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In this article we will discuss about:- 1. Meaning of Crossing Over 2. Double Cross-Over 3. Cytological Basis 4. Cytological Evidence 5. Somatic Crossover 6. Different Theories on the Mechanism 7. Theories that Explain the Happenings 8. Chiasma Formation—the Theories 9. No Cross-over in Drosophila Males 10. Experimental Conditions.
Contents:
- Meaning of Crossing Over
- Double Cross-Over
- Cytological Basis of Crossing Over
- Cytological Evidence of Crossing Over
- Somatic Crossover
- Different Theories on the Mechanism of Crossing Over
- Theories that Explain the Happenings during Cross-Over
- Chiasma Formation—the Theories
- No Cross-over in Drosophila Males
- Cross-over Frequency under Experimental Conditions
1. Meaning of Crossing Over:
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Linkage is an exception to Mendel’s principles of independent assortment and crossing over is in the same way an exception to linkage.
Crossing over means breaks in the linkage of genes within the chromosome and a bodily transshipment of genes from one chromosome to the corresponding position in its mate (Fig. 2.13). The phenomenon of crossing over closely resembles independent assortment of Mendel but it is a different thing.
Independent assortment is concerned with the whole chromosome while crossing over involves parts of chromosome. It is a sort of shuffling of genes between homologous pairs of chromosomes which always brings forth new combination.
The gametes containing the new combinations are known as cross-over or a-combination gametes. The gametes in which the linked genes remain in their original combinations are called non-cross-over gametes.
A case of crossing over in Drosophila:
A gray long female obtained by making a cross between gray long and black vestigial fly is back crossed to a black vestigial male. It is expected that in such a cross the two original kinds will be produced in the F2 generation.
But in actual experiment four kinds of offsprings—gray long and black vestigial like the grand parental combinations and two new combinations gray vestigial and black long appeared. The percentage of these four types were: gray long 41–5, black vestigial 41-5, gray vestigial 8’5 and black long 8’5.
The percentages show that free and random assortment of all gametes have not occurred because had it been so the ratio would have been 1 : 1 : 1 : 1.
The appearance of new combinations is the resultant outcome of breaks in the linkage of the genes within the chromosomes. This incompleteness of linkage leading to exchange of position of genes from one chromosome to the corresponding position of its partner is due to the phenomenon of crossing over (Fig. 2.14).
From the experiment mentioned above it appears that there are 83 per cent (41.5 + 41.5) of non-crossing over and 17 percent (8.5 + 8.5) of crossing-over.
The percentage of cross-over varies between different genes. But for each pair of genes the percentage remains constant. According to Morgan the cross-over percentage is related to the relative distance on the chromosomes between the two pairs of alleles.
Greater the distance, greater will be the amount of crossing over between them. In a simple way it may be stated that breaks occur more frequently in long chromosomes than in short one and between distant points on the same chromosome.
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Interference and coincidence:
In crossing over, not only single pair of isolated genes are involved but also the whole blocks of genes which lie close together. Their proximity interferes mechanically with the crossing over of neighbouring genes owing to the limited flexibility of the chromosomes. In other words, crossing over at a particular region of a chromosome tries to prevent another crossing over close to it.
This phenomenon is called interference. It is because of interference that there are no or few double cross-overs within a section of chromosome 10 units or less in length. The amount of interference becomes less when the distance between two genes increases and there may be no interference when the distance is too great.
The double cross-overs are nothing but coming together or ‘coincidence’ of two single cross-overs. Thus when double crossovers occur in expected numbers the coincidence is said to be 100 per cent and in such cases the interference is 0. When there is no double cross-overs the interference is 100 per cent and coincidence is 0. Thus coincidence is inversely proportional to the amount of interference.
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2. Double Cross-Over:
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Crossing over just once is known as single cross-over and the resultant gametes are called single cross-overs. But sometimes crossing over occurs at two points in the same chromosome pair. This is known as double cross-over and the gametes so formed are called double cross-overs.
The amount of double cross-over between two loci increases with the distance apart of the loci. But as a rule double cross-overs are fewer than single cross-overs. Crossing over may also occur at three loci in the same chromosome pair (triple cross-over) but they are still fewer.
A case of double cross-over in Drosophila:
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A double cross-over involves three linked genes in the same chromosome. In Drosophila yellow body (y), miniature wing (m) and forked bristles (f) are three recessive mutations in the X chromosome. The normal fly has gray body, long wings and straight bristles.
If we indicate the mutant genes by the symbols and their normal alleles by + signs then yellow, minature and forked female will be ymf/ymf, apure female will be represented a +++/+++, and a pure male will be represented as +++. A cross between ymf/ymf ♀ x +++ ♂ may give a female of genotype ymf/+++.
When reduction division takes place in the female, the following possibilities of gamete formation will be encountered (Fig. 2.15).
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We can now calculate the distances between y m and f.
Percentage of single crossover between y and m = 30%.
Percentage of double crossover between y and m = 6%.
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Total percentage of crossover between y and m = 36%.
Similarly,
Percentage of single crossover between m and f =14%.
Percentage of double crossover between m and f= 6%.
