2- Modes of inheritance[1].pptx

8/18/2019 2- Modes of inheritance[1].pptx

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Modes of Inheritance

Dr shrikant rokade


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BASIC DEFINITIONS

A gene is a hereditary factor, transmitted from parents to

offspring, that inflences traits among the offspring!

"hysically, a gene consists of a se#ence of DNA $ases that encode

a specific protein! The physical location of a gene on a

chromosome is termed a locs! %ariation in the DNA se#ence at a

locs prodces different forms of a gene, called alleles!


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Chromosomes &s 'enes

'enes constitte distinct regions on

the chromosome

Each gene codes for a protein

prodct

DNA (NA protein

Differences in proteins $rings a$ot

differences $et)een indi&idals and

species


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'ene—$asic nit of inheritance ,that controls the

development of one trait. A segment of DNA that codes for

one protein!.. A sequence of nucleotides on a chromosome

(chromatin).

*ocs+++location of a gene on a chromosome

Allele—different forms of a gene ,or a gene that occpies a specific

position on a specific chromosome !One allele comes from

each parent ,Represented by a single letter. Two alleles

produce a trait genetically determined characteristics-! Traits

can $e dominant or recessi&e


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Dwarfism = D

Normal height = d

EXAMPLES OF ALLELES


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Some alleles may reslt in a missing or a$normal protein, casing

disease!

.hen a specific site on a chromosome has mltiple alleles in a

poplation, it is termed a polymorphism “mltiple forms”-!


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The $ody’s cells can $e di&ided into t)o grops/

gametes sperm and o&m-, )hich are transmitted to offspring, and

somatic cells cells other than gametes-, )hich are not transmitted!

Nearly all somatic cells are diploid, containing 01 pairs of

chromosomes one mem$er of the pair from the father, the other

mem$er from the mother-! These 01 pairs consist of 00 pairs of

atosomes and one pair of se2 chromosomes the 3 and 4!

chromosomes-!

'ametes are haploid, ha&ing deri&ed only one mem$er of each

chromosome pair dring meiosis


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The specific DNA se#ence at a locs is termed a genotype!

In diploid somatic cells, a genotype may $e homo5ygos homo5ygote-

at a gi&en locs, indicating that the indi&idal inherited the same allele

from $oth parents!

If he or she inherited different alleles from each parent, the genotype is

hetero5ygos hetero5ygote-!

The genotype is o$ser&ed physically as a phenotype, )hich reflects the

interaction of the genotype )ith the en&ironment and )ith genes at

other loci!


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GENOTYPE

One’s genetic make up

Represented by alleles

E.g. HH & Hh are genotypes


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PHENOTYPE

Outward epression o! a genotype

"enotype determines the phenotype

#lue eyes$ blonde hair$ number o! !ingers are phenotypes.


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If only one copy of an allele is re#ired for its phenotypic e2pression, the allele

is dominant i!e!, it is o$ser&a$le in the hetero5ygos state-!

If t)o copies of the allele are re#ired for its e2pression i!e!, the disease

phenotype is o$ser&a$le only in the homo5ygos state-, it is recessi&e!

The e2pression of the recessi&e allele is ths 6hidden” in the hetero5ygote!

The terms dominant and recessi&e pro&ide a con&enient classification of

genetic diseases!

If t)o different alleles are $oth phenotypically e2pressed in a hetero5ygos

genotype, the alleles are said to $e codominant!


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EXPRESSION OF DOMINANT & RECESSIE ALLELES

ominant alleles are e!pressed

Recessive alleles are not e!pressed in the presence of a

dominant allele

– Recessive alleles are only e!pressed if " recessive

alleles are present


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HOMO!YGO"S

#oth alleles ali$e

AA %omo&ygous ominant

aa %omo&ygous Recessive


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HETERO!YGO"S

Alleles are different

Aa %etero&ygous


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CODOMINANT

Two different alleles are both dominant

A ' allele for type A blood

# ' allele for type # blood

A# ' results in type A# blood

OO ' results in type O blood

ote AA or AO group A * ## or #O group #


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A7TOSOMA* T(AIT

Alleles are located on an autosome

%omologous pairs +"".


