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