Therefore, total percentage of crossover between m and f =20%.
Thus the distance between y and m = 36 and the distance between m and f = 20. Since the genes are in the order y mf the distance between y and f = 36 + 20=56 (Fig. 2.16).
The above calculation shows that in getting the distance double crossovers have been counted twice. This appears to be little confusing. But it is to be remembered that a double crossover is equivalent to two single crossovers—one between the genes y and m and another between the genes m and f. The double crossovers are, therefore, considered twice in getting the total amount of crossovers between y and f.
3. Cytological Basis of Crossing Over:
During the prophase stage of first meiotic division, the two members of each pair of chromosomes, i.e., maternal and paternal chromosome come and pair. This pairing is called synapsis. Pairing occurs not only between homologous chromosomes but also between homologous parts of the chromosomes. Each chromosome then becomes duplicated and as a result a tetrad consisting of four chromatids is formed.
During late prophase of first meiotic division the two centromeres tend to go apart. But the chromatids attached to the centromeres do not, as a rule, separate uniformly along their entire length. At one or more points along the tetrad, two of the four chromatids appear to lie across each other forming chiasma.
At each chiasma, two of the four chromatids break and then rejoin, so that newly oriented chromatids are formed out of sections of original ones. Because of this chiasma formation maternal and paternal chromosomes cannot transmit as individual units.
They are compelled to exchange sections. The make-up of the chromosomes before and after meiosis gats changed to some extent because of this segmental interchange. Walter has explained the phenomenon as “Jack and Jill have exchanged heads and although nothing is missing they are now different individuals than they were before”.
4. Cytological Evidence of Crossing Over:
Crossing over involves segmental interchange between homologous chromosomes. But normally crossing over cannot produce permanent visible alternation in the structure of a chromosome. Thus, it is almost impossible to differentiate between a non-crossover chromosome and a crossover chromosome.
An experiment by Stern, however, gives cytological evidence in favour of crossing over. The experiment is a classic one and demonstrates visible results of crossing over. It forms a direct correlation of cytological and genetical crossover and was published in 1931.
The X chromosome of Drosophila is rod-shaped and a female possesses a pair of such rod-shaped X’s. But Stern obtained a female in which one X chromosome is broken into two. One part of this broken X houses the mutant gene carnation (Carnation = car which is recessive and imparts dark red eyes) and the gene Bar eye (Bar or Narrow eyes, dominant).
Both the broken segments had centromeres. In one it was the original centromere while the other derived its centromere probably from the fourth chromosome. Since these fragments had a centromere each they could be distributed in the normal manner in ceil division.
The unbroken X chromosome had a fragment of the Y chromosome attached to one of its ends and contained the normal alleles (+’s) of Carnation and Bar (Fig. 2.17).
Now if there be no crossover the two X chromosomes will go to the two gametes in their changed (changed from normal) make-up and if there be crossover between Carnation and Bar the broken X chromosome bearing the Bar gene will have the Y chromosome attached to it and the unbroken one will lose the Y chromosome though it will have the carnation gene.
The Y chromosome here will act as the marker and thus it will be possible to distinguish the crossovers microscopically.
Now if the X chromosome bearing Car + is added to each of those four classes of eggs produced by the hybrid female only female off-springs will result. When the chromosomes of these off-springs are examined under microscope it is found that (Fig. 2.18).
(a) The offsprings which appear Carnation Bar are with broken X chromosome.
(b) The offsprings which appear red round (Normal) are with the unbroken X with the Y chromosome attached to it.
(c) The red Bar offsprings are with broken X with Y chromosome attached to the part bearing the Bar gene.
(d) The offsprings which appear Carnation round are with unbroken X without the attached Y chromosome.
Thus it becomes evident that when two genetic non-crossover classes bear non-crossover X chromosomes of the mother but the two genetic crossover classes bear the crossover X’s of the mother. This is the cytological basis of crossing over.
5. Somatic Crossover:
Pairing of chromosomes is restricted to the germ cells and it takes place during the first maturation division. Somatic crossover is a rare phenomenon. In Drosophila such a rare instance of somatic cross-over has been shown by Stern.
Such somatic crossovers occur in one or two cells during the course of development of the fly. But these cells give rise to a cluster of cells through the process of division resulting the formation of a patch or spot on the body with the crossover cells. The somatic cells on the other parts of the body will be normal. Thus, the fly will be a mosaic of crossover and non-crossover tissues.
A somatic crossover cannot be investigated by taking into consideration the offsprings of the fly since the somatic cells do not give rise to offsprings. So in detecting somatic crossover in any organism it is the organism itself to be examined for any ‘crossover spot’.
In Drosophila it has been possible to detect such spots with the use of certain genes. The suitable genes for the purpose are yellow body colour (y) and ‘singed’, i.e., short and curly bristles (sn). Both the genes y and sn are recessive mutants and are located on the X chromosomes. Normal alleles for the genes are indicated by + signs.
Stern got a fly of genotype y+ /+sn. That is one chromosome of the fly is with y+ and its homologous chromosome is with + sn (Fig. 2.19). Both the chromosomes are telocentric (Note the dot end of the chromosome).