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3+*IN8ED T(AITS

Alleles are on the - chromosomes.

ore alleles have been found on the - set chromosome than

the / chromosome at this time.

0emales have two - alleles therefore they are often “carriers”

of a trait.

ales have only one - chromosome, therefore they usually

e!press the trait.


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AUTOSOMAL TRAITS


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E2amples of Some Common Atosomal Dominant Diseases

Fre#ency9:;;;$irth Disease


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E2amples of Some Atosomal recessi&e diseases

Fre#ency9:;;;$irth Disease

;!= Cystic fi$rosis

;!= (ecessi&e mental retardation

;!0 Congenital deafness

;!: "henyl>etonria

;!: (ecessi&e $lindness

;!: Adrenogenital syndrome


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Ethnic association )ith some atosomal recessi&e diseases

Ethnic grops- Disease

Mediterraneans, Thais, $lac>s, Middle East,

"oplation, Indians, chineseBeta+thalassemia

7S and African Blac>s, Asian Indians,Mediterraneans Specially 'ree>s- and

Middle East poplations

Sic>le cell disease

Ash>ena5i e)s Tay+Sachs disease

Ash>ena5i e)s 'acher disease

Cacasians Cystic fi$rosis

?opi Indians Al$inism


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Atosomal recessi&e inheritance

:! SIC8*E CE** ANEMIA

Recessive alleles, codes for hemoglobin with + amino acid which

is incorrect!

This results in red blood cells with a sic$le shape, reduced

o!ygen carrying capacity, and often “plugging up” small blood

vessels.

%omo&ygous recessive, ss have sic$le cell anemia.

%etero&ygous, 1s are carriers.


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SIC#LE CELL ANEMIA

R#2s sic$le shaped

Anemia

3ain

1tro$e

4eg ulcers

5aundice

6all stones

1pleen, $idneys * lungs


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A*BINISM

4ac$ of pigment

– 1$in

– %air

– 7yes


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Tyrosine DO"A

Melanin"igment

Tyrosinase

A a

AA Normal pigmentation

Aa Normal pigmentation

aa Al$ino


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"87 DISEASE

3henylalanine e!cess

ental retardation if untreated


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SEX-LINKED TRAITS


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E2amples of Some ?man 3+lin>ed recessi&e traits

789fre#ancey9:; ;;; males Trait

;; (ed+green color $lindness

= Fragile 3+ syndrome

1 Dchenne msclar dystrophy

0 ?aemophilia A factor %III-

;!1 ?aemophilia B factor I3-


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An E2ample of hman 3+ lin>ed dominant traits

%itamin D resistant ric>ets


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-lin$ed recessive trait

HEMOPHILIA

#lood clotting impaired

ore common in males

Recessive allele, h

carried on - chromosome


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'enotype—alleles fond at a locs

"henotype—physically o$ser&a$le featres

?omo5ygote—alleles at a locs are the same

?etero5ygote+—alleles at a locs are different

Dominant—re#ires only one copy of the mtation to prodce

disease

(ecessi&e—re#ires t)o copies of the mtation to prodce disease

Smmary


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A mtation is an alteration in DNA se#ence ths, mtations

prodce ne) alleles-!

.hen mtations occr in cells gi&ing rise to gametes, they can $e

traitsmined to ftre generations!

Missense mtations reslt in the s$stittion of a single amino

acid in the polypeptide chain e!g!, sic>le cell disease is cased $y a

missense mtation that prodces a s$stittion of &aline for

gltamic acid in the glo$in polypeptide-!

Nonsense mtations prodce a stop codon, reslting in prematre

termination of translation and a trncated protein!


