Factors affecting crossing over

 The frequency of crossing over is influenced by several factors which are briefly discussed below

i. Distance- The distance between genes affects the frequency of crossing over. The greater the distance between genes is the higher is the chance of crossing over and vice versa.

ii. Age- Generally crossing over decreases with advancement in the age in the female Drosophila.

iii. Temperature- The rate of crossing over in Drosophila increases above and below the temperature of 22°C.

iv. Sex- The rate of crossing over also differs according to sex. There is a lack of crossing over in Drosophila male and female silk moths.

v. Nutrition- The presence of metallic ions like calcium and magnesium in the food caused a reduction in recombination in Drosophila. However, the removal of such chemicals from the diet increased the rate of crossing over.

vi. Chemicals- Treatment with mutagenic chemicals like alkylating agents was found to increase the frequency of crossing over in Drosophila females.

vii. Irradiation- Irradiation with X-rays and gamma rays was found to enhance the frequency of crossing over in Drosophila females.

viii. Structural changes- Structural chromosomal changes especially inversions and translocations reduce the frequency of crossing over in the chromosomes where such changes are involved.

ix. Centromere effect- Generally genes that are located adjacent to the centromere show a reduced frequency of crossing over.

x. Cytoplasmic genes- In some species, cytoplasmic genes also lead to a reduction in crossing over. For example, Tifton male-sterile cytoplasm in pearl millet.