsFurther subdivided into bands
AutosomesAutosomes
DominantDominant
RecessiveRecessive
KaryotypingKaryotyping
Method employed for genetic Method employed for genetic analysisanalysis
and study of chromosomesand study of chromosomes
Tijo and Leven in 1956Tijo and Leven in 1956
Karyotype is the photomicrograph Karyotype is the photomicrograph of an idividuals chromosome of an idividuals chromosome arranged in a standard mannerarranged in a standard manner
Tissue selectedTissue selected
Circulating T lymphocytesCirculating T lymphocytes Fibroblasts from skinFibroblasts from skin Bone marrow cellsBone marrow cells Chorionic villiChorionic villi Cells from amniotic fluidCells from amniotic fluid
PreparationPreparation 5 ml of peripherel venous blood is 5 ml of peripherel venous blood is
drawn under sterile conditionsdrawn under sterile conditions Combined with heparinCombined with heparin Add to small volume of culture Add to small volume of culture
mediummedium Add Phytohaemagglutinin(PHA) Add Phytohaemagglutinin(PHA) Culture under sterile condition at Culture under sterile condition at
37degree centigrade37degree centigrade
for 72 hoursfor 72 hours
Colchicine is added to arrest divisionColchicine is added to arrest division
Hypotonic saline is addedHypotonic saline is added Fixed by adding mixture of glacial Fixed by adding mixture of glacial
acetic acid and methanolacetic acid and methanol Spread on a chilled slide by droppingSpread on a chilled slide by dropping from a heightfrom a height Mounted on a slide- Metaphase Mounted on a slide- Metaphase
spreadspread Staining and banding doneStaining and banding done
Photo micrographs takenPhoto micrographs taken Cut and arranged in a systematic mannerCut and arranged in a systematic manner Modern computerised technique Modern computerised technique
availableavailable Structural and numerical variations Structural and numerical variations
studiedstudied Banding done –G banding or Giemsa,GTGBanding done –G banding or Giemsa,GTG Trypsin added denatures proteinTrypsin added denatures protein Stained with geimsa - Dark & light bandsStained with geimsa - Dark & light bands
Heparin Heparin prevents clottingprevents clotting
Culture media and fetal calf serum Culture media and fetal calf serum
help to nourish the lymphocyteshelp to nourish the lymphocytes
PhytoheamagglutininPhytoheamagglutinin stimulates stimulates cell cell
division in lymphocytesdivision in lymphocytes
Colchicin Colchicin added prevents formation added prevents formation of spindles –arrests cell division at of spindles –arrests cell division at metaphasemetaphase
Chromosome BandingChromosome Banding
G banding G banding Most commonly usedMost commonly used First treated with trypsin First treated with trypsin
denatures chromosome proteindenatures chromosome protein Giemsa staining gives unique Giemsa staining gives unique
light and dark bandslight and dark bands
Q BandingQ Banding
Stained with quinacrine mustardStained with quinacrine mustard
Observed under ultraviolet Observed under ultraviolet fluorescent microscopefluorescent microscope
R bandingR banding
Preheated before staining with Preheated before staining with giemsagiemsa
Reverse g banding pattern Reverse g banding pattern
C banddingC bandding
Centrometric and regions with Centrometric and regions with secondary constriictions are secondary constriictions are stainedstained
Fluorescent in situ Fluorescent in situ hibridization - FISHhibridization - FISH
Sex Chromatin- Barr Sex Chromatin- Barr bodybody Dark stained chromatin mass in Dark stained chromatin mass in
femalefemale Interface nucleusInterface nucleus Attached on one side of nuclear Attached on one side of nuclear
membranemembrane Buccal mucosa mostly used for Buccal mucosa mostly used for
examinationexamination
Lyon’s hypothesisLyon’s hypothesis
Mary F. Lyon - 1962Mary F. Lyon - 1962
One of two X chromosome One of two X chromosome undergo inactivation on 15undergo inactivation on 15thth or or 1616thth day of embryogenesis day of embryogenesis
Either maternal or paternalEither maternal or paternal
Normal male XYNormal male XY
Normal female- XXNormal female- XX
Turner syndrome- XOTurner syndrome- XO
Klinefelter Syndrome – XXYKlinefelter Syndrome – XXY
Triple X Syndrome - XXXTriple X Syndrome - XXX
Genetics Continued Genetics Continued ….….
