CHROMOSOMAL

MAPPING

INTRODUCTION Number of genes exceeds the number of

chromosome in different species ,many genes located on same chromosome

Number of inklage groups directly proportional to bivalents of species

Sometimes crossing over occurs ,closely linked genes have less chance of departure

Each gene has definite order and location in a linkage group or chromosome, as the crossing over frequency has found to be constant

Eg. in Drosophilia,3 genes(white eyes, yellow body and cut wings)white and yellow always 1% cross over frequency

Thus %crossing over appears to be closely related to physical distance between genes

LINKAGE MAP All genes, linkage groups and their number is

known then by using crossing over as the tool, relative distance between the genes in a linkage group and their order could be determined

This may give diagrammatic representation of chromosome showing the genes as points separated by distances proportional to amount of crossing over

This is said to be genetic map, cross over map or linkage map

LINKAGE MAP CONSTRUCTION

Determination of linkage

groupDetermination

of map distance

Determination of gene order

DETERMINATION OF LINKAGE GROUP

By hybridization experiments b/n wild and mutant strains

Helps to determine how many phenotypes remains completely linked and consequently their genes during the course of inheritance

Different linkage groups of a species could be worked out

DETERMINATION OF MAP DISTANCE

It is measured using map unit or Morgan unit

One map unit or 1 cM = 1 % cross over(recombinants)

can be calculated using the formula,

Total no of recombinants produced in coupling phase or repulsive phase/total number of gametes formed in coupling phase or repulsive phase *100

Each chiasma=50% cross over .if mean chiasma is known for a chromosome pair ,then the map distance would be =mean number of chiasma*50

WHY RECOMBINANTS CANNOT EXCEED 50 % ?

Even if every meiocyte had a crossing over between two linked genes only 2 /4 chromatids in a bivalent was involved in crossing over(2/4=50 %)

TWO POINT TEST CROSS

In which f1 hybrid crossed with a double recessive parent

Crossing over at two points ,so called two point test cross

Acac*acac

F2 hybrid 37 % ACac 37 % aCac of which 26 % recombinants aCac and ACac.

26 Cm

DISADVANTAGES OF TWO POINT TEST CROSS

Underestimate map distance when >10cM

(double crossovers cancel each other)

Provide no info of relative position of two linked genes

Do not allow detection of double cross over

THREE POINT TEST CROSS

It involves three gene

It helps to know gene order or gene sequence

Two single cross over and double crossing over occurs between different genes

Genotype Observed Type of Gamete

ABC 390 Parental

abc 374 Parental

AbC 27 Single-crossover between genesC and B

aBc 30 Single-crossover between genesC and B

ABc 5 Double-crossover

abC 8 Double-crossover

Abc 81 Single-crossover between genesA and C

aBC 85 Single-crossover between genesA and C

Total 1000

STEPS

Determine the parental genotype

Determine the gene order:  The double-crossover gametes are

always in the lowest frequency. From the table the Abc and abC genotypes are in the lowest frequency. a double-crossover event moves the middle allele from one sister chromatid to the other

ACB order

.linkage distance between AC and CB is measured as usual two point test cross but here double crossover is also included

So the distance between genes A and C is 17.9 cM [100*((81+85+5+8)/1000)], and the distance between C and B is 7.0 cM [100*((27+30+5+8)/1000)].

DETERMINING GENE ORDER After determining the relative distance between

the genes of a linkage group ,it becomes easy to place genes in linear order

Eg :ABC linear order any one could be in middle If suppose distance b/n A-B =12,B-C=7, A-C=5

B-A-C, in this case distance between BC is not equitable n so A cannot be in middle

A-B-C ,in this case distance between AC is not equitable n so B cannot be in middle

A-C-B ,this is correct order

COMBINING MAP SEGMENT

Finally different segments of maps of a complete chromosome are combined to form a complete map of 100 cM long for a chromosome

INTERFERENCE In most higher organisms it has been found

that one chiasma formation will reduce the formation of other in an immediately adjacent region on the chromosome because of physical inability of chromosome to bend back upon them within minimum distance

This tendency of one cross over interfering on other cross over is called interference.

Thus interference reduces the double crossover frequencies than excepted through map distance

COEFFICIENT OF COINCIDENCE The strength of interference varies in different

segments of the chromosome and is usually expressed in terms of a coefficient of coincidence

Coefficient of coincidence=% observed double cross over /%expected double cross over

Coincidence n interference are inversely proportional to each other

Eg study made in 3 genes of corn ,c (colorless aleurone), sh( shrunken grains) and wx (waxy endosperm)

CROSSING OVER FREQUENCIES BETWEEN 3 GENES OF CORNregions genes Percentag

e crossover

Map distance

1 C-sh 3.4 3.4+0.1=3.5

11 Sh-wx 18.3 18.3+0.1=18.4

Double cross over

C-sh-wx

0.1

PICTORIAL REPRESENTATION OF GENETIC MAP OF DROSOPHILIA

PHYSICAL MAPPING Linkage map distances between genes are

not necessarily proportional to physical linear measurements

Special cytological techniques have been used to determine location of gene on the chromosome

Polytene chromosomes of salivary glands of Drosophilia were very useful-T-H Painter first geneticist

Bridges did extensive and detailed investigation

DEFINITION

A cytological map is a graphic representation of the location of genes on a chromosome, based on correlating the genetic recombination results of testcrosses with the structural analysis of chromosomes that have undergone changes, such as deletions or translocations, as detected by banding techniques.

BRIDGES SYSTEM 5000 single cross bands-4 pairs of salivary

gland chromosome in D.melanogaster

Some genes have been associated with individual bands

bridges sys of designating parts of chromosomes with numbers ,sub division with letters and numbering bands within subdivision had made it possible for investigators to discuss precise location

EXAMPLE

The gene (w) for white eyes is in bands 3C2.

In linkage units this gene is located at 1.5 in the X chromosome

Linkage data do not correspond exactly with cytological locations, but the linear sequence of genes can be verified from salivary preparations

USES OF GENETIC MAPS

The chromosome maps display the exact location , arrangement and combination of genes in a linkage group or chromosomes

They are useful in predicting results of dihybrid and trihybrid crosses

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