The linkage is caused due to linked genes borne on the same chromosome. Morgan pointed out that the phenomenon of complete linkage occurs rarely because sometimes the linked genes show the tendency to separate during meiosis and new combinations are formed.
This is due to interchange of parts between two homologous chromosomes for which the term “crossing over” is used.
Thus, 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.”
In the linkage experiment with maize, it is seen that the genes for seed colour C and full seed S remain associated in the parental combination in about 96 per cent but break apart in about 4 per cent (see Fig. 5.8). This recombination of linked genes to interchange parts between homologous chromosomes is termed as crossing over.
Crossing over takes place in the segment of the chromosome between the loci of the genes C and S in some cells but not in others, so that about 96 per cent of the gametes contain the parental gene combination and 4 per cent contain recombination’s.
Mechanism of Crossing Over:
During the zygotene stage of the first prophase of meiosis, the homologous maternal and paternal chromosomes start pairing and lie closely side by side. This phenomenon is called synapsis. This pairing of homologous chromosomes is brought about by the mutual attraction between the allelic genes. The paired chromosomes are known as bivalent. A recent study reveals that synapsis and chiasma formation is facilitated by a highly organised structure of filaments called synaptonemal complex. Synapsis is followed by the duplication of chromosomes which change the bivalent nature of chromosome pair into tetravalent.
During this each of the homologous chromosomes in a bivalent split longitudinally into two sister chromatids attached to the undivided centromere. Thus, four chromatids are formed which remain side by side as two pairs. Later, in pachytene stage crossing over takes place during which the non-sister chromatids of homologous pair twist over each other, the point of contact of cross over chromatids being called as chiasma (Fig. 5.9).
In crossing over two or three chromatids are involved and accordingly two or more chiasmata are formed. At each chiasma the chromatid breaks and the broken segment rejoin a new chromatid (Fig. 5.10A & B). Thus exchange of parts of chromatids brings about alteration of original sequence of genes in the chromosome.
After crossing over is completed, the non-sister chromatids repel each other due to lack of attraction between them. The repulsion or separation of chromatids starts from the centromere towards the end just like a zipper and this separation process is named as terminalization. The process of terminalization continues through diplotene, diakinesis and ends in metaphase I.
At the end of terminalization the twisting chromatids separate so that the homologous chromosomes are separated completely and move to opposite poles in Anaphase I. The crossing over thus brings about alteration of the linear sequence of gene in chromosomes that produce gametes and thus add new combination of character in progeny.
Theories of Crossing Over:
(i) Contact First Theory (by Serebrovsky):
According to this theory the inner two chromatids of the homologous chromosomes undergoing crossing over first touch each other and then cross over. At the point of contact breakage occurs. The broken segments again unite to form new combinations (Fig. 5.11).
(ii) The Breakage-First Theory (By Muller):
According to this theory the chromatids under-going crossing over first of all break into two without any crossing over and after that the broken segments reunite to form the new combinations (Fig. 5.11).
(iii) Strain Theory (by Darlington):
According to this theory the breakage in chromosomes or chromatids is due to strain caused by pairing and later the breakage parts again reunite.
Types of Crossing Over:
(i) Single Crossing Over:
In this type of crossing over only one chiasma is formed all along the length of a chromosome pair. Gametes formed by this type of crossing over are called single cross over gametes (Fig. 5.10A and B).
(ii) Double Crossing Over:
In this type two chiasmata are formed along the entire length of the chromosome leading to breakage and rejoin of chromatids at two points. The gametes produced are called double cross over gametes (Fig. 5.14B).
(iii) Multiple Crossing Over:
Factors Influencing Crossing Over:
In Drosophila, crossing over is completely suppressed in male but very high in female. Also there is a tendency of reduction of crossing over in male mammals.
Gowen first discovered that mutation reduces crossing over in all the chromosomes of Drosophila.
Inversion is an intersegmental change in the chromosome. In a given segment of chromosome crossing over is suppressed due to inversions.
Plough has experimentally shown that when Drosophila is subjected to high and low temperature variations, the percentage of crossing over in certain parts of the chromosome is increased.
5. X-ray Effect:
Muller demonstrated that X-ray irradiations increase crossing over near centromere. Similarly Hanson has shown that radium increases crossing over.
Bridges has demonstrated that the age also influences the rate of crossing over in Drosophila. When the female becomes older the rate of crossing over increases.
High calcium diet in young Drosophila decreases crossing over rate where as diet deficient of metallic ions increases crossing over.
8. The frequency of crossing over is less at the ends of the chromosome and also near the centromere in comparison to other parts.
Significance of Crossing Over:
1. Crossing over provides direct proof for the linear arrangement of genes.
2. Through crossing over segments of homologous chromosomes are interchanged and hence provide origin of new characters and genetic variations.
3. Crossing over has led to the construction of linkage map or genetic maps of chromosomes.
4. Linkage group and linear order of the genes help to reveal the mechanism and nature of the genes.
5. Crossing over plays a very important role in the field of breeding to improve the varieties of plants and animals.