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Medelian Principles AGB121


• Johann Gregor Mendel (1822-1884) regarded, as ‘Father of Genetics’ was pioneer in postulating the principles of basic genetics.

Law of Dominance

Law of segregation Law of Independent Assortment Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Law of Dominance • Phenomenon by which one trait appears and other does not appear even though both factors are present is called dominance.

Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Law of segregation • Allelic genes in a zygote do not blend or contaminate each other but segregate and pass into gametes.

Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Law of Independent Assortment: • It states that “in a cross involving two pairs of genes, the inheritance of one pair of genes is independent of inheritance of other pair of genes”.

Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Dihybrid Cross

Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Gene Interaction AGB121


Kinds of gene interaction • Intra allelic interaction : Interaction of alleles of the same pair which are affecting same character. 1.Dominance 2. Codominance 3. Over dominance • Interallelic interaction – Between alleles. Alleles at one locus influences the expression of another allele at different loci (Epistasis) Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Modification to Mendelian Ratios Incomplete Dominance

X white

Red

Roan Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Snapdragon Flower

In poultry Andalusian fowls Crossing Black and white birds produced Blue Andalusian Dr R Jayashree, Asst. Professor, veterinary College Bangalore fowls


Over dominance The phenotype of heterozygote measured quantitatively is not always equal or intermediate to homozygotes, instead, it exceeds the phenotypic measurement of both homozygous parents. Examples: •Sickle cell anaemia in Human •In drosophila white-eyed gene is in heterozygous condition causes marked increase in amount of certain fluorescent pigments over white and wild type. •Over dominance is mainly used with certain biological fitness like size, productivity and viability. Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Co-dominance • It is a case where the heterozygotes produce both the effect of dominance and recessive or both substances. Ex: ABO blood group in human beings • Parental blood groups AB IAIB X IAIB F1 Gametes

IA

IB

IA

IAIA

IAIB

IB

AIB BIB I I Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Epistasis It is the inter-allelic interaction between two pairs of genes resulting in less number of phenotypes than the usual four Dominant Epistasis - 12:3:1 Recessive epistasis-9:3:4

Duplicate genes with cummulative effect-9:6:1 Duplicate dominant genes-15:1 Duplicate recessive genes-9:7 Dominant & recessive interaction-13:3 Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Dominant Epistasis -12:3:1 In dog coat colour is Black –B_ (BB or Bb) and Brown – bb

Gamete

BI

Bi

bI

bi

BI

BBII

BBIi

BbII

BbIi

Bi

BBIi

BBii

BbIi

Bbii

bI

BbII

BbIi

bbII

bbIi

bi

BbIi

Bbii

bbIi

bbii

12 White B-I-(BBII,BBIi,BbII,BbIi) 3 Black B-ii(Bbii,Bbii) 1 Brown bbii Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Dominant Epistasis -12:3:1 Squash fruits can be white yellow or green Gamete

CS

Cs

cS

cs

CS

CCSS

CCSs

CcSS

CcSs

Cs

CCSs

CCss

CcSs

Ccss

cS

CcSS

CcSs

ccSS

ccSs

cs

CcSs

Ccss

ccSs

ccss

White fruiting- 12(CCSS,CCSs,CcSS,CcSs,ccSs,ccSS,ccSs), Yellow fruiting -3, (CCss, Ccss) Green fruiting- 1,(ccss) Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Recessive Epistasis (9:3:4) Coat colour inheritance in Labrador dogs Pure breeding labrador dog are black- BBEE Pure breeding labrador are golden yellow-bbee

Gamete

BE

Be

bE

be

BE

BBEE BBEe

BbEE

BbEe

Be

BBEe BBee

BbEe

Bbee

bE

BbEE BbEe

bbEE

bbEe

be

BbEe

bbEe

bbee

Bbee

Black furred - 9 - B_E_BBEE,BBEe,BbEE,BbEe) Brown furred – 3- bbE- (bbEE, bbEe Jayashree, Asst. Professor, veterinary Golden furred - 4- Dr RB-ee,(BBee,Bbee,bbee) College Bangalore


Recessive Epistasis (9:3:4) Coat colour in mice

Black =9- B_C(BBCC,BbCC,BbCc,BBCc) Brown = 3 -bbCC,bbCc Albino =4 – BBcc, Bbcc, bbcc

Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Duplicate genes with cummulative effect (9:6:1) In squash fruit shape of the fruit Gamete

AB

Ab

aB

ab

AB

AABB

AABb

AaBB

AaBb

Ab

AABb

AAbb

AaBb

Aabb

aB

AaBB

AaBb

aaBB

aaBb

ab

Aabb

Aabb

aaBb

aabb

Disk shaped = 9 - A_B_, Sphere shaped = 6 A-bb,aaB_, Long = 1 aabb Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Duplicate dominant genes (15:1) In poultry feathered shank is FFSS,ffSs,FfSs,FFss and In Buff rocks unfeathered shank is ffss

Gamete

FS

Fs

fS

fs

FS

FFSS

FFSs

FfSS

FfSs

Fs

FFSs

FFss

FfSs

Ffss

fS

FfSS

FfSs

ffSS

ffSs

fs

FfSs

Ffss

ffSs

ffss

Feathered =15 Clean shank = 1 Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Duplicate recessive genes (9:7) In man deafness and normal hearing is inherited in this fashion

Gamete

DE

De

dE

de

DE

DDEE DDEe DdEE

DdEe

De

DDEe DDee

DdEe

Ddee

dE

DdEE DdEe

ddEE

ddEe

de

DDEe DDee

ddEe

ddee

Normal hearing D_E_=9(DDEE,DdEE,DDEe,DdEe) Deafness D_ee,ddE_,ddee =7 (DDee,Ddee,ddEE,ddEe.ddee) Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Duplicate Recessive genes(9:7) In poultry a colour gene W is required for the expression of colour by the birds which carry dominant gene CC,Cc and when cc is present the birds are white. Gamete

CW

Cw

cW

cw

CW

CCWW CCWw CcWW CcWw

Cw

CCWw

CCww

CcWw

Ccww

cW

CcWW

CcWw

ccWW

ccWw

cw

CcWw

Ccww

ccWw

ccww

Coloured feathering = 9 C_W_ (CCWW,CcWW,CCWw,CcWw) White feathering = 7 CCww,Ccww Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Dominant and Recessive epistasis(13:3) Feather colour in White leghorn and White silkie C_ allele is required for expression of colour and cc for no colour Allele I inhibits the formation of colour and allele i allows expression of colour

Gamete

CI

Ci

cI

ci

CI

CCII

CCIi

CcII

CcIi

Ci

CCIi

CCii

CcIi

Ccii

cI

CcII

CcIi

ccII

ccIi

ci

CcIi Ccii ccIi Dr R Jayashree, Asst. Professor, veterinary

ccii

College Bangalore


Two genes for one trait

Gamete

PR

Pr

pR

pr

PR

PPRR PPRr

PpRR

PpRr

Pr

PPRr PPrr

PpRr

Pprr

pR

PpRR PpRr

ppRR

ppRr

pr

PpRr Pprr

ppRr

pprr

P_R_ = Walnut comb (Double dominant) =9 P_rr = Pea comb( One locus dominant )=3 R_pp = Rose (Other locus dominant)=3 pprr = Single comb (Double recessive) =1

Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Multiple Alleles A set of three or more alleles occupy the same locus of the homologous chromosomes and affect the same character are called as multiple alleles.

Phenotypes Genotypes Black (Agouti) CC, Cch, Ccch, Cc Himalayan ChCh, ChCch, Chc Chinchilla Cchcch,chc Albino cc Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Multiple allele

Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Multiple allele- Examples ABO blood group systems.

Where I A = IB > i Blood group

IA IA

Iai

A

IBIB

Ibi

B

I AI B

AB

ii

ODr R Jayashree, Asst. Professor, veterinary College Bangalore


SEX CONTROLLED INHERITANCE

Sex Linked

Sex Influenced Sex limited

Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Sex linked Inheritance (X –linked)

Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Criss-cross inheritance

Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Sex Influenced/Sex controlled traits Baldness in Human

BB

-

Both men and women are bald

Bb

-

Men bald, women non bald

bb

-

Both men and women non bald.

Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Horned condition in sheep

Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Sex limited Traits • Genes controlling the sex traits are present in both the sexes but the expression of the gene is limited to one sex only. It is mainly due the effect of sex hormones. The genes are present in the autosomes.

Dr R Jayashree, Asst. Professor, veterinary College Bangalore


Course Information