Types of crossing over

 Depending upon the number of chiasmata involved, crossing over may be of three types, viz., single, double and multiple

i. Single Crossing Over- It refers to the formation of a single chiasma between non-sister chromatids of homologous chromosomes. Such cross-over involves only two chromatids out of four.

ii. Double-crossing over It refers to the formation of two chiasmata between non-sister chromatids of homologous chromosomes. Double crossovers may involve either two strands or three or all four strands. But only two strands involve in the crossing-over process at each cross-over point. Again, there are two types in this; complementary and reciprocal. In reciprocal crossing over the same two strands involve in both the cross overs and in complementary cross over different pairs of chromatids are involved in both cross overs.

iii. Multiple crossing over - The presence of more than two crossovers between non-sister chromatids of homologous chromosomes is referred to as multiple crossing over. The frequency of such type of crossing over is extremely low.