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Morgan formulated the chromosome theory of linkage according to which:
a. Genes lie in a linear order in the chromosomes.
b. The genes that are linked, stay on the same chromosome.
c. The distance between the linked genes in the chromosome determines the strength of linkage. The closer the genes are located, stronger is the linkage.
Linkage is of two types – complete and incomplete linkage.
a. Complete Linkage:
When the genes are very close to each other they have no chance of separation from each other and are always transmitted together, generation to generation. The genes are then called linked genes and the phenomenon of inheritance of completely linked genes is called complete linkage.
b. Incomplete Linkage:
The genes that are located far away from each other show a tendency to separate during meiotic prophase by crossing over. These genes are known as incompletely linked genes and this phenomenon of inheritance is called incomplete linkage.
Incomplete linkage was demonstrated by Bateson and Punnett in Sweet pea. They crossed a dominant Sweet pea plant homozygous for blue flowers and long pollen (BBLL) with a double recessive plant homozygous for red flowers and round pollen grains (bbll).
The F1 were heterozygous for blue flowers and long pollen grains (BbLl). When the F1 hybrid was test crossed with a double recessive parent (bbll), instead of the normal 1:1:1:1, an unexpected phenotypic ratio of 7:1:1:7 was obtained (Fig 3).
When independent assortment takes place, the frequency of the four F2 phenotypes is 25% each, with the parental types equal to 50% and the recombinants equal to 50%. In Sweet pea, the parental types accounted to 87.4% and the recombinant to 12.6%. This shows that the genes do not assort independently and that they are linked.
It may be concluded that since new combinations or recombinants are produced, linkage is incomplete and crossing over has occurred. If the genes were completely linked, the frequency for recombinants would have been zero per cent. (Table 1).
Crossing over is the mutual exchange of segments between non-sister chromatids of homologous chromosomes. The chromatids in which crossing over has occurred are called crossovers or recombinants and the chromatids that remain intact are called non- crossovers or parental chromatids (Fig. 4).
The process of crossing over occurs by a mechanism called the breakage and reunion theory by Darlington. It comprises of four steps – synapsis, tetrad formation, exchange of chromatids and disjunction.
a. Synapsis:
The pairing of homologous chromosomes during prophase of meiosis I is called synapsis. Synaptic forces attract the homologous chromosomes and bring them close together. The paired homologous chromosomes are called bivalents.
b. Tetrad Formation:
The chromatids of each bivalent slightly separate and become visible and this group of four chromatids is called a tetrad- Crossing over occurs in the four stranded stage.
c. Exchange of Segments or Crossing Over:
Adjacent non-sister chromatids break at homologous sites and mutually exchange small segments and rejoin. The crossing of two chromatids is called chiasma formation and the resultant cross is called chiasma or chiasmata.
d. Disjunction:
After crossing over is completed, the synaptic forces end and the homologous chromosomes move apart. The chiasma moves in a zipper like fashion towards the end of the tetrad. The movement of chiasma is called terminalisation.
Types of Crossing Over:
Crossing over may be of the following types – single, double and multiple, according to the number of points at which it occurs.
a. Single Crossing Over:
When only one chiasma occurs, it is called single crossing over. It produces two non-crossover chromatids and two crossover chromatids (Fig. 5).
b. Double Crossing Over:
When the crossing over occurs at two points in the same chromosome pair it is called double crossing over. It produces four crossovers. It may be reciprocal or complementary.
i. Reciprocal double crossing over takes place between two non-sister chromatids and produces two crossover and two non-crossover chromatids like the single crossing over (Fig. 6).
ii. Complementary crossing over is when both the chromatids taking part in the second chiasma are different from those involved in the first chiasma. It produces four crossovers but no non-crossovers. In complementary crossover, all four chromatids are involved.
c. Multiple Crossing Over:
When crossing over occurs at three, four or more points in the same chromosome pair they are called triple, quadruple or multiple crossing over. The number of chiasmata formed depends on the length of the chromosome. Also, each chromosome within a species has a characteristic number of chiasmata. The further apart, two genes are located on a chromosome, greater the opportunity for a chiasma to occur between them.
Significance of Crossing Over:
Crossing over is a natural phenomenon seen in living organisms and practically in all higher plants and animals.
The significance of crossing over is as follows:
a. It introduces new combinations of traits during sexual reproduction which may prove helpful in a changed environment.
b. The variation introduced are the raw materials for evolution and the formation of new species.
c. The frequency of crossing over helps in mapping of chromosomes, which determines the location of specific genes on the chromosomes.