Linkage and crossing over involve the exchange and recombination of genetic material between homologous chromosomes. Crossing over occurs during prophase I of meiosis through the breakage and rejoining of non-sister chromatids, producing new combinations of genes. It increases genetic variation and is important for evolution and the development of new species through natural selection. The frequency of crossing over is used to map the linear positions of genes on chromosomes.

1. Linkage and crossing
over
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S.SRIVIDHYA

2. Linkage
 Genes far apart on the same assort independently are
not linked
 The position of the gene – locus
 Occurs in the prophase of meiosis 1 where homologous
chromosomes break at identical locations and rejoin with
each other
 Two genes are said to be under linkage, or linked, when
they are located on the same chromosome.
 Example: peas T=tall; t=short R=red; r=white

3. Used as a mapping tool and genomic
studies
First identified in drosophila (Morgan)
Linkage group = number of chromosomes
in one set (n)
Human(2n=46) n=23; linkage group =23
Drosophila(2n=8) n=4

4. Gene mapping
Gene mapping : how genes are located
in a chromosome (linear)
Units :
map units – 10 map units apart
Recombination frequency –
10%(chances of getting separated)

5. Types of linkages
Complete linkage
Do not get separated very closed
Example : drosophila
9:3:3:1 (not) ; 8:8
Incomplete
Can separate because distance is more

6. Crossing Over
 Crossing over is a recombination of genes due to exchange of genetic
material between two homologous chromosomes
 It is the mutual exchange of segments of genetic material between
non-sister chromatids of two homologous chromosomes, so as to
produce re-combinations or new combinations of genes.
 It occurs in the pachytene stage, at four strand stage with the help of
enzymes (endonuclease, exo-nuclease, R-protein or recombinase;
 Stern and Hotta,(1969, 1978).
 There is breakage of chromatid segments, exchange of nonsister
chromatid segments and later their fusion in new places.

7. Types of Crossing Over:
 (i) Single Crossing Over:
 Crossing over occurs at one point between two non-sister chromatids of a homologous
chromosome pair. There are two parental types and two recombinants,
 (ii) Double Crossing Over:
 Crossing over occurs at two points in a homologous pair of chromosomes,
 (a) Reciprocal Double Crossing Over:
 Two points of crossing over occur between the same non-sister chromatids,
 (b) Complementary Crossing Over:
 The two crossing overs involve three or all the four chromatids so that the number of cross overs
is three or four with occurrence
of one or no parental type,
 iii) Multiple Crossing Over:
 Three or more points of crossing over occur in the same homologous chromosome. Double
cross-overs and parental types may or may not occur.

8. Factors Influencing Crossing Over
(and Linkage):
 1. Distance:
 Nearer the genes lesser the recombination
 2. Age:
 Increase in age decreases the degree of crossing over in most of
the cases.
 3. Sex:
 Male Drosophila shows little crossing over. The phenomenon of
crossing over is quite common in the female fly. Negligible
crossing over is also reported in one sex of some other
heterogametic organisms.

9.  4. X-Rays:
 Exposure to X-rays increases the incidence of crossing over.
 Whittinghill produced a number of cross-overs in male Drosophila
with the help of X-rays.
 5. Temperature:
 Variations in temperature increase the frequency of crossing over.
 6. Chemicals:
 A number of chemicals present in the food have been found to
change the degree of crossing over in animals.

10. Is crossing over important for the
diversity of biological evolution?
New species are produced
Mutation and Evolution
From single cell to human
Variation
Struggle for exhistance

11. Importance:
 1. Crossing over is a means of introducing new combinations of genes and
hence traits.
 2. It increases variability which is useful for natural selection under changed
environment.
 3. Since the frequency of crossing over depends upon the distance
between the two genes, the phenomenon is used for preparing linkage
chromosome maps.
 4. It has proved that genes lie in a linear fashion in the chromosome.
 5. Useful re-combinations produced by crossing over are picked up by
breeders to develop useful new varieties of crop plants and animals. Green
revolution has been achieved in India due to this selective picking up of
useful re-combinations. Operation flood or white revolution is also being
carried out on the similar lines.