Paper Code BT 203 : Genetics

UNIT-1

MENDELS LAWS OF INHERITANCE:-LAW OF SEGREGATION OR LAW OF PURITY OF GAMETES

It states that in the heterozygous condition, a dominant and a recessive gene remain together throughout the life without contaminating and mixing together, eventually it separates or segregates from each other ,each gamete receives single gene or allele , either dominant or recessive gene.

LAW OF INDEPENDENT ASSORTMENT: OR LAW OF RECOMBINATION OF GENES: This law states that, when the gametes is formed the members of different pair of alleles(gene) segregate quite independently of each other and that all of possible combination of genes concern will be among the progeny.

LAW OF SEGREGATION :- EXAMPLE:- MONOHYBRID CROSS: DEFINITION:- A hybridization cross between the pure breed of homozygous tall and pure breed of homozygous dwarf plants differing in a single pair of phenotypic traits is called monohybrid cross.

EXPERIMENT:- EXPLANATION:- MENDEL crossed a homozygous tall plant with a homozygous dwarf plant.The F1 heterozygotes (Tt) were found to be tall plant.When the F1 hybrids were allowed to cross among themselves or self fertilization, they produce tall and dwarf plants in the ratio 3:1 in the F2 generation. In this cross the homozygous tall plant has two dominant genes(TT) and the homozygous dwarf plant has two recessive genes(tt). Because their gametes may have only one allele. So the gametes of tall plant have single allele(T) and the gametes of dwarf plant have only one allele(t). During fertilization, the gametes of both genotypes

are fused to form a heterozygote of the genotype(Tt) and phenotype of tall plant , showing dominance of ‘T’ allele over recessive allele ‘t’ . During developmental stages , each gene replicating and living together but during gametogenesis both alleles (genes) segregate from each other in their original forms to produce 50% gametes with a dominant gene (T) and 50% gametes with a recessive gene(t). Phenotypic ratio=3:1 Tall plants(T)-3 and Dwarf plants(t)-1Genotypic ratio=1:2:1 Homozygous tall(TT) – 1 Heterozygous tall(Tt) – 2 Homozygous dwarf(tt) – 1

EXPERIMENT:-

Parents -- TT x tt Gametes -- T x t F1 -- Tt Heterozygous hybrid

P2 ---- Tt x Tt (self fertilization)

F2 ------ TT Tt Tt tt PUNNETS SQUARE OR CHECKER BOARD:

T tT TT Tt t Tt T

PHENOTYPIC RATIO ----- 3:1 GENOTYPIC RATIO ----- 1:2:1

LAW OF INDEPENDENT ASSORTMENT:-

EXAMPLE: DIHYBRID CROSS:- The cross made between two pure breeding plants, where the inheritance of two different inherited characters are studied at the same time, is called dihybrid cross.

EXPERIMENT:- Seed shape is determined by a single gene that has alleles

llFor round ,R (or) wrinkled seeds ,r The round phenotype is completely dominent over wrinkled. Seed colour can be yellow or green. Again this is determined by two alleles of a single gene. Yellow,Y is completely dominent over green,y . The homozygous pure bread round yellow(RRYY) plant was crossed with another homozygous pure bread wrinkled green(rryy). During gametogenesis, the homozygous genes of each parent plant segregate from each other to produce the gamete with genotype of RY and ry. The RY and ry gamete unit to produce the F1(first filial) generation which is heterozygous with the phenotype of round and yellow and genotype of RrYy displaying the completed dominance of R&Y allele over the r&y alleles. However, when two of the F1 individuals are crossed or self fertilized, each produces four kinds of gametes in equal numbers,RY,Ry,rY,ry. These gamete un it to form 16 combinations in F2(second filial) generation, the phenotypic & genotypic ratio can be studied by Punnete sqare or checker board. The phenotypic ratio is 9 round yellow, 3 round green, 3 wrinkled yellow and one wrinkled green. Phenotypic ratio = 9:3:3:1 Experiment- Parents—YYRR x yyrr Gametes--RY ry F1 -- RrYy(heterozygous) P2—RrYy x RrYy(self fertilisation) G2—RY Ry rY ry F2 Generations- Punnete square or checker board-

RY Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy

Phenotypic ratio- 9:3:3:1 9—round yellow 3—round green 3—wrinkled yellow 1—wrinkled green genotypic ratio—1:2:2:4:1:2:1:2:1 NON MENDELIAN INHERITANCE- INCOMPLETE DOMINANCE:-

MODIFICATION OF MONOHYBRID RATIO (OR) MODIFICATION OF MONOHYBRID 3:1PHENOTYPIC

RATIO

1. Incomplete dominance : Dominant allele fails to mask the phenotypic effect of the another allele in the heterozygous condition incomplete dominance (or) partial dominance.

Eg: four-o-clock plant (Mirabilis jalapa) Flower color trait P1 Red x White RR rr

R Rr (pink flower) F1

F1 Selfed Rr (pink) x Rr(pink)

R r

R RR(red) Rr(pink)

r Rr(pink) rr (white)

R

r

R r

R r

Phenotypic ratio :- Red : Pink : White 1 : 2 : 1Genotypic ratio :- RR : Rr : rr

Eg: Andulasian fowl Black (BB) White (bb) Blue (Bb)

2. Co dominance : In co dominance , due to lack of dominant recessive relationship both the allele have capability to express them phenotypically in the heterozygous condition. Eg: cattle coat colour P2 Red coat x White coat RR rr

Rr (roan) (reddish grey) ( Mixture of red & white hairs ) F1 x F1

R r

R RR (red)

Rr (roan)

r Rr (roan)

rr (white)

R r

Phenotypic ratio:- Red : Roan : White 1 : 2 : 1Genotypic ratio :- RR : Rr : rr 1 : 2 : 1

eg :- blood group

3. Hetero dominance:- The extreme phenotypic expression of F1

hybrid (heterozygote) than their parents is called hetero dominance or over dominance or super dominance.

Eg :- Panicle length Medium x Small (10 cm) (5 cm)

MM mm

Tall (15 cm) MmF1 selfed Mm x Mm

1 - MM (medium panicle) – 10 cm 2 - Mm (tall panicle) - 15 cm

M m

M MM(medium) Mm(tall)

m Mm(tall) mm (small)

1 - mm (small panicle) - 5 cm

Phenotypic ratio :- 1 : 2 : 1Genotypic ratio:- 1 : 2 : 1

Lethal allele :- The genes usually cause death of the organism in zygotic , embryonic stages (or) early stage after the birth. Thus modify the 3:1 phenotypic ratio in to 2:1.

(a)Dominant lethal : ‘Y’ yellow gene ‘incompletely dominant’ , and in homozygous (YY) cause lethality. Eg : ‘yellow’ lethal in mice by Cuenot(1905) F1 yellow x yellow Yy Yy

Y y

Y YY (yellow)

Yy (yellow)

y Yy (yellow)

yy (black)

YY Yy yy Yellow yellow black 1 : 2 : 1 [DIE] eg :- Creeper – shortened deformed leg in chicken

creeper (CC) creeper(Cc) normal(cc)

Recessive lethal : Lethality occurs when the individual carries homozygous recessive alleles. Eg : - Snapdragon Auria or golden – yellowish green plant

F1 Auria x Auria Cc Cc Cc : Cc : cc Auria Auria white 1 : 2 : 1 (lethal) eg :- maize (zea mays) F1 Green x Green Gg Gg GG : Gg : gg Green : Green : white 1 : 2 : 1 (albino/lethal) 3Genic Interaction (or) Non-Allelic Interaction: The expression of a single character by the interaction of more than one pair of gene is called gene interaction or interaction of genes. Bateson & Punnet proposed factor hypothesis to explain gene interaction. According to this hypothesis some characters are produced by the interaction of two or more pairs of factors(genes). Two types:-non allelic gene interaction -allelic gene interaction

The genic interaction occurring between genes located in different locus of the same chromosome or different chromosomes is called as non-allelic gene interaction. The genic interaction between the two alleles of a single locus is known as allelic gene interaction.

