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