

Useful notes on the mechanism of crossing over!

According to Morgan the linkage is caused due to linked genes being carried out by the same chromosome. He also pointed out that the phenomenon of complete linkage occurs rarely because sometimes the linked genes show the tendency to separate during meiosis. This way, the crossing over may be defined as “a mechanism of the recombination of the genes due to interchange of chromosomal segments at the time of pairing.”


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The cross-over between linked genes allows their recombination during meiosis. The mechanism of cross-over is as follows:

Mechanism of crossing over:

The figure shows that crossing over, leading to recombination of linked genes, is due to interchange of sections of homologous chromosomes. At meiosis, the homologous (maternal and paternal) chromosomes come together and pair, or synapse, during prophase. The pairing is remarkably precise and is evidently brought about by mutual attraction of the parts of the chromosomes that are similar or homologous because they contain allellic genes.

Further, during the transition between the pachytene and the diplotene of the prophase of meiosis, the paired chromosomes divide each into two chromatids, so that the bivalent is now composed of four chromatids. At about the time when the chromosomes are first seen to be divided, the chromatids establish one or more exchange, or Chiasmata, per bivalent.

At each chiasma, two of the chromatids have become broken and then rejoined, so that the new chromatids are now formed of sections of the original ones. Owing to chiasma formation, maternal and paternal chromosomes are not transmitted as indivisible units; they exchange sections, and the new chromosomes that arise as a result of meiosis carry genes that before meiosis were located in different members of the pair of homologous chromosomes. Janssens (1909) for the first time discovered the process of chiasma formation. This is a highly precise process since the two chromatids at a chiasma exchange exactly equivalent segments, so that, with very rare exception, neither chromatid gains or loses any genes.



Absence of crossing over in Drosophila males:

In most of the organisms, recombination of linked genes takes place both in females and males. Similarly, in most organisms, Chiasmata formation takes place in both female and male meiosis. But in Drosophila the situation is very different. In Drosophila crossing over rarely or never takes place in the male. However in the silkworm moth it seems to be absent in the females.

Recombination and crossing over:

The cytological observations have established one important fact about crossing over that the exchanges of chromosome segments which cause recombination of linked genes occur at the four-strand, or tetrad stage, in the prophase of meiotic division. Here the homologous chromosomes have paired, each homologue has split into two chromatids held together by undivided centromere, and Chiasmata are observed in the tetrad.

It is revealed by careful observation that at each chiasma only two of the four chromatids crosses over whereas the two other chromatids preserve their original continuities. Genetically it means that of the four chromosomes which emerge from meiosis, only two can show crossing over at any one level.


Recombination is a process which can occur in a variety of situations but the most popular one is in relation to meiosis. Thus, this is a meiotic process that generates haploid product whose genotype is different from the two haploid genotypes constituting the diploid. The product of recombination is a recombinant.

Chiasma – type hypothesis:

Morgan suggested that the formation of recombinants can be explained by the occurrence of crossing over (i.e., chiasma formation) during meiosis I. This led to chiasma-type hypothesis. The points in support of this hypothesis are as follows:

1. During prophase I (i.e., zygotene), homologous chromosomes pair (i.e., synapsis), with each chromosome consisting of two chromatids.

2. During pachytene of prophase I two non-sister chromatids break at homologous sites and then rejoin.


3. Such rejoining takes place between non-sister chromatid and leads to physical exchange.

4. This physical exchange leads to reshuffling of genes and thus recombination takes place.

Synaptonemal complex and the process of crossing over:

Since it is an established fact that DNA is the material of the gene it becomes clear that crossing over involves break and rejoining of DNA. During synapsis, a molecular scaffold known as Synaptonemal complex aligns the DNA molecules of the two homologous chromosomes side by side.

With the result the DNA strand of one chromosome, i.e., a chromatid becomes able to pair with the corresponding DNA strand of the other where their base sequence match. Break and new synthesis of one DNA strand using a strand of other chromatid DNA as the template results in crossing-over.

Crossing-over involves the breaking and rejoining of chromosomes in the Synaptonemal complex, which can be observed by cytological methods. However, no crossing-over has been observed in male Drosophila during sperm formation. Likewise no recombination of genes occurs during meiosis.


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