GENE INTERACTION:

Factor hypothesis: The expression of an allele of one gene will sometimes alter the expression of alleles of another gene (non-allelic) is called Gene interaction orNon-allelic interaction

William Bateson and R.C.Punnett in fowl comb shape. Domestic breeds Shape of comb

Wyandottes RoseBrahmas Pea

Leghorns Single

Wyandottes and brahmas were crossed and all F1 chickens had Walnut comb, a phenotype not expressed in either parents .

When F1 chickens were mated among themselves (Walnut) in F2 dihybrid ratio was observed (9:3:3:1).

Walnut birds -9Rose -3Pea -3Single -1

Represents:

Walnut (R_P_) two dominant combinations

Single (rr pp) recessive combination.

OBSERVATION MADE:1. F1 progeny differs from those of the parent (that is non rose or pea).2. Two phenotype (walnut and single) not expressed in the original parents appeared in F2. Rose x Pea RRpp rrPP

RrPp walnut F1

F1 crossed

Gametes RP Rp rP rP

RP RRPP W

RRPp W

RrPP W

RrPp W

Rp RRPp W

RRPP R

RrPp W

Rrpp R

rP RrPP W

RrPp W

rrPP P

rrPp P

rP RrPp W

Rrpp R

rrPp P

rrpp s

Walnut -9 (R_P_) -interraction (non allelic). Rose -3 (R_pp) -R>r Pea -3 (rr P_) -P>p Single -1 (rrpp) -r,p recessiveness.

Biochemical basis of Gene Interaction : Generally phenotypic expression of a trait resulted from product of many genes and their interaction with environment. In a biochemical pathway many steps of reaction will occur to convert precursor to end product. Gene 1 G2 G3

Precursor(p) enzyme 1 A enzyme2 B enzyme3 C end product

Any change in gene 1,2,3 or absence of any of these genes , will not produce end product and fails to produce desired phenotype.

Epistasis : Any gene that mask the expression of another non-allele gene is called epistasis. The gene mask the effect of another gene is called epistatic gene (or) suppressor. The gene, which is suppressed by a epistatic gene is called hypostatic gene.

1. Dominant Epistasis: Black dog x White dog iiBB IIbb

white dog F1

IiBb Gene ‘I’ dominant epistatic inhibitor prevent the effect gene B (or) b.Gene B (black) dominant over b (brown). “ Dominant allele of a gene suppress the expression of another gene (either dominant or recessive) produce new phenotype is called dominant epistsis.”

F1 white dog x white dog IiBb IiBb

Gametes IB Ib iB ib

IB IIBB IIBb IiBB IiBb

Ib IIBb IIbb IiBb Iibb

iB IiBB IiBb iiBB iiBb

ib IiBb Iibb iiBb iibb

9 I_B_ : white dog 3 I_bb : white dog 3 ii B_ : black dog 1 ii bb : brown dog

phenotypic ratio: 12 : 3 : 1 white : black : brown

2. Recessive epistasis (or) supplementary gene interaction : Recessive alleles of one gene locus masked the phenotypic expression of alleles of another gene. Due to this gene interaction the dihybrid phenotypic ratio of 9:3:3:1ismodified to 9:3:4.

gamets AB Ab aB ab

AB AABBagouti

AABbAgouti

AaBBagouti

AaBbagouti

Ab AABbagouti

AAbbAlbino

AaBbagouti

Aabbalbino

aB AaBBagouti

AaBbAgouti

aaBBblack

aaBbblack

ab AaBbagouti

AabbAlbino

aaBbblack

aabbalbino

Eg: mice coat colour Albino x Black AAbb aaBB

AaBb- Agouti (grey) F1

F1 x F1 AaBb x AaBb

Ab

ab

black

9 A_B_ Agouti3 A_bb Albino Agouti : Black : Albino3 aa B_ Black 9 : 3 : 4 1 aabb Albino

3. Duplicate genes with cumulative effect (9:6:1):

When two different genes governing same trait come together in dominant condition the effect are get increased . Eg: Pig coat colour Sandy type x Sandy type (partial colour) RRss rrSS

RrSs ( All red) F1

F1 x F1

Gamets RS Rs rS rs

RS RRSSred

RrSsred

RrSSred

RrSsred

Rs RRSsred

RRsssandy

RrSsred

Rrsssandy

rS RrSSred

RrSsred

rrSSsandy

rrSssandy

rs RrSsred

Rrsssandy

rrSssandy

rrsswhite

F2 phenotype: 9 R_S_ Red 3 R_ss Sandy 3 rr S_ Sandy 1 rrss white 9 : 6 : 1 Red : Sandy : White

4. Duplicate gene action : Single character is controlled by two or more non allelic genes

independently. G.H Shull (1914) in Shepherd’s purse weed capsule shape

(triangular and oval). Triangular shape controlled by two dominant genes T and D

independently. Triangular x oval P1 TTDD ttdd

TtDd (triangular) F1

F1 x F1 TtDd x TtDd

gamets TD Td tD td

T D

t d

TD TTDDtri

TTDdtri

TtDDtri

TtDdtri

Td TTDdtri

TTddtri

TtDdtri

Ttddtri

tD TtDDtri

TtDdtri

ttDDtri

ttDdtri

td TtDdtri

Ttddtri

ttDdtri

ttddOval

F2 Phenotypic ratio 15:19 T_D_ -Triagular

3 T_dd -Triangular 3 tt D_ -Triangular 1 tt dd -Oval

Bio chemical basis:

Two different genes(non-allalic) produce enzymes that are able to catalyze the same reaction by duplicate pathway.

enzyme 1(gene1)

Precursor End product enzyme 2(gene 2)

5. Complimentary Gene Interaction (or) Duplicate Recessive Genes(9: 7)

Two different dominant genes complement each other and produce a new phenotype , called Complimentary gene action. Eg. Sweet Pea (Lathyrus odoratus) flower colour by Bateson and Punnett.

White flower x White flower CCpp ccPP CcPp (Red flower) (F1) F1 x F1 CcPp x CcPp

CP Cp cP cp

CP CCPP Red

CCPp Red

CcPP Red

CCPp Red

Cp CCPp Red

CCpp White

CcPp Red

Ccpp White

cP CcPP Red

CcPp Red

ccPP white

ccPp white

Cp CcPp Red

Ccpp White

ccPp white

Ccpp white

Dihybrid F2 ratio modified to 9 : 7 from 9 : 3 : 3 : 1

9 C_P_: Red 93 C_pp:3 cc P_: white 71 cc pp:

Biochemical basis : The pigment anthocyanin is responsible for the production of red colour flower. This anthocyanin is produced from colourless substance called chromogen.

