Question

The process of crossing over occurs between
A. Sister chromatids
B. non-sister chromatids
C. brother chromatids
D. non-brother chromatids

Answer 1

Hint: Crossing over occurs between prophase 1 and the metaphase 1. The different characters from the chromosomes get crossed over to give a recombinant different from both the parents. Crossing over was mainly studied in the female egg cells.

Complete answer: Chromosomal crossing over is the exchange of genetic material during sexual reproduction between two homologous non-sister chromatids during meiosis that will result in recombinant chromosomes. Crossing over was studied by Thomas Hunt Morgan.
Before starting the explanation of the question, there is a fact that there is no term like brother chromatids in biology. This work was studied on the egg cells as they were large and easy to handle so scientists named the chromatids on a feminine name that is sister and non- sister chromatids.
Crossing over will create new combinations of genes that are not found in either parent. Before meiosis, each chromosome is replicated and the replicas are called sister chromatids. During meiosis, the homologous chromosomes are separated to form gametes, which has only one member of each pair of chromosomes.
When meiosis starts, each chromosome composed of two chromatids is paired with its homologue. The chromatids of two homologous chromosomes are called non-sister chromatids.
Meiosis 1 separates homologues from each other while meiosis 2 separates sister chromatids from each other. During crossing over in meiosis 1, segments are exchanged between non-sister chromatids.

So the answer is option B: non-sister chromatids.

Note: The pairing of homologues in meiosis 1 ensures that each gamete receives one member of each pair. The point of attachment of chromatids is called chiasma. The pairing of homologues brings together the almost identical sequences on each chromosome, setting this as a stage for crossing over. Crossing over can also occur between sister chromatids but their recombination will not have any genetic variation because the DNA sequences are identical between chromatids. Crossing over helps to preserve and maintain the genetic variability in the species.