12th Class Biology Principle Of Inheritance And Variation Crossing Over

The process by which exchange of chromosomal segment take place is called crossing over. Crossing over may be defined as "the recombination of linked genes" brought about as a result of interchange of corresponding parts between the chromatid of a homologous pair of chromosomes, so as to produce new combination of old genes. The term was given by Morgan and Cattle. Janssen (1909) observed chiasmata during meiosis-I (Prophase). Morgan proposed that chiasmata lead to crossing over by breakage and reunion of homologous chromosomes. Crossing over results in new combination while non-cross over result in parental type, which leads to variations.

Crossing over and chiasma

There are two views extended to explain the relationship between crossing over and chiasma formation. They are summarised here under :

Chiasma type theory : According to Janssen, 1909 the act of crossing over is followed by chiasma formation. He suggests that the crossing over takes place at the pachytene stage and the chiasma appear at diplotene.

Classical theory : According to Sharp, 1934, crossing over is the result of chiasma formation. According to this view, the chiasma are organised at pachytene and crossing over takes place at diplotene stage. On the basis of evidence available from molecular biology, that is untenable and hence rejected.

Mechanism of crossing over

There are different views put forward to explain the mechanism of crossing over.

Copy choice hypothesis : According to Belling, 1928 the chromomeres represent the genes joined by interchromomeric regions. The chromomeres duplicate first and then the interchromomeric regions. The synthesis of these regions may occur in such a way that the chromomeres of the chromatid of a homologue get connected of the chromatid of the other homologue at a specific location. As a result, the adjacent chromatids of a pair of homologue are exchanged.

Precocity hypothesis : According to Darlington, the pairing of homologues occurs to avoid singleness of a chromosome. The pairing need of a chromosome could be nothing less than the replication of DNA. The crossing over takes place due to torsion on chromosome created by coiling of the two homologues around each other.

Cross over value : The percentage of crossing over varies in different materials. The frequency of crossing over is dependent upon the distance of two genes present on a chromatid.

Coincidence : Coincidence or coefficient of coincidence is inverse measure of interference and is expressed as the ratio between the actual number of double cross over and the expected number of such double cross. That is:

\[Coincidence=\,\frac{Actual\text{ }number\text{ }of\text{ }double\text{ }cross\text{ }over}{Expected\text{ }number\text{ }of\text{ }double\text{ }cross\text{ }over}\]

Factors controlling frequency of crossing over

Primarily, frequency of crossing over is dependent upon the distance between the linked genes, but a number of genetic, environmental and physiological factors also affect it. These are:

Temperature : High and low temperature increase the frequency of crossing over.

X-ray : Muller has discovered that exposure to X-ray and other radiations increases the frequency of crossing over.

Age : The frequency of crossing over decreases with increasing age in female Drosophila.

Chemicals : Certain chemicals which act as mutagens do affect the frequency of crossing over. Gene mutations may affect the frequency of crossing over. Some increase the frequency, whereas some may decrease it.

Sex : Crossing over in Drosophila males is negligible. Males of mammals also exhibit little crossing over. In silk-moth, crossing over does not occur in females.

Chiasmata formation : Chiasmata formation at one point discourages chiasmata formation and crossing over in the vicinity. This phenomenon is known as interference.

Inversions : Inversions of chromosome segments suppresses crossing over.

Distance : Distance between the linked genes is the major factor which controls the frequency of crossing over. The chances of crossing over between distantly placed genes are much more than between the genes located in close proximity.

Figure depicts that chance of crossing over between a and c are double as compared to the chances between a and b or b and c.

Nutritional effect : Crossing over frequencies are affected by concentration of metallic ions, such as calcium and magnesium.

Genotypic effect : Crossing over frequencies between the same two loci in different strains of the same species show variation because of numerous gene differences.

Chromosome structure effect : Changes in the order of genes on a chromosome produced by chromosomal aberrations usually act as cross over suppressors.

Centromere effect : Genes present close to the centromere region show reduced crossing over.

Interference : If there are two doubles crossovers, then one crossover tries to influence the other by suppressing it. This phenomenon is called as interference. Due to this phenomenon, the frequency of crossing over is always lower than the expected.

Significance of crossing over

This phenomenon is of great biological significance, which are as under:

(i) It gives evidence that the genes are linearly arranged on a chromosome. Thus, it throws light on the nature and working of the genes.

(ii) It provides an operational definition to a gene. It is deemed as the smallest heritable segment of a chromosome in the interior of which no crossing over takes place.

(iii) The crossing over is helpful in the chromosomal mapping. The percentage of crossing over is proportional to the distance between two genes.

(iv) It is the main cause of genetic variations. It's occurrence during the act of meiosis produces variations in the heritable characters of the gametes.

(v) This phenomenon has also found it's utility in breeding and evolving new varieties. The linkage of undesirable characters can be broken by temperature treatment, using X-ray or chemicals. Thus, new recombinants can be prepared.

Chromosomal maps

A linkage or genetic chromosome map is a linear graphic representation of the sequence and relative distances of the various genes present in a chromosome. A chromosome map is also called a linkage map or genetic map.

The percentage of crossing over between two genes is directly proportional to their distance. The unit of crossing over has been termed as by Haldane as centi Morgan (cM). One unit of map distance (cM) is therefore, equivalent to 1% crossing over. When chiasma is organised in between two gene loci, only 50% meiotic products shall be crossovers and 50% non-crossovers. Thus, the chiasma frequency is twice the frequency of cross over products i.e., chiasma % = 2 (cross over %) or crossover \[%={\scriptscriptstyle 1\!/\!{ }_2}\] (chiasma %).

Accordingly, Sturtevant, 1911 prepared the first chromosomal map. Infact this map is a line representation of a chromosome where the location of genes has been plotted as points at specific distances. These distances are proportional to their crossing over percentage. Suppose there are three genes on a chromosome say, A B and C which could be arranged as A, B, C, A, C, B or B, A, C. A three point test cross confirms as to which gene is located in the centre. By determining the crossing over value between A and B, B and C as also between A and C, the linkage maps can be prepared. Broadly speaking, a chromosomal map can be prepared from the following results of crossing over between the genes A, B and C :

(i) 4% crossing over taking place between A and B.   (ii) 9% crossing over taking place between A and C.

Hence the genes be located as above and there should be 13% crossing over between B and C and the genes may be arranged as under :

If there is 5% crossing over between B and C, the genes are arranged in the following manner and there should be 9% crossing over between A and C.

Uses of chromosomal map

(i) Finding exact location of gene on chromosomes.

(ii) Knowing recombination of various genes in a linkage group of chromosomes.

(iii) Predicting result of dihybrid and trihybrid cross.