By action of an enzyme the chromogen is converted into anthocyanin. Dominant gene ‘C’ Chromogen (colourless) Dominant gene ‘P’ Enzyme Presence of both ‘C’ and ‘P’ dominant genes the chromogen produced by gene ‘C’ and it converted into anthocyanin by an enzyme produced by gene ‘p’. chromogen enzyme anthocyanin (Red colour) If both the genes are in recessive condition‘c’ and ‘p’ , chromogen and enzyme is not produced. So, absence of any one of the dominant gene the metabolic reaction will not be completed and fails to produce anthocyanin pigment which results in white colour flower.

6. Dominant and recessive interaction (or) Inhibiting Gene action :

Dominant allele of a gene inhibit the expression of another gene irrespective of its dominant or recessive nature .Thus modify the F2 dihybrid ratio of 9 : 3 : 3 : 1 to 13 : 3. Eg : colour of Leghorn type poultry by Bateson and Punnett (1908).

White x coloured IIcc iiCC

IiCc (White) F1

F1 x F1 IiCc x IiCc

IcAb

iCab

Gametes IC Ic iC Ic

IC IICC White

IICc White

IiCC White

IiCc White

Ic IICc White

IIcc White

IiCc White

Iicc White

iC IiCC White

IiCc White

iiCC coloured

iiCc coloured

ic IiCc White

Iicc White

iiCc coloured

Iicc White

Phenotypic ratio – 13 : 3 White : colored

MULTIPLE ALLELE The genes having more than two alternative or allelic forms occupying same locus and controlling single trait is called multiple alleles. CHARACTERS OF MULTIPLE ALLELES: 1.Multiple alleles always occupies the same locus in the chromosome. 2.No crossing over within multiple alleles. 3.Multiple alleles always influence same character. 4.multiple alleles produce series of dominance and recessive interaction. 5.Wild type alleles shows dominance to all other types of alleles.

EXAMPLES OF MULTIPLE ALLELES1. Rabbit coat colour.2. Blood group.

3. Drosophila wings type.4. Self sterility in tobacco.

MULTIPLE ALLELISM IN RABBIT COAT COLOUR Rabbit coat colour is controlled by multiple alleles. There were four varieties of rabbits -Agouti, chinchilla, Himalayan and albino.

1. Agouti : This is wild type rabbit and its body is brownish grey in colour. It is represented by C+ and shows dominant reaction with all other types.

2. Chinchilla : The rabbit is silvery grey in colour represented by Cch mutant allele. It is dominant than Himalayan and Albino.

3. Himalayan : Rabbit shows ears, nose, tips of the limbs are coloured rest of the body is white. It is represented by allele Ch mutant. It is dominant than Albino.

4. Albino: Rabbit shows lack of pigmentation represented by mutant allele ‘C’. It is recessive to all other types.

Dominant reaction of multiple allele C+>Cch>Ch>CPhenotype and Genotype - rabbit coat colourPhenotype Genotype DominanceAgouti C+C+, C+Cch, C+Ch, C+C C+>Cch,

C+>ChC+>C

Chinchilla CchCch, CchCh, CchC Cch>ChCch>C

Himalayan ChCh,ChC Ch>C

Albino CC Recessive to all types

CROSS 1:Eg: Agouti X Chinchilla

C+C+ CchCch

C+Cch (Agouti) F1C+C+, C+Cch, CchCch

1 : 2 : 1

---------------- Agouti : chinchilla

CROSS 2:Eg: Agouti X Himalayan

C+C+ ChCh

C+Ch (Agouti) F1C+C+,C+Ch, ChCh

1 : 2 : 1 ------------

Agouti : Himalayan

CROSS 3:Eg: Agouti X Albino

C+C+ CC C+C (Agouti) F1

C+C+, C+C, CC --------------- 1 : 2 : 1 Agouti Albino

All the above crosses indicates that Agouti type allele is dominant than all other types .(C+>Cch>Ch>C)

CROSS 4:Eg: Chinchilla X Himalayan

CchCch ChCh

CchCh (Chinchilla) F1CchCch:CchCh:ChCh

1 : 2 : 1 ------------ Chinchilla Himalayan

CROSS 5:Eg: Chinchilla X Albino

CchCch X CC CchC Chinchilla F1

CchCch:CchC:CC

1 : 2 : 1 ------------ Chinchilla Albino

The above two crosses confirm that the allele for chinchilla (Cch) is dominant to Himalayan(Ch) and Albino© types

CROSS 6: Eg: Himalayan X Albino

ChCh CC ChC Himalayan F1

ChCh : ChC : CC 1 : 2 : 1

----------Himalayan Albino

This cross suggest that Himalayan type is dominant to Albino.

BLOOD GROUPING INHERITANCE Human blood contains two principal components, 1. cells (red,white,platelets) 2.liquid (plasma) The plasma is composed of clotting protein fibrinogen and serum.When the serum of one individual is mixed with serum of the other person , some cases the red blood cells gets clumped . The clumping or agglutination of red blood cells takes place due to the antigen antibody reaction. This was first observed by landstainer(1900).

ANTIGEN ANTIBODY REACTION Whenever any foreign molecules (micro/macro) (protein) is injected in the blood of man and other higher vertebrates , then the blood will react to eliminate/neutralize that foreign substance from it. Such foreign substances are called antigen (agglutinogens). The antigen may be plant or animal proteins, bacterial or viral toxins. In response to antigen appearance in blood, another protein molecule produced which will superficially

interact with antigen is called antibody (agglutin or immune body).The interaction between antibody and antigen changes form of the antigen and is destroyed, inactivated or eliminated from blood circulation . The antibodies are highly specific for a particular antigen. The types of antibodies are:

1. Acquired antibody: Acquired antibodies are produced by plasma cells only during the time of entry of foreign antigens in the blood.

2. Natural antibody: Natural antibodies are produced by the blood, even in the absence of appropriate antigens.

Eg:ABO blood group in the human being.

ABO BLOOD GROUP IN HUMAN BEING Land Steiner(1900) identified two kinds of antigens called A and B antigens from surface of the red blood cells. Accordingly he recognized three types of blood groups namely A,B,andO.

In 1902, Land steiner’s student Von Decastle and Sturli identified fourth type of blood group called AB.BLOOD GROUPS ANTIGEN(RBC) ANTIBODY(SERUM)A A ANTI-BB B ANTI-AAB A,B NONEO NONE ANTI-A

ANTI-B

The terminal sugar present in the antigen A in galactosamine and in antigen B is galactose.Four blood group have different agglutising properties. To determine the blood group agglutinisation test is performed.

On a glass slide is placed a drop of type A serum (containing anti-B antibodies)a separate drop of type B serum(containing anti-A antibodies).

When a drop of type ‘O’ blood is added to each drop there is no agglutination or clumping of red blood cells in either drops. This shows that ‘o’blood group has neither A nor B antigen

If a drop of type B blood is added , agglutination occurs with type A serum. Type A blood agglutinated with type-B serum

AB red blood cells agglutinated with both serum GROUP ‘O’

GROUP ‘A’

GROUP ‘B’

GROUP ‘AB’

AB red blood cells agglutinated with by both sera.

Group O

Group A

Group B

Group AB

A A

O O AB AB

B BBased on antigen,antibody reaction in blood the possible blood transfusion are:

Blood group antigen antibody accept blood agglutinised from A A Anti-B A or O B, AB

B B Anti-A B or O A,AB

AB A,B none A,B,AB or O None

O none Anti-A O A,B and Anti-B AB

AB blood group persons are called as UNIVERSAL RECIPIENT,Because they can receive blood from all blood groups.

