Question

What is crossing over? Give its significance.

Answer 1

Hint: Crossing over is a process where there is exchange of genetic material between the non sister chromatids of the homologous chromosomes which takes place in the pachytene stage of prophase-I.

Complete answer:
Crossing over is a process where there is exchange of genetic material or the segments during sexual reproduction between the non-sister chromatids of the homologous chromosomes. It takes place in pachytene stage of prophase-I of meiosis during synapsis and this synapsis start before the formation of synaptonemal complex and it is not completed until neat the prophase-I. Crossing over occurs between these chromatids in the region chiasma when the matching regions on matching chromosomes break and get reconnect afterwards to the other chromosome . It is one of the final phases of the genetic recombination.

Significance of crossing over:
(i) Creation of Variability: It leads to the formation of new combinations of genes or the recombination and therefore it is a genetic mechanism to create variability and that is important for improvement of genotypes through the selection.

(ii) Locating Genes: This process is useful for locating genes in the chromosomes.

(iii) Linkage Maps: It plays a vital role in the preparation of linkage maps or chromosome maps which is important as it provides information regarding the frequency of the recombination and sequence of the genes which are further required in the preparation of the linkage maps.

(iv) Linear Arrangement of Linked genes: It gives us the evidence for the linear arrangement of linked genes in a chromosome..

(v) Useful for Plant and animal breeders: Useful recombinations are used by the plant and animal breeders. They try to break up the linkage by crossing over in order to get combinations of genes which are recombinants.

Note: Crossing over creates new combinations of genes which you call as recombinants and these genes are formed in the gametes that are not found in either parent, contributing to the genetic diversity. Significance is creation of variability, locating genes, linkage maps etc.