Role of genetic Role of genetic counselorcounseloris to is to provide all provide all information regarding information regarding the disorder, including the disorder, including risks and optionsrisks and options, so , so that it becomes helpful that it becomes helpful to patients for taking to patients for taking their own decisionstheir own decisions
Genetic counsellingGenetic counselling
Diagnosis of genetic diseaseDiagnosis of genetic disease Mode of inheritanceMode of inheritance Risk of occuranceRisk of occurance Management of genetic diseaseManagement of genetic disease Disputed paternityDisputed paternity Cousins marriageCousins marriage
Very little scope for treatment in Very little scope for treatment in inheritanceinheritance
Avoid or Reduce the incidenceAvoid or Reduce the incidence
Diagnosis of genetic Diagnosis of genetic diseasedisease Family history – detailed pedigree Family history – detailed pedigree
chartchart
Examination of patient - diagnosisExamination of patient - diagnosis
Laboratory investigationsLaboratory investigations
Determination of mode of Determination of mode of inheritanceinheritance
Assesment of risk of occurrenceAssesment of risk of occurrence
Management of genetic diseaseManagement of genetic disease
Genetic counselling in disputed Genetic counselling in disputed paternitypaternity
Cousin marriage and genetic Cousin marriage and genetic counsellingcounselling
First degree relative - 50%First degree relative - 50%
Uncle/niece Aunt/Nephew – 5-Uncle/niece Aunt/Nephew – 5-10%10%
Third degree - 3-5%Third degree - 3-5%
ManagementManagement
Tell about the diagnosis Tell about the diagnosis
Tell about the risk involvedTell about the risk involved
Tell about all options available for Tell about all options available for the managementthe management
Prevent the disorderPrevent the disorder
PRENATAL DIAGNOSISPRENATAL DIAGNOSIS
IndicationIndication
Couples with family history Couples with family history Advanced maternal ageAdvanced maternal age Neural tube defectsNeural tube defects Couples with previous child with Couples with previous child with
chromosomal abnormalitychromosomal abnormality Mother with x- linked recessive Mother with x- linked recessive
disorderdisorder
TECHNIQUESTECHNIQUES
AMNIOCENTESISAMNIOCENTESIS CHORIONIC VILLUS BIOPSYCHORIONIC VILLUS BIOPSY FETOSCOPYFETOSCOPY ULTRASONOGRAPHYULTRASONOGRAPHY MATERNAL SERUM SCREENINGMATERNAL SERUM SCREENING FOETAL BLOOD SAMPLINGFOETAL BLOOD SAMPLING
AMNIOCETESISAMNIOCETESIS
Performed >< 14 -16 wksPerformed >< 14 -16 wks
10 -20 ml of amniotic fluid aspirated10 -20 ml of amniotic fluid aspirated
Through anterior abdominal wallThrough anterior abdominal wall
Foetal karyotypingFoetal karyotyping
Neural tube defects- Alpha feto Neural tube defects- Alpha feto proteinprotein
Level is raisedLevel is raised Leakage from open defectsLeakage from open defects Risk of abortion – 1%Risk of abortion – 1% Termination of pregnancy – 2Termination of pregnancy – 2ndnd
trimestertrimester
CHORIONIC VILLUS CHORIONIC VILLUS BIOPSYBIOPSY Chorionic villus sample aspiratedChorionic villus sample aspirated Under guidance of ultra soundUnder guidance of ultra sound Catheter introduced through cervixCatheter introduced through cervix Even DNA analysis can be carried Even DNA analysis can be carried
outout 10 – 11 weeks 10 – 11 weeks Termination of pregnancy in 1Termination of pregnancy in 1stst
trimestertrimester Risk of abortion – 2-3%Risk of abortion – 2-3%
FetoscopyFetoscopy
Endoscope for visualization of Endoscope for visualization of foetusfoetus
Fibro optic self illuminated Fibro optic self illuminated instrumentinstrument
is inserted in the amniotic cavityis inserted in the amniotic cavity
under local anaesthesiaunder local anaesthesia
INDICATIONSINDICATIONS
Limb malformationsLimb malformations Cleft malformations and the likeCleft malformations and the like Malformation of genitalsMalformation of genitals Skin disordersSkin disorders To obtain skin biopsy To obtain skin biopsy To obtain foetal blood sample To obtain foetal blood sample