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A+ single $ase mtation "oint mtation- may $e/

:! Silent Mtation

– This occrs if the reslting codon still codes for the same aminoacid! It is more li>ely to occr if the change occrs in the third$ase of the codon! If A in the codon for serine, 7CA, is changed toC, the reslting codon, 7CC, still codes for serine!

0! Missense Mtation –

This occrs if the reslting codon codes for a different amino acid!It is more li>ely to occr if the change occrs in the first or second$ase of the codon! If 7 in the codon for serine, 7CA, is changed toC, the reslting codon, CCA, codes for proline!

1! Nonsense Mtation

– This occrs if the reslting codon codes for termination of the

peptide chain! If C in the codon of serine, 7CA, is changed to ',

the reslting codon, 7'A, leads to termination of translation!


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%codon !or proline

U A A

%'ermination codon

Nonsense

mutation

Silent

mutation

U C A

%codon !or serine

U C U

%codon !or serine

Missense

mutation

C C A


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Ncleotide $ases may $e inserted or deleted! .hen the nm$er of

inserted or deleted $ases is a mltiple of three, the mtation is said

to $e in+frame!

If not a mltiple of three, the mtation is a frameshift, )hich

alters all codons do)nstream of the mtation, typically prodcing

a trncated or se&erely altered protein prodct!

Mtations can occr in promoter and other reglatory regions or

in genes for transcription factors that $ind to these regions! This

can decrease or increase the amont of gene prodct prodced in

the cell!


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I+ ADDITION O( DE*ETION OF N7C*EOTIDES

:! Addition or Deletion of One or T)o Ncleotides

• This reslts in a frame shift mtation, leading to a change in

all codons after on the1’ side- the addition or the deletion!

0! Addition or Deletion of 1 Ncleotides

• This leads to addition or deletion of one amino acid to the

peptide chain! The reading frame is not changed!

B+ Other mtations/ These can alter the amont or strctre

of the protein prodced $y translation!


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(eletion o! base

)ddition o! base

mR*)

+ , ) + , , + ) + " " , +

-er

-er 'yr "ly

)ddition o! ++

+ , ) , + ) + " " , +

-er

ro /et )la0’1End2’1End

(eletion o! ,

+ , ) , + ) + " " , +

-er

3eu 'rp


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II+ Splice site mtations/ Mtations at splice sites can alter the )ay in)hich introns are remo&ed from prem(NA molecles, prodcing

a$errant proteins!

III+ Trincleotide repeat e2pansion/ Occasionally, a se#ence ofthree $ases that is repeated in tandem )ill $ecome amplified in

nm$er, so that too many copies of the triplet occr!

– If this occrs )ithin the coding region of a gene, the protein )ill

contain many e2tra copies of one amino acid!

– If the trincleotide repeat e2pansion occrs in the ntranslated

portion of a gene, the reslt can $e a decrease in the amont of

protein prodced !

'andem repeats o! ,)" triplets

coding !or glutamine

Huntington disease 4ragile15 syndrome

2’ ))))

%,)"66107%,"8129

2’ ))))


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Mtations can also $e classified according to their phenotypic

effects!

Mtations that case a missing or decreased protein prodct are

termed loss+of+fnction!

Those that prodce a protein prodct )ith a no&el or a$normal

fnction are termed gain+of+fnction!


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The recrrence ris>

The recrrence ris> / is the pro$a$ility that the offspring of a

cople )ill e2press a genetic disease! For e2ample, in the mating of

a normal homo5ygote )ith a hetero5ygote )ho has a dominant

disease+casing allele, the recrrence ris> for each offspring is :90,

or =;! It is important to remem$er that each reprodcti&e e&ent

is statistically independent of all pre&ios e&ents! Therefore, the

recrrence ris> remins the same regardless of the nm$er of

pre&iosly affected or naffected offspring!

Determining the mode of inheritance of a disease e!g!, atosomal

dominant &s! atosomal recessi&e- ena$les one to assign an

appropriate recrrence ris> for a family!


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