What is the significance of crossing over in evolution

Hence, out of four chromatids the two adjacent chromatids are recombinants and other two are original chromatids. Thus four types of gametes are obtained. Crossing over leads to the production of new combination of genes and provides basis for obtaining new varieties of plants. It plays an important role in the process of evolution. The crossing over frequency helps in the construction of genetic maps of the chromosomes. It gives us the evidence for linear arrangement of linked genes in a chromosome.

Gene mapping. Genes are arranged linearly in a chromosome. The point in a chromosome where the gene is located is called locus. The diagrammatic representation of location and arrangement of genes and relative distance between linked genes of a chromosome is called linkage or genetic map. The unit of genetic map is Morgan or centimorgan. When the percentage of crossing over between two linked genes is 1 per cent, then the map distance between the linked genes is one morgan.

There is a greater probability of occurrence of crossing over, when the two genes are farther apart in a chromatid. The probability of crossing over between two genes is directly proportional to the distance between them. When two genes are nearer, the probability of occurrence of crossing over between them is limited.

However, double or multiple crossing over may involve all four, three or two of the four chromatids, which is very rare. Crossing over leads to re-combinations or new combinations between linked genes. Crossing over generally yields two recombinant types or crossover types and two parental types or non-crossover types. Crossing over generally leads to exchange of equal segments or genes and recombination is always reciprocal. However, unequal crossing over has also been reported.

The frequency of recombinants can be worked out from the test cross progeny. Cases of two strand crossing over, somatic crossing over, sister strand crossing over and unequal crossing over are also known.

However, frequency of such cases is extremely low, i. Crossing over differs from linkage in several aspects Table 9. 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. Chiasma was first discovered by Janssens in Depending on the position, chiasma is of two types, viz. 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. Later on interstitial chiasma is changed to terminal position by the process of chiasmaterminalization.

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. The movement of chiasma away from the centromere and towards the end of tetrads is called terminalization. The total number of chiasmata terminalized at any given stage or time is known as coefficient of terminalization.

Generally, chiasma terminalization occurs between diplotene and metaphase I. There are three theories to explain the mechanism of chiasma terminalization, viz:. According to this hypothesis, terminalization takes place due to localized repulsion force in centromere and generalized repulsion force on chromosome surface during diplotene stage. Thus tension force becomes greater than the force binding the chromatids at the point of exchange resulting in terminalization.

According to this theory, all bodies having a definite shape resist any change that leads to alter their shapes. Chiasma forces the chromosome out of shape by its binding force.

This leads to the development of repulsion at the point of exchange resulting in terminalization of chiasma. There are two main theories to explain the relationship between crossing over and chiasma formation, viz.

This theory states that first chiasma is formed and then crossing over takes place. The genetic crossing over occurs as a result of physical strain imposed by chiasma formation. The chiasma is formed at diplotene stage of meiosis and crossing over occurs between diplotene and anaphase. In this case, 1 : 1 relationship between chiasmata and crossing over is not observed because chiasma may not lead to breakage and subsequent genetic crossing over.

This theory was proposed by lanssens and later on elaborated by Belling and Darlington. According to this theory, first crossing over occurs and then chiasma is formed.

The crossing over occurs sometimes during early meiotic stages, perhaps at pachytene, when homologous chromatids are closely paired. As the meiotic cell moves towards metaphase and reductional division, a chiasma is formed at the point where crossing over has occurred. Thus according to this theory each chiasma represents one genetic cross over. This theory remains at present the most accepted explanation for the relationship between genetic crossing over and cytological observed chiasmata.

Molecular Mechanism of Crossing Over : There are two important theories viz:. This theory was proposed by Belling. This theory states that the entire recombinant section or part arises from the newly synthesised section. The non-sister chromatids when come in close contact they copy some section of each other resulting in recombination. According to this theory, physical exchange of preformed chromatids does not take place. The non-sister chromatids when come together during pairing, copy part of each other.

Thus, recombinant chromosome or chromatids have some alleles of one chromatids and some of other. The information may be copied by one strand or both the strands. When only one strand copies, non-reciprocal recombinant is produced. If copy process involves both strands of chromosomes, reciprocal recombinants are produced. Assume, there are two chromosomes, viz. When their chromatids come in close contact they copy each other and result in Ab and aB re-combinations besides parental combinations Fig.

According to this theory breakage and reunion does not occur, while it has been observed cytological. Generally crossing over takes place after DNA replication but here it takes place at the same time. This theory states that crossing over takes place due to breakage and reunion of non-sister chromatids. The two segments of parental chromosomes which are present in recombinants arise from physical breaks in the parental chromosomes with subsequent exchange of broken segments Fig.

The breakage results due to mechanical strains that result from the separation of paired homologous chromosomes and chromatids in each chromosome during pachytene stage.

The broken ends of non-sister chromatids unite to produce chiasmata resulting in crossing over. The term interference was coined by Muller which refers to the tendency of one crossover to reduce the chance of another crossover in its adjacent region. Interference is affected by gene distance on the chromosome.

Joan E. Bailey-Wilson, Ph. Featured Content. Introduction to Genomics. Polygenic Risk Scores.