Types of Crossing Over:
Depending upon the number of chiasmata involved, crossing over may be of three types, viz., single, double and multiple as described below:
i. Single Crossing Over:
It refers to 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 formation of two chiasmata between non-sister chromatids of homologous chromosomes. Double crossovers may involve either two strands or three or all the four strands. The ratio of recombinants and parental types under these three situations are observed as 2:2:3:1 and 4 : 0, respectively.
iii. Multiple Crossing Over:
Presence of more than two crossovers between non-sister chromatids of homologous chromosomes is referred to as multiple crossing over. Frequency of such type of crossing over is extremely low.
