2. 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
3.
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
4.
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
6. 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
7.
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
8. 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 %)
9. 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
10. 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
11. 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
16. Determine
Determine
the parental genotype
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
17. 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)].
18.
19. 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
20. Finally
different segments of maps of a
complete chromosome are combined to
form a complete map of 100 cM long for a
chromosome
21. 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
22. 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)
25. 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
26. 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.
27. 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
28. 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
29.
30. 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
Notas del editor
If yellow were in the middle, yellow phenotype would show up in DCO.If echinus were in the middle, echinus phenotype would show up in DCO.white is actually in the middle since white phenotype shows up in actual DCO data