The O blood group persons are called universal donor, because they can donate their blood to any other group. But they can receive from only O group and not from other groups

MULTIPLE ALLELES IN BLOOD GROUP ANTIGENS ORGENETICS OF ABO BLOOD GROUP IN HUMANS There are four types of blood in human being .A,B,O blood group are identified by Landsteiner. AB group was identified by van De Cartelle and Struli . The ABO locus has three types of alleles I A ,IB and IO

IA is dominant over IO IB is dominant over IO

In heterogeneous condition IA and IB are codominant(IA IB) IO is recessive ( IO IO) homozygotes produce no antigen GENOTYPE PHENOTYPE IAIA,IAIO A IBIB,IBIO B IAIB AB IOIO O

If Blood group A ( IAIA ) male carries blood group B (IBIB) the blood group of children are ; IAIO x IBIO

IAIB , IBIO

AB Group , B Group

Other blood group system in human : Apart from A,B,O type many blood groups are reported like Rh,MNS,Lewis,P,Xg, etc. ABO,Rh factor forms to be important for blood transfusion and other types shows little importance.

Rh FACTOR ALLELES IN HUMANS:

The Rh factor was discovered in 1940 by K.Landsteiner and A.S .Wiener in monkey Macca rhesus. Later similar types also identified in human being , besides A and B antigens. There are two groups of human being, the individuals whose blood cells react with Rh antibody are termed as Rh positive and those which do not react are Rh positive. The symbol ‘Rh’ came from the first two letters of the species name of the monkey. The Rh+ person contains Rh antigen present on the surface of RBC, whereas Rh- person does not contain Rh antigen. The Rh antigen has no natural antibodies-however, Rh antibody can be produced artificially. An Rh- person develops Rh antibody when he receives blood from a Rh+ person, even in small quantity(0.05ml).The antibody once formed remains throughout the life. GENETICS OF Rh BLOOD TYPE: The Rh factor is controlled by a pair of alleles R and r R allele produce Rh+ blood type r allelle produce Rh- blood type The Rh+ blood type is found to be composed of several antigens, which indicates possibility of multiple allelism of gene ‘R’. There were two hypothesis proposed to explain inheritance of R gene.1 .WIENER’S HYPOTHESIS: Wiener postulated a number of (around eight) multiple allelles at single locus for ‘R’ gene.2.FISHER’S HYPOTHESIS: Fisher rejected the Wiener’s multiple allelism for ‘R’ gene. He proposed that at least three pairs of pseudoalleles remain closely linked to each other and inherited as block (CDE/cde), due to possible recombination presence of anyone of the dominant gene ‘D’ produce Rh+ type. All recessive ‘cde’ gene produce Rh- type. This hypothesis was widely accepted . ERYTHROBLASTOSIS FOETALIS: If a woman is Rh- and her husband is Rh+ , blood from the foetus may pass through the placenta into the maternal blood stream and stimulate the formulation of antibodies (anti-Rh antibodies). The first born baby would not have any affect due to Rh factor. Then, when this women becomes pregnant a second time, some of these antibodies may pass through the placenta into the child’s bloodstream and cause clumping of its red blood cells, this condition is called erythroblastosis foetalis. More frequently the

baby is born alive but dies after birth. In extreme cases so many RBC are destroyed and the baby dies before birth.

Unit-II

CHROMOSOME: Chromosomes are the rod shaped dark stained bodies seen during metaphase stage of mitosis , when the cells are stained with a suitable basic dye and viewed under a light microscope. It has two sister chromatids joined by centromere and four arms. Chromosomes were first described by Strasburger (1875) Term chromosome was first used by Waldeyer (1888). Chroma = colour , Soma = body

CHROMOSOME NUMBER: Each species has a constant somatic and gametic chromosome number. Somatic chromosome number is the number of chromosomes found in somatic (vegetative) tissues of a species, represented by 2n (two copies of each chromosomes).

The two copies of a chromosome are ordinarily similar morphology, gene content and gene order called homologous chromosomes. Gametic chromosome number is one half of the somatic number, is represented by (n) and it is number of chromosomes found in the gamete of a species. Somatic Gametic Pea (Pisum sativum) 14 7 Rice (oryza sativa) 24 12 Maize (zea mays) 20 10 Fruitfly(Drosophilamelanogaster) 8 4 Man(Homo sapiens) 46 23

CHROMOSOME SIZE: The chromosome size shows variation depending up on the stage of cell division. The size of mitotic metaphase chromosomes of various animal and plant species generally varies between 0.5micro meters and 32 micro metres in length and between 0.2micro metres and 0.3 micro metre in diameter . In general plants have longer chromosomes than animal. The species have lower chromosome number have longer chromosome than those of higher chromosome number .

EUKARYOTIC CHROMOSOME STRUCTURE OR MORPHOLOGY: The meiotic metaphase is ideal stage to study chromosome morphology,The structural features are1,Chromatid2. centromere3.Telomere4.Secondary structure

CHROMATID:

The chromosome is longitudinally divided into two identical parts known as chromatid. Two chromatids are joined by centromere. The chromatids of a chromosome formed by replication and single chromatid is referred to as sister chromatids. Chromatids of the homologous chromosomes are referred as non-sister chromatids.

chromatids centromere Telomere

CENTROMERE: The region where two sister chromatids of a chromosome joined together is known as centromere .It appears as constriction (narrow region) in chromosome is termed primary constriction, The centromere helps in the movement of chromosomes in anaphase, hence it is known as kinetochore.

TELOMERE: The two ends of a chromosome are known as telomeres. Telomeres are highly stable and they do not unite with telomeres of other chromosomes.

SECONDARY CONSTRICTION: In some chromosomes, a second constriction is present in addition to primary constriction (centromere), the additional constriction is known as secondary constriction.

The DNA generally present near telomere is known as satellite chromosome.

Secondary consrtiction

Based on position of the centromere the chromosome divided into four classes:1. metacentric2. sub metacentric3. acrocentric4. telocentric

In metacentric chromosome , centromere is located in the centre of the chromosome. It appears ‘v’ shaped during anaphase .

In sub metacentric chromosome the centromere is located on one side of the central point

In acrocentric the centromere located close to one end of the chromosome.

During anaphase the sub metacentric and acrocentric appears as ‘j’ shaped. In Telocentric chromosome the centromere is located in one end.

Metaphase:

Anaphase:

KARYOTYPE: Karyotypes are presented by arranging the somatic chromosomes in a descending order of size keeping their centromere in a straight line. Thus the longest chromosome is placed on the extreme left and the smallest on extreme right.

IDIOTYPE: The karyotype of a species may be represented diagrametrically showing all the morphological features of chromosomes is called idiotype. Idiotype generally represented with haploid chromosome.

EUCHROMATIN: The part of a chromosome which take deep stain during cell division but do not stain during metaphase. Euchromatin is genetically active parts of chromosomes.

HETEROCHROMATIN: The parts of chromosome (or chromatid) which take up dark stain during interphase is genetically inactive. These region usually contains repetitive DNA sequences which rarely transcribed.MODELS OF CHROMOSOME STRUCTURE: Two different models of chromosome structure based chromatin fibres1.Multistrand model2.Foled fibre modelFOLDER FIBRE MODEL: Proposed by DuPraw(1965) and is widely accepted. According to this model, chromosomes are made up of chromatin fibres , with average diameter of 230A . Each chromatin fibre contains only one DNA double helix which is in a coiled state.This coiled DNA is coated with histone and non histone proteins. The coils of DNA are stabilized by proteins and divalent cations (ca++ and mg++).

