chromosomal mappin highlighted
TRANSCRIPT
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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