Crossing over refers to the exchange of genetic material between non-sister chromatids during meiosis. This leads to new combinations of genes on chromosomes. There are two main theories that explain the relationship between crossing over and chiasma formation - the classical two-plane theory and the widely accepted one-plane or chiasma type theory. The mechanism of crossing over involves synapsis of homologous chromosomes, formation of tetrads, exchange of chromatid segments at chiasmata, and terminalization. Crossing over is significant as it generates genetic variability and plays an important role in evolution and plant breeding.

1. CROSSINGOVER
Dr. Gayatri D. Hande
Assistant Professor,
Shri. Shivaji Science College
Amravati.

2. Genetics is the study of heredity and variations. Linkage and
Crossing over are the two main principles of inheritance and
variations.
The concept of linkage deals with the detailed study of gene
location on the chromosome and their inheritance that reflect
in the phenotype of an organism.
Linkage keeps the two or more linked genes together over
generations during inheritance.

3. Linkage can be complete or incomplete. The early
experiments to understand the phenomenon of linkage
were studied using Drosophila and sweet pea.
Crossing over is the phenomenon of the exchange of
genes through the recombination of sister chromatids. It
is of great importance due as it leads to variations in the
organisms.

6. Crossing over –
The exchange of chromosomal segments between
two non-sister chromatids of non homologous
chromosomes after crossing over.
F. Janssens was the first person to discover chiasma
formation and the related process of crossing over.
Morgan found the phenomenon of linkage and
recombination.

8. Alleles-Different versions (sequences) of a gene.
Mutant-Newly created allele made by mutagenesis.
Genotype-The complete set of alleles for all genes carried by an
individual.
Wild type-Standard reference genotype. Most common allele for
a certain trait.
Phenotype-Observable trait specified by the genotype.
Point mutation-A change in a single base pair.
Silent mutation-A point mutation in a codon that does not change
the specified amino acid.
Genetics Terms

9.  Crossing-over is a physical exchange between
chromatids in a pair of homologous chromosomes. It
results in a new association of genes in the same
chromosome.
 The role of crossing-over is important for evolution to
take place. In fact, crossing-over and independent
assortment are mechanisms that produce new combinations
of genes.
 Natural selection can then act to preserve those
combinations that produce organisms with maximum
fitness, that is, maximum probability of perpetuation of the
genotype

10. The Concept of Crossing-Over
Following are the important features of crossing-
over:
i) A gene is located on chromosomes at a
particular site called a locus (plural-loci).
ii) The loci of the genes on a chromosomes are
arranged in a linear sequence.
iii) In a heterozygote, the two alleles of a gene
occupy corresponding positions in the
homologous chromosomes, that is, allele A
occupies the same position in homolog 1 that
allele a occupies of a species is fixed or
constant.

12. iv) Crossing-over involves the breakage and
rejoining of two chromatids (of homologous
chromosomes), resulting in reciprocal exchange of
equal and corresponding segment between them
v) Chromosomes with recombined or new
combinations of genes are formed by the
occurrence of crossing-over.
vi) Crossing-over occurs more or less at random
along the length of a chromosome pair. Thus, the
probability of its occurrence between two genes
increases with increasing physical separation of the
genes along the chromosome.

13. Crossing over refers to the exchange of genetic material
or chromosome segments between non-sister chromatids
in meiosis. This genetic process occurs between
homologous regions of matching chromosomes and the
interchange of homologous chromosomes.
Depending upon the number of chiasmata that appeared,
crossing-over can be categorised into three types:
Single cross-over
Double cross-over
Multiple cross-over
Significance of crossing-over are:

14. Crossing over produces a new combination of
genes
Crossing over plays an essential role in the process
of evolution
Crossing over frequency helps in the construction
of genetic maps
Crossing over provides the evidence for a linear
arrangement of linked genes in a chromosome
Crossing over provides an inexhaustible store of
gene variability in sexually reproducing organisms

15. Crossing over can put new alleles together in
combination on the same chromosome, causing them
to go into the same gamete.
When genes are far apart, crossing over happens
often enough that all types of gametes are produced
with 25% frequency.

16. Theories of Crossing Over
There are two theories to explain the relationship
between crossing over and chiasma formation:
1. Classical theory: It is also called a two-plane theory.
This theory was proposed by L.W. Sharp.
2. This theory stated that chiasma is the cause of
crossing over but not the result of crossing over. The
formation of chiasma occurs before the genetic
crossing over.

17. 3. Chiasma type theory: It is also called one plane
theory. It was proposed by Jannsen and later
developed by Belling and Darlington.
According to this theory, chiasma is the result of
crossing over, and crossing over precedes the
chiasma formation. This theory is widely accepted.

18. Mechanism of Crossing Over
The mechanism of crossing over involves the
following steps:
1. Synapsis: The pairing between the
homologous chromosomes (synapsis) takes place
during zygotene. This pair of homologous
chromosomes is called synapsis.

19. 2. Tetrad formation: The two chromatids of a chromosome
are referred to as dyads. A group of four homologous
chromatids (two dyads) of two synapsed homologous
chromosomes is known as a tetrad. The two chromatids of the
same chromosome are called sister chromatids. The two
chromatids, one of the one chromosome and the other of its
homologue, are termed non-sister chromatids.
A highly organized structure of filaments is formed between
the paired homologous chromosomes at the zygotene stage of
meiosis-I called synaptonemal complex. It helps in keeping the
homologous chromosome in a closely paired state.

20. 3. Exchange of Chromatid segments:
The two non-sister chromatids come in contact at certain points.
This is the region where the exchange of genes between the two
non-sister chromatids of a tetrad takes place. The places where
homologous chromosomes are held together and exchange bits
of chromatids are known as chiasma.
In synapsis, the non-sister chromatids of homologous
chromosomes break and recombine. This leads to the formation
of chiasmata. The exchange of fragments is stimulated by the
development of recombination nodules during the pachytene
stage. The unchanged part of the chromatid is called non-
crossover, and the changed parts are called recombinants.

22. 4. Terminalization: The chromatids separate progressively
from the centromere towards the chiasma and get separated
from each other. It is called terminalization. Terminalization of
chiasma begins in the diplotene stage after crossing over, and
completion takes place in the diakinesis stage.

23. Significance of Crossing Over
The phenomenon of crossing over is of great significance that
can be discussed as follows:
1. It provides an inexhaustible store of genetic variability in
sexually reproducing organisms.
2. Since crossing over helps in the development of new
characteristics. Therefore it is of paramount importance in
plant breeding.
3. The new gene combination produced during crossing over
plays an important role in microevolution.
4. The frequency of crossing over is helpful in the mapping of
chromosomes.
5. Crossing over also justifies the linear arrangement of genes.