Each chromatid contains a single long chromatin fibre.The DNA of this chromatin fibre replicates during interphase producing two sister chromatin fibres.

ORGANISATION OF CHROMATIN FIBRES: Two models of chromatin fiber structure have been proposed1. coiled model – Dupraw2. nucleosome-solenoid model

NUCLEOSOME-SOLENOID MODEL: Proposed by Kornberg and Thomas (1974) and is most widely accepted model. The eukaryotic chromatin is composed of a repeating unit called nucleosome. One complete nucleosome consist of 1. nucleosome core2. linker DNA3. Average of one H1 histone molecule4. other associated chromosomal proteins

The nucleosome core of a histone octamer composed of two molecules of each histone (H2a,H2b,H3&H4)A 146bp long DNA molecule coiled around this histone octomer.

The size of linker DNA varies from 8bp to 114bp depending on species.The linker DNA forms the string part of the chromatin fibre. The nucleosome fibre super coiled.The super coiled nucleosome fibre is called solenlid and may be stabilized by H1

histone.

PROKARYOTIC GENOME ORGANISATION:BACTERIA: In Bacteria the nuclear material is not separated from the cytosol by membrane as it is eukaryotic cells. However, the nuclear material is usually concentrated in a specific region of cell called Nucleoid. During cell division , nucleoid DNA becomes adhered to plasma membrane and is described to the daughter cells with out formation of observable chromosomes. Nucleoli are not presenting the nucleoid.The nuclear membrane is found in a highly condensed state called the “folded genome”. In this state the single DNA molecule is arranged in several loops or domains , each of which is highly twisted or super coiled.This structure is associated with RNA and proteins (Histone like protein).

VIRAL GENOME:

All viruses or virions are extremely small, they are diverse in size and in organization. Generally, viruses range in diameter (length) from about 20 to 200nm. Most virions are either rod shaped or guasi spherical and contain a nucleic acid core surrounded by a specific geometric array of protein molecules that form a coat or capsid. Many animal viruses and in some plant viruses, a lipoprotein envelope surrounds the capsid. The nucleic acid is generally DNA in living organisms except some RNA viruses where the genetic information is stored in RNA. Replication of the genetic material in viruses takes place in the host cells.

NON SENSE MUTATION: A substitution (or) change of base in DNA due to mutation leads to generation of one of the stop codon , which will result in premature termination of translation of a peptide chain is called non sense mutation.

METHODS OF CHROMOSOME ANALYSIS:step1: chromosome preparationStep2: chromosome banding

Step3: karyotype analysis

1. PREPARATION OF A KARYOTYPE (CHROMOSOME PREPARATION): 5 ml venous blood

Add phytohaemagglutelin and culture medium Culture at 370c for 3 days

Add colchicine and hypotonic saline

Cell fixed & spread cells in slide by droping

Digest with trypsin and stain with Giemsa

Analyse metaphase spread

Develope karyotype

2. CHROMOSOME BANDING: Several staining methods can be utilized to identify individual chromosomes. First arrest spindle fibre formation using colchicine at metaphase.

1. G (Giemsa) banding:- This is most commonly used method. The chromosomes are treated with trypsin which denatures their protein content and then stained with a DNA binding dye known as Giemsa which gives each chromosome a characteristic and reproducible pattern of light and dark bands.

2.Q(Quinacrine) banding:- This gives a banding pattern similar to that obtained with Giemsa , and requires examination of the chromosomes with an ultraviolet fluorescent microscope.

3.R(Reverse) banding:- The chromosome are heat denatured before staining with Giemsa , this results in light and dark bands which are the reverse of those obtained using normal G banding.

4.C(Centromeric heterochromatin) banding: If the chromosome are pretreated with acid followed by alkali prior to G banding , the centromeres and other hetero chromatic regions containing highly repetitive DNA sequence are preferentially stained.

KARYOTYPE ANALYSIS: KARYOTYPE: Arrangement of metaphase chromosomes in a sequence according to length and position of the centromere.

Karyotype analysis involves first counting the number of chromosomes present in a specified number of cells, something reffered to as metaphase spreads, followed by careful analysis of

the banding pattern of each individual chromosome in selected cells. Usually the total chromosome count is determined i.e., 10 to 15 cells of high quality banding i.e., metaphase spreads.(30 or more & mosaicism suspected) The banding pattern of each chromosome is specified and can be shown in the form of a stylized ideal karyotype known as an ideogram. The karyotype (or) karyogram will show each chromosome pair in descending order of size.

EXAMPLE:HUMAN KARYOTYPE: Denver adopted a system for classifying and identifying human chromosomes. The 22pairs of autosomes were numbered in descending order of length and classified according to the position of the centromere as metacentric sub- metacentric and acrocentric chromosome. Human Karyotype

Denver report Description

1.Group 1-3 Large chromosome with appro- ximately median centromeres.2.Group 4-5 Large submetacentric chromosome. 3.Group 6-12 Medium sized submeta centric chromosome.4.Group 13-15 Large acrocentric chromosom.5.Group 16-18 No.16 ,Metacentric,no.17-18, are small submetacentric chromosome.6.Group 19-20 Small metacentric chromosome7.Group 21-22 Short acrocentric chromosomes

8.Sex chromosomes X and Y

SPECIAL TYPES OF CHROMOSOMES:

Polytene Chromosome: (Giant chromosome):

It is present in certain salivary gland tissues of Dipterian flies(eg.Drosophilia).Extremely large chromosomes are present in the nucleus of the interphase cells in these tissues .In the nucleus many extra replication of a single chromatid or DNAstrand takes place producing thousands of replicates .This type of replication is called as “Endo replication”.All replicates or chromatids of the same chromosome are lined up together in a parallel fashion. This type of parallel duplication is called as “Polyteny” which produces very thick chromosomes .Staining of these chromosomes produce distinct banding patterns. The dark bands consists of chromomere and the interband(lightband)consists of chromonemata. Some bands appear to be swollen along the length of the chromosome.These swollen regions are called as “Puffs or Balbiani rings”.In this region the segment of the chromomere is in the highly extended state and it represents the region of active transcription.

LAMPBRUSH CHROMOSOME:

They are present during the prophase-1 of oogenesis in some vertebrates, mainly amphibians with large yolky eggs.They store large amounts of proteins in the eggs.The lampbrush chromosomes are 800micro.m long having long lateral loops giving a hairy “Lampbrush”appearance.The homologous chromosomes pair up and each chromosome produce 2 chromatids at the lampbrush stage.The structure of lampbrush chromosome consists of a central axial region with numerous pairs of lateral loops. The central axial region consists of 2 chromatids which are highly condensed.The lateral loops represent the transcriptionally active region.Each pair of lateral loop arise from a single chromomere.The loop length varies within asingle chromosome.At the end of meiotic prophase-1 loops begins to disappear and the chromosomes contract and attain the usual small size in metaphase.

