Crossing over refers to the exchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis. It results in new combinations of genes and genetic variation. Crossing over occurs via the formation of chiasmata, where segments are exchanged between chromatids. It can involve two, three, or all four chromatids, and can be single, double, or multiple. Factors like temperature, radiation, age, and nutrition can influence the rate of crossing over. Its significance includes providing evidence for gene order and creating genetic variation important for breeding programs.

1. Crossing over: Concept, Types and
theories
Vaishali S.Patil
Assosiate Professor, Department of Botany
Shri Shivaji College of Arts, Commerce & Science Akola

2. • Crossing over refers to the interchange of parts
between non-sister chromatids of homologus
chromosomes during meiotic prophase
(pachytene). In other words, crossing over results
from exchange of genetic material between non-
sister chromatids involving breakage and reunion
at precise point.
• The term crossing over was first used by Morgan
and Cattell in 1912.

3. 1. Crossing over takes place during meiotic prophase, i.e., during
pachytene. Each pair of chromosome has four chromatids at that
time.
2. Crossing over occurs between non-sister chromatids. Thus one
chromatid from each of the two homologus chromosomes is
involved in crossing over.
3. It is universally accepted that crossing over takes place at four
strand stage.
4. Each crossing over involves only two of the four chromatids of two
homologus chromosomes. However, double or multiple crossing
over may involve all four, three or two of the four chromatids, which
is very rare.
5. Crossing over leads to re-combinations or new combinations
between linked genes.

4. 6. Crossing over generally leads to exchange of equal segments or
genes and recombination is always reciprocal. However, unequal
crossing over has also been reported.
7. The value of crossover or recombinants may vary from 0-50%.
8. The frequency of recombinants can be worked out from the test cross
progeny. It is expressed as the percentage ratio of recombinants to the
total population (recombinants + parental types). Thus,
Cases of two strand crossing over, somatic crossing over,
sister strand crossing over and unequal crossing over are also
known.
The point of exchange of segments between non-sister
chromatids of homologous chromosomes during meiotic
prophase is called chiasma (pleural chiasmata). It is
thought to be the place where crossing over takes place.
Crossing over generally yields two recombinant types or crossover types
and two parental types or non-crossover types.

5. • Depending on the position, chiasma is of two types, viz.,
terminal and interstitial.
•When the chiasma is located at the end of the pairing
chromatids, it is known as terminal chiasma and when it is
located in the middle part of non-sister chromatids, it is
referred to as interstitial chiasma.

6. •Later on interstitial chiasma is changed to terminal position by the
process of chiasma terminalization.(The movement of chiasma away
from the centromere and towards the end of tetrads is called
terminalization.)
•The number of chiasma per bivalent may vary from one to more than
one depending upon the length of chromatids.
•When two chiasmata are formed, they may involve two, three or all the
four chromatids.

7. Types of Crossing Over:
Depending upon the number of chiasmata involved, crossing over
may be of three types, viz., single, double and multiple as described
below:
i. Single Crossing Over:
It refers to formation of a single chiasma between non-sister chromatids
of homologous chromosomes. Such cross over involves only two
chromatids out of four.

8. ii. Double Crossing Over:
It refers to formation of two chiasmata between non-sister chromatids of
homologous chromosomes. Double crossovers may involve either two
strands or three or all the four strands. The ratio of recombinants and
parental types under these three situations are observed as 2:2:3:1 and 4 :
0, respectively.

9. iii. Multiple Crossing Over:
Presence of more than two crossovers between non-sister chromatids of
homologous chromosomes is referred to as multiple crossing over.
Frequency of such type of crossing over is extremely low.

10. Theories of Crossing Over:
(i) Contact First Theory (by Serebrovsky):
According to this theory the inner two chromatids of the homologous chromosomes
undergoing crossing over first touch each other and then cross over. At the point of
contact breakage occurs. The broken segments again unite to form new combinations
(Fig. 5.11A).

11. (ii) The Breakage-First Theory (By Muller): According to this theory
the chromatids under-going crossing over first of all break into two
without any crossing over and after that the broken segments reunite to
form the new combinations (Fig. 5.11 B).
(iii) Strain Theory (by Darlington):
According to this theory the breakage in chromosomes or chromatids is
due to strain caused by pairing and later the breakage parts again
reunite.(16.5)

12. Factors Influencing Crossing Over:
x:
osophila, crossing over is completely suppressed in male but very high in female,
here is a tendency of reduction of crossing over in male mammals.
tation:
first discovered that mutation reduces crossing over in all the chromosomes
osophila.
version:
sion is an intersegmental change in the chromosome. In a given segment of
mosome crossing over is suppressed due to inversions.
mperature:
h has experimentally shown that when Drosophila is subjected to high and
emperature variations, the percentage of crossing over in certain parts of the
mosome is increased.
ray Effect:
r demonstrated that X-ray irradiations increase crossing over near centromere.
arly Hanson has shown that radium increases crossing over.
:
es has demonstrated that the age also influences the rate of crossing over in
ophila. When the female becomes older the rate of crossing over increases.
trition:
calcium diet in young Drosophila decreases crossing over rate where as diet

13. Significance of Crossing Over:
1. Crossing over provides direct proof for the linear arrangement of
genes.
2. Through crossing over segments of homologous chromosomes are
interchanged and hence provide origin of new characters and genetic
variations.
3. Crossing over has led to the construction of linkage map or genetic
maps of chromosomes.
4. Linkage group and linear order of the genes help to reveal the
mechanism and nature of the genes.
5. Crossing over plays a very important role in the field of breeding to
improve the varieties of plants and animals.