Question

Crossing over occurs between:
A. Two non-sister chromatids of a homologous pair of chromosome
B. Two chromatids of any chromosome
C. Two chromatids of same chromosome
D. All of the following

Answer 1

There are two types of cell division:
1. Mitosis
2. Meiosis
Mitosis is the division in which there is no exchange of genetic material. It takes place in the somatic cells.
Meiosis is the division in which there is the exchange of genetic material. This exchange takes place at the homologous chromosomes. Homologous chromosomes are those chromosomes which have the same length and same position of the centromere. These chromosomes pair up at the time of meiosis. Crossing over takes place at these homologous chromosomes.

Complete answer:
To solve the given question, we must be well aware of the crossing over.
Crossing over is the exchange of genetic material between the non-sister chromatids of homologous chromosomes. Crossing over takes place at the pachytene stage of prophase 1. The alleles are assorted on the parental chromosomes. This gives rise to different combinations of alleles on the chromosomes. This causes genetic variations in the population.
Homologous chromosomes always occur in pairs. The pairing of these chromosomes always creates a bivalent or a tetrad. A tetrad structure is the one in which four arms of the chromosomes are visible.
Crossing over is also known as recombination.
Synapsis is also the pairing over of the arms of the non-sister chromatids. Crossing over takes place after synapsis. Recombinase enzymes help in the process. After crossing over the visible crossovers are called chiasma. This is crossing over.

So, the correct answer is option A.

Note: It must be noted here that the exchange of chromosomes is never between the unrelated chromosomes. The main advantage of crossing over is increasing the types of phenotypes in the population. Further. The chances of survival of the offspring are also increased.