SEX DETERMINATION

Sexually reproducing organisms may be classified into :

1. Monoceious:Both male and female reproducing organism produced in single plant

and in different flowers. Eg: maize

2. Hermaphrodite: Both male(androecium) and female(gynoecium)reproducing organs are produced in single flower Eg: paddy

3. Dioecious: Male and female gametes are produced in different individuals.

Eg: papaya Primary sexual characters:

The characters formed during birth such as sex cells , reproductive organ that differentiate sex.

Secondary sexual characters:

The secondary sexual characters are developed during course of development.

Mechanism of sex determination :

i. Genetically controlled mechanisms:

a.Sex chromosomes

1)heterogametic male 2)heterogametic female

Eg : human Eg: bugs

b. Gene balance Eg: Drosophilac. Male haploidy or Haplodiploidy Eg: honey bee d. Single gene effects Eg: neurosporaii. Metabolically controlled sex determination Eg: pigeons

iii. Hormonally controlled sex determination Eg: higher animals

iv. Environmentally controlled sex determination Eg: bonellia

In case of dioecious organism there are two types of chromosomes:

1) Autosomes: The chromosomes and their genes determines the somatic characters of the individuals are known as “autosomes” (A) and they do not have any relation with the determination of sex.

2) Sex chromosomes : The chromosomes which are responsible for the determination of sex are known as “sex chromosomes”.

Eg: X and Y chromosomes

CHROMOSOMAL SEX DETERMINATION : In dioecious diploid organisms following two systems of sex chromosomal determination have been recognized ; i)Heterogametic males. ii)Hetrogametic females.

Heterogametic males : In this type , female has two ‘x’ chromosomes and male has one ‘x’ and one ‘y’ chromosomes.

During gametogenesis female produce only one type of gametes containing one ‘x’ chromosome . sex produce similar type of gametes called Homogametic sex

Incase of male it produce two types of gametes carrying x and y chromosomes in equal proportion. The sex which produces two different types of gametes in terms of sex chromosomes called Heterogametic sex

TYPE: XX-XY:

Eg: man other mammals

Female is having homomorphic X chromosomes (XX)Male is having heteromorphic sex chromosomes produce X and Y type of gametes. So,

sex of the gametes or individuals are determined by gametes produced by males.

MALE FEMALE

XX-XO TYPES :

Eg: Bugs and grasshoppers Female and homogametic and is having two X chromosomes (XX) Male is having only one ‘X’ chromosome refered as ‘XO’ and it produce

two types of gametes ,half with ‘X’ chromosome and alf without ‘X’ chromosome. Type of sex is determined by the male gametes.

HAPLOID – HAPLODIPLOIDY MECHANISM : Eg: bees, wasps ,ants

The male is determined by unfertilized haploid and female is developed from fertilized egg(2n)

In this case meiosis is normal in female and no spermatogenesis in males due to their haploidy (n)

Among diploid female, the female fed by royal gelly developed into queen and other females acts as workers .

ENVIRONMENTAL SEX DETERMINATION : Eg: bonellia viridis – marine worm

The larvae are released by female into water . The larvae reared in isolation and develop into female. Some larvae swim and contact with female proboscis develop into a male.

The female produces a hormone like substances that influence the larvae towards maleness.

SEX CHROMOSOME

The sex chromosome ( X and Y ) are of unequal size, shape (heteromorphic) and show difference in staining duality.In man and drosophila the ‘X’ chromosomes found to be straight, rod like and comparatively larger than ‘Y’ chromosome.‘Y’ chromosome of man and drosophila are smaller than X chromosome , however in ‘Y’ chromosome of drosophila , one end remain slightly curved or bent to one side. The inheritance of ‘X’ an ‘Y’ linked gene is called sex linked inheritance .The inheritance of ‘X’-linked genes called X-linked inheritance. The inheritance of ‘Y’-linked genes is called ‘Y’-linked inheritance .

Examples for sex linked inheritance: i) Colour blindness ii) Haemophilia

iii) Eye colour in drosophila iv) Hypertrichosis (hair in the ear pinna) v) Ichthyosis hystrix (scales on skin)

INHERITANCE OF ‘Y’-LINKED GENES :

The genes present in the differential region of Y chromosomes are called “Holandric genes” (Holos-whole; andros-male) . The Y linked genes inherit directly male to male (father to son) not to female (daughter).

Eg: Hypertrichosis in man Ichthyosis (scales on body ) Hypertrichosis leads to development of excessive hairs in the pinna of ear .

INHERITANCE OF X –LINKED GENES :

The genes are present in ‘X’ chromosome’s differential region . It represented twice in female (XX) and once in male (XY). The colour blindness is a sex linked character discovered by Wilson in 1911 . It is a hereditary disease , affected person cannot distinguish red colour (protonopia) and green colour ( deuteronopia).

Colour blindness is governed by recessive gene/character . The recessive genes present the proper development of colour sensitive cells in the retina. The genes for colour blindness located in X-chromosome and absent in Y-chromosome.

GENETICAL NATURE:

Some portion of X and Y chromosomes are identical having homologous genes called homologous regions, remaining region of X and Y chromosomes have different types of genes called Non-homologous or differential regions. During meiosis pairing and crossing over occurs only in homologous regions of X and Y chromosomes. So genes present in this region do not inherit along with their genes due to crossing over. Such genes are called partially or incompletely sex linked genes. The genes reside in differential or non-homologous regions of X and Y chromosomes always inherit together. Because of the differential regions of X and, they do not undergo crossing over. Such genes are called completely sex linked genes.

The completely sex linked genes may be of the following types:

1.Holandric genes: The genes which remain confined to differential region of Y chromosomes only are called holandric genes or Y-linked genes . The Y-linked genes inherit along with ‘Y’-chromosome and they phenotypically express only in male sex .

2.Sex linked genes : The genes which present in the differential region of X-chromosome only are called sex

linked (or) x-linked genes. The phenomenon of inheritance of Y-linked and X-linked genes is called sex-linkage. The characters controlled by the genes located on sex chromosome (other than sex

characters ) are called sex linked characters. Sex linked inheritance was first discovered by T.H.Morgan in 1910 in drosophila eye

colour . Male has only one gene on ‘X’ chromosome for colour blindness, presence of the gene for

a character is called Hemizygous. This character is common in male but rare in female . Colour blindness character follows typical pattern of inheritance from father to grand son

and it appears only in alternate generations. This is called criss-cross inheritance. This character is never transmitted from father to son. The female carrying one recessive gene (XCXc ) is called carrier.

The carriers are normal in their vision.

Similar pattern of criss-cross inheritance was observed in haemophilia in man and eye colour in drosophila.

Example 2:HAEMOPHILIA (Bleeder’s disease): Discovered by John Cotto (1803). This disease is characterized by delayed blood clotting

due to the absence of anti-haemophilic globulin which plays major role in blood clotting. In Normal person blood clots within 2 – 8 minutes , whereas in haemophilia person the

clotting is delayed ( 20 min – 24 hours). This ‘X’ linked recessive gene appeared as mutant in Queen Victoria and transmitted to

her decendants. This disease is common in Royal families of England and Russia. This disease is also called as Royal disease.

Example 3 : Similar inheritance of X linked recessive gene was observed in Drosophila for eye colour. Red (normal) female (XWXW) Red (carrier) female (XWXw) Red (normal) male (XWY) White male (XwY) SEX-LIMITED GENES:

Sex limited genes express characters in only one sex. The genes may be located on any chromosome. Their expression in vertebrate is governed by the sex hormones .

