Question

Crossing over occurs in
A. Four strand stage
B. Three strand stage
C. Two strand stage
D. Single strand stage

Answer 1

Between prophase I and metaphase I crossing over happens and is the mechanism in which two homologous non sister chromatids hook up with each other and share various genetic material segments to generate two recombinant chromosome chromatids.

Complete answer:
Crossing over is known as the allocation of genetic material between the non -sister chromatids of two homologous chromosomes during sexual reproduction which ends in recombinant chromosomes.
It is one of the final stages of genetic recombination, which happens during a period called synapsis in the parenchyma stage in prophase I of meiosis. Before the synaptonemal starts forming synapsis takes place and is not finished until the end of prophase I.
At the four strand stage mainly the crossing over happens. It happens in the pachytene flower. The genetic material interaction between the bivalent non -sister chromatids is called crossing over.
Segment interchange happens at four strand stages seen between chromosomes as well as between chromatids. Even two strand crossing will lead to four recombinants, two parental variations and two recombinants will be produced by four strand crossing over all four gametes generated from a mother cell.

Hence, the correct answer is option (A).

Note: Crossing over value is known as the similar level of crossing over between two gene loci. For a given range of environmental and genetic factors, recombination appears to be stable in certain areas of linking structure and the same applies to the cross over factor used in the development of genetic maps.