fromfrom
umbilical cordumbilical cord
ULTRASONOGRAPHYULTRASONOGRAPHY
Non-invasiveNon-invasive
Routinely done at 12 weeksRoutinely done at 12 weeks
Localization of placentaLocalization of placenta
Diagnosis of muliple pregnancyDiagnosis of muliple pregnancy
Diagnosis of malformationDiagnosis of malformation
Structural abnormalitiesStructural abnormalities
To ascertain foetal ageTo ascertain foetal age
Nuchal translucency - Nuchal translucency - NTNT Accumulation of fluid behind the Accumulation of fluid behind the
baby’s neckbaby’s neck
Maternal serum Maternal serum screeningscreening Obtained at 16 weeksObtained at 16 weeks AFPAFP Neural tube defectsNeural tube defects Multiple pregnancyMultiple pregnancy Congenital nephrotic syndromeCongenital nephrotic syndrome Abdominal wall defectAbdominal wall defect
TRIPLE TESTTRIPLE TEST
Level of AFP reducedLevel of AFP reduced
HCG increasedHCG increased
Unconjugated oestriol is reducedUnconjugated oestriol is reduced
Foetal blood samplingFoetal blood sampling
CordocentesisCordocentesis HaemophiliaHaemophilia ThalassaemiaThalassaemia Sickle cell diseaseSickle cell disease Immune deficiancy disordersImmune deficiancy disorders High risk abortionHigh risk abortion
TREATMENTTREATMENT
PRENATAL TREATEMENTPRENATAL TREATEMENT
POSTNATAL TREATEMENTPOSTNATAL TREATEMENT
REPLACEMENT OF GENE REPLACEMENT OF GENE PRODUCTPRODUCT
DRUG TREATMENTDRUG TREATMENT
TISSUE REMOVAL AND TISSUE TISSUE REMOVAL AND TISSUE TRANSPLANTTRANSPLANT
STEM CELL TRANSPLANTAIONSTEM CELL TRANSPLANTAION
DIETARY RESTRICTIONDIETARY RESTRICTION
GENE THERAPYGENE THERAPY
Germ line gene therapy Germ line gene therapy
Somatic cell gene therapySomatic cell gene therapy
THE ENDTHE END
GENETICS – Cont- 2
Chromosome disordersChromosome disorders
Number Number StructureStructure Different cell linesDifferent cell lines
Responsible for significant Responsible for significant morbiditymorbidity and mortalityand mortality
50 % of Spontaneous abortions50 % of Spontaneous abortions
Upto 1% congenital and Upto 1% congenital and childhood disabilitieschildhood disabilities
Cause for malignanciesCause for malignancies
Normal chromosome number - Normal chromosome number - 4646
Changes in chromosome numberChanges in chromosome number AneuploidyAneuploidy – Not an exact – Not an exact
multiple of haploid numbermultiple of haploid number PolyploidyPolyploidy – Multiple of haploid – Multiple of haploid
numbernumber
NumericalNumerical Aneuploidy MonosomyAneuploidy Monosomy TrisomyTrisomy TetrasomyTetrasomy Polyploidy TriploidyPolyploidy Triploidy
TetraploidyTetraploidy
StructuralStructural Translocation ReciprocalTranslocation Reciprocal
RobertsonianRobertsonian DeletionDeletion InsertionsInsertions Inversions ParacentricInversions Paracentric
PericentricPericentric RingsRings IsochromosomesIsochromosomes
Diferrent cell lines – MixoploidyDiferrent cell lines – Mixoploidy Mosaicism Mosaicism ChimerismChimerism
monosomymonosomy
Absence of a single chromosomeAbsence of a single chromosome
Monosomy for an autosome is Monosomy for an autosome is incompatible with survival to termincompatible with survival to term
Lack of contribution of an X or Y Lack of contribution of an X or Y chromosome result in 45X chromosome result in 45X
Normal meiosisNormal meiosis
Non dysjunction in Non dysjunction in meiosismeiosis
CausesCauses
Chromosomal aberrations are Chromosomal aberrations are present in 10% of all spermatozoa present in 10% of all spermatozoa
and 25 % of mature oocytesand 25 % of mature oocytes Advanced maternal ageAdvanced maternal age RadiationRadiation Delayed fertilization after ovulationDelayed fertilization after ovulation Chemicals in the environmentChemicals in the environment VirusesViruses
PolyploidyPolyploidy
Contain multiples of haploid Contain multiples of haploid number of chromosomesnumber of chromosomes