Eg 1: In man the beard is produced by sex limited gene. Woman normally donot have beared inspite of presence of beared gene .

Eg 2: Milk production in cattleEg 3: plumage pattern in fowl

In domestic leghorn fowl, male have long curved , fingered feathers on tail and neck(cock feathered). Female are shorter , straighter, without fringe (hen feathered).

H-(dominant) allele –Hen feathering h -(recessive ) allele –Cock

But their expression influenced by sex hormone,

GENOTYPE MALE FEMALEHH Hen feathered Hen featheredHh Hen feathered Hen featheredHh Cock feathered Hen feathered

SEX INFLUENCED GENES The sex influenced gens are influenced by the bearer. They are located on

autosomes. The sex influenced genes express more frequently in one sex than in other

Eg:baldness in man This character dominant in man and recessive in women Heterozygous (Bb) the females are normal inspite of presence of dominant allele and express in male. In male gene for baldness can operate in presence of male

harmone

GENOTYPE PHENOTYPEMALE FEMALE

BB bald BaldBb bald NormalBb normal normal

Eg: horned condition sheep

UNIT-III

LINKAGE

Definition:Genetic marker located on the same chromosome thus tend to remain together

during sexual reproduction . That is they do not exhibit independent assortment. Such genetic marker are said to be linked, and the phenomenon, or transmission pattern, of linked genes is called Linkage.

Genetic markers are said to be linked whenever over 50% of the gametes produced contain Parental combinations of the markers and the less than 50% of the gamets contain recombinant combinations of the markers. There are rare cases when genes located on different chromosomes exhibit linkage and fairly common cases when genes located on the same chromosome assort independently.

The effect of linkage were first evident in the results of a dihybrid cross in sweet peas that were reported by W. Bateson and R.C. Punnett in 1906. However they did not interpret their esults in erms of the behavior of genes located on the same chromosome. T.H. Morgan was the first to relate linkage to the segregation of homologous chromosomes and the occurrence of crossing over between homologous chromosomes during meiosis. Many of our current concepts about linkage , crossing over and chromosome mapping have evolved from the work of Morgan and his students C.B. Bridges, H.J. Muller and A.H. Sturtevant.

ARRANGEMENT OF LI NKED GENES

1.Coupling phaseThe two dominant or recessive genes are coming from same parent enter in to the

same gamete and inherit together for many generations called coupling phase linkage. In this case two dominant genes located on one chromosome of homologous pair and two recessive genes located in other pair of homologous chromosome. This type of arrangement is called cis arrangement.

2.Repulsion phase:The dominant or recessive genes coming from two parent tend to separate each

other and enters in to the different gametes called repulsion phase linkage. In this case one dominant and recessive genes located on same chromosome of homologous pair.This type of arrangement is called trans arrangement.

The term coupling and repulson was coined by Bateson and punnett(1906).

Example: Sweet pea →Blue (B)>Red(b)

Pollen shape→long type (L)>round type (l)

Coupling phase:Blue long type X Red round

The F1 resulted is Blue long type and then the test cross resulted in 7:1:1:7 with more of parental type.

The blue long type is crossed with red long resulted in blue long in F1.The F1 is test crossed with recessive parent it produces possible recombination and resulted in 1:7:7:1 with more of parental type.

FACTORS AFFECTING THE LINKAGE1. Distance between the genes:

Closely linked genes shows strong linkage while genes widely located shows deep linkage.2. The factors like age, high temp, X-ray treatment decrease the strength of the linkage.

Linkage groupThe genes or loci on one chromosome comprise a linkage group. One linkage

group corresponds to a pair of homologous chromosomes. The number of linkage group corresponds to a number of pairs of chromosomes in a species.SSyntenic

The genes on the same chromosome whether they show linkage or not they are called as syntenic groups.

Recombination:Recombination is production of gene combination which are found in the parents.

It is produced by:(1) Assortment of Non-homologous chromosome.(2) By crossing over between homologous chromosomes during meiosis.

For linked genes the frequency of recombination can be used to estimate the genetic map distance. The frequency of recombination between two loci is between 0-0.5. It depends on how closely the loci are linked to each other on the chromosome.

CROSSING OVER

Definition:It is a process by which the party of homologous chromosomes are

interchanged and crossing over takes space during prophase-1 of meiosis(tetrad stage).The cross-shaped structure in which the two of the four chromatid of homologous

chromosomes were appear to exchange the parts.it is detached in cytological studies of meiosis in many organisms.These cross-shaped structures were first detected in amphibians by F-janssens and these structures are called chiasmate.

Features of crossing over

The losi of genes on a chromosome are arranged in a linear sequence. The two alleles of a gene in a heterozygote occupy corresponding positions in

homologous chromosomes. Crossing over involves the breakage of each of two homologous chromosome and

exchange of parts takes place. Chromosome with recombinant combinations of linked genes are formed by the

occurance of crossing over in the region between the two loci. The probability that crossing over will occur between the two loci increases with

increasing distance b/w the two loci on the chromosomes.

Experiments on crossing over

1.Stern’s experiment -Drosophila2.Creightons Mc clintock experiment - Maize1.Stern’s experiment:Stern’s experiment is on cytological basis of crossing over. Stern crossed a female with 2x chromosomes and the 2x chromosomes are morphologically distinguishable. 1X chromosome was having a short segment of Y chromosome attached to it.

The other ‘X’chromosome was a sort chromosome in which the segment has been translocated to chromosome four.

Observed results:Stern observed

1. Round carnation eyes with normal length chromosomes.2. Round shaped red eyes with short ‘x’chromosome with attached

‘y’segment. Stern observe males with round carnation, normal length ‘x’ chromosome and males with car shaped red eyes with short ‘x’chromosome along with the attached ‘y’ segment.

POSTREPLICATION TETRAD STAGE

Proof that crossing over occurs after duplication of chromosomes. This proof can be obtained by studying the fungi of the class known a. Ascomycetes bread mould Neurospora crassa has been particular importance in genetics studies.In most organisms such as drosophila a maize one cannot recover and analyse the genotypes of all four haploid products of single meiotic event.

One has to perform test cross. But in case of Neurospora crassa, one can abe to isolate x determine the genotypes of all four products of meiotic events. The data from cross in which the genotypes of all the products of meiosis have been determined re called tetrad data.

Asexual reproduction occurs by mitotic division of haploid cells to form spores called conidia. Hyphal fussion can also occur between mycelia. If the fussion occurs between cells with nuclei are genetically identical. The resulting cells are called Homokaryons. If the nuclei are of two different genotypes the resulting cells are Heterokaryons.

Neurospora crassa undergoes sexual and asexual reproduction. During sexual reproduction the products of meiosis are maintained in a tube like structure called as Ascus. Each ascus contains four pairs of ascospores, with each pair being identical twins.

An analysis of tetrad data in which pair of allele of two genes located at same chromosomes shows that crossing over occurs after replication in the four strand or tetrad stage. If the crossing over occurs prior to replication that is two strand all the products of meiotic event would have recombinant gametes.

Before Chromosome Replication

If the crossing over occurs after replication in the tetrad stage, only two of the four products of each meiotic division will be recombinant. The other two products will have parental combinations.

.

The results of analysis of four pairs of ascospores in neurospora crassa clearly indicates that crossing over occues after replication because 50% of recombinant ascospores and 50% parental ascospores.

Different types of crossing over (No.of strands involving)The double crossing over can occur in three different ways.

