What is Crossing Over?
Mutual exchange of blocks of homologous genes between a pair of homologous chromosomes is known as crossing over. Crossing over may be defined as the phenomenon of interchange of segments between non-sister chromatids of a homologous pair of chromosomes, resulting in a recombination of genes during Prophese I of meiosis. Crossing over involves breaking and rejoining of chromosomes in the synaptonemal complex.
During zygotene and pachytene stages of the prophase I of meiosis, pairing takes place in such a way that similar parts of chromosomes lie side by side, probably by mutual attraction between allelic genes. During diplotene, each chromosome splits longitudinally into two chromatids so that each bivalent is now composed of four chromatids. The chromatids originating from the same chromosome are called sister chromatids. During the process of crossing over, the two non-sister chromatids come in contact at certain points.
At which stage Crossing over occurs?
During the zygotene and pachytene stages of the prophese I of meiosis, there is pairing or synapsis between homologous chromosomes. In this pairing, similar parts of the chromosomes remain side by side by mutual attraction between allelic genes. This is known as two-strand stage or bivalent. Further, in diplotene stage of prophase I of meiosis, each chromosome splits longitudinally into two chromatids. Now bivalent is composed of four chromatids. This stage is known as the four-strand stage or tetrad. The chromatids evolving from the same chromosome are known as the sister chromatids. Crossing over is detected at four strand stage. It has been detected that crossing over takes place between the maternal and paternal chromatids.
Example of Crossing Over
Crossing over is now illustrated by taking the example of Drosophila melangaster. In Drosophila, two pairs of characters are involved in a dihybrid cross. Mutant fly having recessive characters of pink eyes (r) and curled wings (s) is crossed with a wild fly having dominant characters of red eyes (R) and 
straight wings (S). In this cross, all the progeny of Ft hybrid show red eyes and straight wings having genotype RrSs. Now a female from these F1hybrids is crossed with a double recessive male of P generation which is known as a test cross. This F1 hybrid female will produce four types of gametes. When F1 female gametes mated with single type of male gametes, the F2 generation consists of 49% flies with red eyes and straight wings, 49% with pink eyes and curled wings, 1% with red eyes and curled wings and 1% with pink eyes and straight wings.
This result in F2 generation shows that the two types are non-cross overs which combine to form 98% and two types of new combinations or re-combinations of the remaining 2% are produced due to crossing over. This experiment shows that both the genes for each allelic pair are situated in the same chromosome. They connected together in 98% gametes having no chromosomal interchange but in the remaining 2% gametes there is interchange between their non-sister chromatids of the homologous chromosomes. This interchange of segments of chromatids occurs due to crossing over.
Types of Crossing Over
Depending on the number of chiasmata of the homologous chromosomes, crossing over may be of three types, such as single crossing over, double crossing over and multiple I crossing over.
i) Single Crossing Over: Here only one chiasma is formed between the non-sister I chromatids ; as a result single crossing over occurs. Resultant effect is, two parental types and two recombination types. This single crossing over is very common occurrence.
ii) Double Crossing Over : Here two chiasmata are formed. Hence, two crossing I over occur at two points in the same tetrad. This type of crossing over is less frequent.
iii) Multiple Crossing Over : Here three or more chiasmata are formed in the-tetrad. Corresponding to the number of chiasmata, crossing over occur; though this type does not occur frequently.
Importance of Crossing Over
Crossing Over is a widespread phenomenon in plants and animals. Hence, this phenomenon is of great importance in Genetics. Importance of crossing over is given below:
(a) Crossing over provides direct evidence that the genes are arranged in a liner fashion on the chromosome.
(b) Crossing over helps in the tracing of linkage groups.
(c) In the construction of gene map helped by the percentage of crossing over.
(d) As variation is an essential feature of evolution, this phenomenon gives rise to new combination of genes which results variations in the offspring.