Now let us assume that in the developing fly crossover has occurred in one such cell and that between sn and the centromere. The split chromosome halves attached to a given centromere would no longer remain alike. In each instance one of the sister chromatids would be y + and the mates of these chromatids would be + sn.
Now if during the line-up of the chromosomes at metaphase the two chromatids with y+ face one pole and the two chromatids with + sn face the opposite pole, at the end of the division we would get two cells—one with genotype y+/y+ and the other with genotype + sn/+ sn.
By further division, each of the cells would give rise to cluster of cells. If these two clusters lie close to the surface of the body, y+/y+ clusters would form a yellow spot and +sn/+sn would form a spot with singed bristles. The two spots would lie close together since they have been derived from sister cells and they would in this way give rise to twin spots. The causes for somatic crossover are not known.
6. Different Theories on the Mechanism of Crossing Over:
A. Classical theory:
This theory explains that during the early prophase stage of Meiosis the chromosomes split up longitudinally. Each chromosome forms two sister chromatids. The two non-sister chromatids of the homologous pairs of chromosomes coil round each other.
At their points of contact the chromatids break first and cross. The theory thus states that crossing over does not produce chiasmata but actually chiasmata are caused by crossing over.
B. Chiasms type theory:
The theory states that breaking up of chromatids occurs at pachytene stage. After breaking up the chromatids unite again and form a chiasma. Thus according to it the chiasma is the result of crossing over.
C. Copy choice theory:
This theory is based on the fact that synthetic activity and duplication of chromosomes are intimately associated with recombination. In the mechanism according to the theory the sister chromatids duplicate their genetic parts and non-sister chromatids develop fibres at random.
The entire recombinants are formed from newly formed sections. The theory takes for granted that duplication occurs during late meiotic prophase but now it has been established that DNA replication occurs long before synapsis.
7. Theories that Explain the Happenings during Cross-Over:
Contact first theory:
According to this theory the chromatids destined to undergo cross-over touch each other first and then cross-over to give rise to chiasma. After this, breakage takes place at the point of contact and new attachment of chromatid parts takes place.
Breakage first theory:
Muller advocated the theory. According to him the chromosomes destined to cross-over first break into two segments then reunion occurs between non-sister chromatids to give new arrangement.
Strain theory:
This theory was advocated by Darlington. The theory states that the chromosomes break as a result of strain at the time of pairing. A sort of strain develops when two chromatids pair, twist round each other and this results in breakage and reunion.
Belting’s theory:
Belling believes that crossing over occurs between newly duplicated genes and that there is no breakage or reunion during crossing over.
Significance of crossing over:
a. Crossing over supports the fact that genes are arranged in a linear fashion on chromosomes.
b. Crossing over provides opportunities for reshuffling of genes and thus brings variations which play major role in the process of evolution.
c. By calculating the cross-over frequency it is possible to plot the genes on the chromosomes.
8. Chiasma Formation—the Theories:
The process of chiasma formation was first correctly understood by a Belgian Cytologist, Janssens (1909). He suggested that a chiasma represents an exchange of parts between homologous chromosomes, lie thought that the exchange involves the whole chromosomes or in other words both chromatids of each homologous chromosome exchange parts with both chromatids of the other.
But from the present-day knowledge, we know that the exchange at any point is between single chromatid— one of paternal and one of maternal origin—while the other two chromatids remain unaffected. Fig. 2.20 gives a schematic idea of exchange of genes between chromatids during crossing over.
Many speculations have been made to assign causes for breakage of chromatids. The followers of two-plane theory advocate that chiasmata causes crossing over. While the followers of one-plane theory claim that chiasmata is the consequence not the cause of crossing over.
But the real cause behind the breakage and subsequent rejoining of chromatids is still not known. However, the process is a highly precise one because the two chromatids in a chiasma exchange mirror image segments and no gain or loss of genes occurs (Fig. 2.21).
9. No Cross-over in Drosophila Males:
Recombination of linked genes occurs in most of the organisms that furnish materials for genetic studies. That is formation of chiasmata is universal in males and females of these organisms. The situation is, however, different in case of Drosophila males where crossing over rarely or never occurs. This is because linkage is complete in male Drosophila.
A similar situation is encountered in silk-moth where no cross-over occurs in females. Cytological studies of spermatogenesis in Drosophila males show that homologous chromosomes pair as usual. But no chiasmata is established at least in the autosomal bivalents. At the first meiotic divisions the pairs of chromatids go straight to the two poles and at the second division single chromatid passes to each cell.
10. Cross-over Frequency under Experimental Conditions:
The cross over frequency in the chromosomes may be influenced by a number of physiological and external environmental factors. In old females of Drosophila the amount of crossing over is less than what is encountered in its young age.
X’rays (Muller), temperature and chemical composition of food affect the cross-over frequency. Frequency of crossing over which is nil in case of Drosophila males in normal circumstances is increased by X’rays.
Crossing over is a feature that appears during gametogenesis. Under certain circumstances it has been seen in somatic cells. This somatic crossing over occurs in Drosophila (Stern) and in maize (Jones) Its significance is not yet understood.
