The frequency of cross between genes is basically dependent upon the distance between the two, but a number of other genetic, environmental and physiological factors are known to affect the frequency of cross over.
The following are some of them:
1. Sex of the organism:
In certain individuals, cross over may be reduced, suppressed or even totally eliminated altogether. The following table gives a comparison between the male and female individuals of a species with regard to the frequency of cross over.
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2. Age of the organism:
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Bridges (1917) in his studies on Drosophila has shown that with the increasing age in the flies, the frequency of cross over particularly of those genes close to centromere would be substantially reduced.
3. Temperature:
The rate of cross over will be fairly normal at a temperature range between 17 to 29 (Plough 1917). Any alteration in this range, either decrease or increase will correspondingly decrease or increase the frequency of cross over. This is however subject to certain limit.
4. Decreased Hydration:
Lowering of water content in the surroundings is known to reduce the frequency of chiasma. It may even result in complete suppression of cross over in extreme dehydrated conditions (Sybenga 1972).
5. Distance between the genes:
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This is perhaps the most important factor influencing the frequency of cross over between two gene loci. As has been repeatedly pointed out, proximity between genes reduces the chances of cross over while distance increases the rate of cross over.
6. Ionizing radiation:
Organisms subject to the effect of ionizing radiations show increased cross over frequency as seen in the case of Drosophila. One of the reports (Sybenga 1972) indicates that even in Drosophila males, crossing over may be induced by radiation.
Work on Lilium and Chlamydomonas (Lawrence 1965) have indicated radiation effects on chiasma frequency.
7. Chemicals:
Certain chemicals which induce chromosomal aberrations are also known to induce or suppress crossing over.
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Ethyl methane sulphonate is known to induce somatic cross over. Colchicine also prevents cross over by preventing pairing between chromosomes. High dose of selenium will also reduce crossing over frequency.
8. Chromosomal aberrations:
Inversion in chromosomes alters the gene sequence and thus reduces pairing and crossing over. This is particularly true in inversion heterozygote, when any cross over in the inverted portion would result in duplications and deficiencies in the meiotic products.
In addition to the above, some of the cytoplasmic factors are also known to have an influence on the frequency of cross over.