69 – Triploidy69 – Triploidy 92 - Tetraploidy92 - Tetraploidy Survival beyond mid pregnancy is Survival beyond mid pregnancy is
rarerare Result in spontaneous abortionResult in spontaneous abortion
Causes of polyploidyCauses of polyploidy
Failure of maturation of meiotic Failure of maturation of meiotic division in an ovum or spermdivision in an ovum or sperm
Fertilization of ovum by two Fertilization of ovum by two spermssperms
TrisomyTrisomy
Presence of an extra chromosomePresence of an extra chromosome Trisomy 21 –Down syndromeTrisomy 21 –Down syndrome
Trisomy 13 – Patau syndromeTrisomy 13 – Patau syndrome
Trisomy 18 – Edwards Trisomy 18 – Edwards syndromesyndrome
Trisomy 21
Seguin 1846- Furfuraceous idiocy Langdon Down 1866 – mongolian
idiocy Clinical lecture reports of the
london hospital Chromosomal basis - Lejune and
colleques - 1959
Down syndrome Incidence 1 in 700 New born has severe
hypotonia,sleepy, Excess nucheal skin Craniofacial charecteristics Brachycephaly Upward sloping of palpebral fissure Small ears Protruding tongue Limbs Single palmar crease – 50 %
Trisomy 21Trisomy 21
Cardiac ASD VSD Common
atrioventricular canal,PDA Others Anal atresia Duodenal atresia Hirschsprung Short stature Strabismus Wide gap between 1st and 2nd toes
Natural history
IQ ranging from 25 – 75 Average around 40 – 45
Happy and very affectionate Most develop alzeimers disease in
later life Attributed to an amyloid precursor
protein gene dosage effect
Trisomy – 95% Translocation – 4% Mosaicism -1%
Critical region distal end of long arm
21q22q
Chromosome 21 is a gene poor chromosome
High ratio of AT to AG sequences
Recurrence risk Straight foreward trisomy 1/200
to 1/100 Familial translocation (1-3%) for
male carriers 10 – 15 % for female carriers 21q22q translocation 100 %
chance of recurring
Trisomy 13
First described in 1960 Share many features in common
with trisomy 18 Incidence1 in 5000 Very poor prognosis Die within first few days or within
weeks
Trisomy 13Trisomy 13
Trisomy 18Trisomy 18
Trisomy 8Trisomy 8
Trsomy 9pTrsomy 9p
Disorders of sex chromosomes Turner syndrome
Klinefelter syndrome
Klinefelters syndrome
47,XXY
First described in 1942
Incidence is 1 in 1000 male births
Clinical features Clumbsiness, mild learning
disabilities Verbal skills are poor
Verbal IQ reduced to 10 – 20 points
Slightly taller than normal
long lower limbs
30% show moderately severe gynaecomastia
All are infertile( azospermia) small soft testis
Increased incidence of leg ulcers ,osteoporosis, carcinoma of
breast
Treatment with testosterone From puberty onwards Benificial for secondary sexual
charectors Long term prevention of
osteoporosis
Chromosomal findings Equal chance inheritance from
mother and father A small percentage will show
mosaicism ie 46XY/47XXY Rarely male with more than two X
chromosomes can be encountered ie48XXXY,49XXXXY Severely
retarded Physical charecteristics more
marked
Klinefelters syndromeKlinefelters syndrome
Turners Syndrome
First described in 1938 Absence of barr body consistant
with presence of only one X chromosome
Noted in 1954 Cytogenetic conformation in 1957 Common at conception and
spontaneous abortions
Incidence 1in5000 Clinical features On ultrasound scanning – hydrops Or swelling localised to neck Neck webbing Stunted growth Streak ovaries Cubital valgus
Low posterior hair line Increased carrying angles Widely spaced nipples Coarctation of aorta 15% Normal intelligence
Short stature is partly due to haploinsufficiency fo the SHOX gene
Which is located in a pseudoautosomal region
Primary amenorrhoea and infertility Oestrogen replacement therapy At adholesence development of
secondary sexual charectors Longterm prevention of
osteoporosis
Turner syndrome 16 Turner syndrome 16 weeksweeks
47,XXY47,XXY
Disorders of Disorders of chromosome structurechromosome structure DeletionDeletion Breakage occurs in a part of Breakage occurs in a part of
chromosome chromosome Broken part is subsequently lostBroken part is subsequently lost Because it has no centromereBecause it has no centromere