1. The two strand double crossing over2. Three two strand double crossing over3. Four two strand double crossing over

1. The two strand double crossing overIt occurs when both crossovers involve the same two chromatids.

2. The three strand double crossing overCrossovers are those in which the second crossover involves one of the same two

chromatids as the first crossover plus one different chromatid. Three strand double crossovers can occur in twice as many as two strand or four strand crossovers.

3. The two strand double crossing overIt occur when the second crossover involves the two chromatids not involved in

the first crossover.

GENE MAPPING OR LINKAGE MAP

The F1 double heterozygotes produced over 50 % of g amets in parental combination and less than 50 % of gamets contain recombinant. This result is contrast to the prediction of independent assortment. The recombination of linked genes produced by crossing over.

Linkage groups- each linkage group corresponding to one of the pairs of homologous chromosomes in the genoma of that species.

Example:Drosophila melanagester-4 linkage groups corresponding to 4 pairs of chromosome.

Maize-10 linkage groups (2n=20)Mouse-20linkage groups (2n=40)

An important features of all linkage maps is their linearity, all genes in given linkage group can be shown to map in a linear array.

TWO POINT CROSSRecombinant combination of alleles of two linked genes are produced by crossing

over in the nterval between the segregating loci. The rational behind the genetic mapping is hat the probability of a crossover occurring between two loci is a function of the length of the interval separating the loci.

A.H. Sturtevant suggested that frequency of recombinant gametes produced be used as an index of the distance between two loci on a chromosome.

Linkage MapA linear or circular that shows the relative position of genes on a chromosome as

determined by genetic analysis.1map unit = 1 percent recombination

Example :Long wing / Gray body x Short wing / Black body

P1 = Vg+ b+ vg b

↓F1 = Vg+ Vg b+ b (Longwing graybody)

F2 = Long wing – 415Short black – 405Short gray - 92Long black - 88

Recombinant frequency = 180/ 1000 = 0.18

Percentage of C.O = 18 or 18 cMGenes vg (short wing) and b (black body) in Drosophila yield 18 % recombinant

test cross progeny. So both the genes are placed 18 map units apart on linkage group. Therefore, the linkage map distance by the frequencies of crossing over or recombinant chromosome produced during meiosis.

Property of additivity:The P & Q are linked with 8 map units and P & R are linked and are 3 map units

apart.

P----3cM---------R----------------------------------Q----------------------8cM-----------------------------

0r

R----3cM---------P-----------------8cM--------------------------------Q

---------------------------------11cM------------------------------------

The maximum crossover frequency that can be result from cross over between linked genes is 50%.

THREE POINT CROSS:A cross in which three pairs of alleles are segregating. It is used to detect double

crossover which is not recognized in two point cross. It also used to detect the order of genes on the chromosome.

In this experiment first homozygous are crossed to produce triple heterozygous or trihybrids (ABC/abc) then the trihybrid are test crossed to homozygous recessive (abc/abc). If there is no linkage it produce 1:1:1:1:1:1:1:1.

Example:Three point cross in Drosophila melanagaster.

Cu/cu+ = Curled versus straight wingse/e+ = Ebony versus gray bodyst/st+ = Scarlet versus red eyes.

P1 = cu e st (curled,ebony& scarlet) X cu+ e+ st+ (straight,gray&red)(homozygous) (homozygous)

↓F1 = cu+ cu e+ e st+ st X cu e st/cu e st

(heterozygous) (homozygous)

F2 =

Genotypes No. of progenyCu e st+/ cu e st 366Cu+ e+ st / cu e st 380Cu e st / cu e st 24Cu+ e+ st+ / cu e st 30Cu+ e st / cu e st 89Cu e+ st+ / cu e st 105Cu e+ st / cu e st 2Cu+ e st+ / cu e st 4Total 1000

Determining order of the genesBy little practice you will order the three gene loci by simply looking at the

parentel and double crossover genotypes. Based on observed double crossover the middle gene can be identified from three possible gene order. In this case the middle gene is cu/cu+ the order is st-cu-e.

Calculating between st & cuThe recombinant genotypes are st cu & st+ cu+

Distance between } = ( Single crossover Double crossover )st & cu Frequencies + frequencies

------------------------------------------------ X 100Total

= (24 + 30 + 2 + 4)-------------------- X 100 = 6 mapunits

1000

Distance between } = ( Single crossover Double crossover )cu & e Frequencies + frequencies

------------------------------------------------ X 100Total

= (89 + 105 + 2 + 4)--------------------- X 100 = 20 mapunits

1000

st------------------cu----------------------------------------------------------e6cM 20cM

Interference :Fewer double crossovers are observed than expected crossovers, if the crossovers

are independent then the phenomenon is called chromosome interference or chiasma interference. This was first observed by A.H. Muller (1916). The degree of interference is measured by the coefficient of coincidence (cc).

Coefficient of coincidence = Observed double crossover frequency--------------------------------------------Expected double crossover frequency

The coefficient of coincidence is 1 indicates no interference. If the coefficient of coincidence value between 0-1 indicates positive interference. So the interference is the occurrence of one crossover reduces the occurrence of another crossover nearby.

CC + interference = 1

Example:

Observed double crossover frequency = 0.005

Expected double crossover frequency = 0.015

0.005CC = ------ = 0.33

0.015

Interference = 1- 0.33 = 0.67

In certain microorganism (bacteriophage) coefficient of coincidence is greater than 1 thus indicates that the occurrence of one crossover increases the likelihood of additional crossover occurring nearby. This type of interference is called Negative interference.

SOMATIC CELL HYBRIDIZATION AND GENE MAPPING SOMATIC CELL HYBRIDIZATION was first demonstrated by G.BARSKS and colleagues in 1960 ,in mouse cells. Somatic cell are diploid body cell or non gorm live cells of an organism . SOMATIC CELL HYBRIDIZATION is fusion of somatic cells which are growing in vitro. Cell fusion produces binucleate hybrid cells (heterokaryons) which is followed by nuclear fusion to produce uninucleate hybrid cells called as synkaryons . normally the spontaneous cell fusion frequency is very low (1 cell fusion per million cells).the frequency of cell fusion can be increased by addition of 1)uv inactivated sondai virus 2) by using the chemical polyethylene glycol . 3)by using electro fusion These agents stimulate cell fusion by increasing 1) the cell contact and 2)by altering cell membranes Example Human karyotype Denver adopted a system for classifying and identifying human chromosomes. The 22 pairs of autosomes were numbered in descending order of length and classified according to the position of the centromere as metacentric,sub metacentric,and acrocentric chromosome. Denver report Description 1)group 1-3 2)group 4-5 3)group 6-12 4)group 13-15 5)group 16-18 6)group 19-20 7)group 21-22 8)sex chromosomes x & y

Large chromosome with approximately median centromeres. Large sub metacentric chromosome Medium sized submetacentric chromosome Large acrocentric chromosomes No.16 –metacentric ,no-17,18 are small submetacentric chromosomes Small metacentric chromosomes Short acrocentric chromosomes

TYPES OF SOMATIC CELL HYBRIDS 1)MONOSPECIFIC HYBRIDS:- when the two cells of the same species are fused they are called as monospecific hybrids. 2)INTERSPECIFIC HYBRID:- when 2 cells from different species are fused they are called as interspecific hybrids. MOUSE – HUMAN SOMATIC CELL HYBRIDS

-