Factors Affecting Crossing 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 effect it. These are:

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

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

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

4. 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.

5. 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.

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

7. Inversions: Inversions of chromosome segments suppresses crossing over.

8. 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.

9. Cytoplasm: Factor for crossing over is present in cytoplasm and is inherited to the offsprings (Thoday and Boam and Lawrence).

10. Nutritional Effect: Crossing over frequencies are affected by concentration of metallic ions, such as calcium and magnesium. Levine fed young Drosophila females on a high calcium diet and found a decrease in crossing over

11. Genotype Effect: Crossing over frequencies between the same two loci in different strains of the same species show variations because of numerous gene differences.

12. Chromosome Structure Effect: Changes in the order of genes on a chromosome produced by chromosomal aberrations usually acts as cross over suppressors.

13. Centro-mere Effect: Genes present close to the centro-mere region show reduced crossing over.