Single break Single break Two breaks Two breaks If more than 2 % loss ,lethalIf more than 2 % loss ,lethal
Two kinds Two kinds Microscopic or chromosomal Microscopic or chromosomal
deletiondeletion eg ; deletion of short arm of eg ; deletion of short arm of
chromosome 5 chromosome 5 (cri – du – chat(cri – du – chat syndrome)syndrome)
Short arm of chromosome 4 (Short arm of chromosome 4 (wolf –wolf – hirschhornhirschhorn syndrome) syndrome)
Cri-du chat syndromeCri-du chat syndrome
1 in 50000 births1 in 50000 births 5p-5p- Under developed larynx=cat like Under developed larynx=cat like
crycry MicrocephalyMicrocephaly severe mental and physical severe mental and physical
retardationretardation Microcephaly, CHDMicrocephaly, CHD
Cri- du -chatCri- du -chat
Sub microscopic micro deletionsSub microscopic micro deletions
Diagnosed by FISH studiesDiagnosed by FISH studies
Prader-willi 15 Angelman 15 Willms tumour 11 Williams 7 Langer giedion 8 Miller dieker 17 Di - george
Duplication Duplicated part of a chromosome
within a chromosome segment may be attached to
chromosome May be a separate segment No loss of genetic material Less harmful May involve part of a gene ,whole
gene, or a series of genes
DuplicationDuplication
InversionInversion
Involes a single chromosome Involes a single chromosome which breaks up at two pointswhich breaks up at two points
Broken segment rearranges itself Broken segment rearranges itself by inverting its positionby inverting its position
inversion may be pericentric or inversion may be pericentric or para centricpara centric
Paracentric inversionParacentric inversion
Ring chromosomeRing chromosome
Rare abnormalityRare abnormality Chromosome forms a closed circleChromosome forms a closed circle Break near ends Break near ends Broken and sticky ends Broken and sticky ends
subsequently fuse with each other subsequently fuse with each other Usually chromosomal mosaicsUsually chromosomal mosaics
IsochromosomesIsochromosomes
Incorrect splitting of centromereIncorrect splitting of centromere
IsochromosomesIsochromosomes
TranslocationTranslocation
Exchange of genetic material Exchange of genetic material between two chromosomes occurs between two chromosomes occurs
ReciprocalReciprocal – if two nonhomologous – if two nonhomologous chromosomes exchange pieceschromosomes exchange pieces
RobertsonianRobertsonian -Breakage of two -Breakage of two acrocentric chromosomes at or acrocentric chromosomes at or close to centromeres with close to centromeres with subsequent fusion with long arms. subsequent fusion with long arms. Short arms are lostShort arms are lost
Reciprocal translocations are balanced re arrangements
No detectable phenotypic effects in carrier
Reciprocal Reciprocal translocationtranslocation
Robertsonian Robertsonian translocationtranslocation
MosaicismMosaicism
If nondysjunction occurs during If nondysjunction occurs during an early cleavage division of a an early cleavage division of a zygotezygote
an embryo with two or more cell an embryo with two or more cell lines lines
with different chromosome with different chromosome compliments are producedcompliments are produced
Robertsonian translocation
Incidence 1in 1000 Break at or near the centromere in
two acrocentric chromosomes and subsequent fusion of their long arms
Short arms of both the chromosomes are lost
Chromosome number is reduced to 45
ChimerismChimerism Chimera is an individual who have Chimera is an individual who have
two or more genetically distinct cell two or more genetically distinct cell lines which are derived from more lines which are derived from more than one zygotethan one zygote
Persons with populations of blood Persons with populations of blood cells of two genotypes that are cells of two genotypes that are from different zygotesfrom different zygotes
Anastamoses >< blood vessels of Anastamoses >< blood vessels of fused placeta of DZ twins may fused placeta of DZ twins may occuroccur
Erytrocyte mosaicismErytrocyte mosaicism