Question

In sexually reproducing organisms, there are three main sources of genetic variation. Explain two of these sources and briefly describe what it is and how it leads to genetic variation. 

Answer 1

In sexually reproducing organisms it is meiosis that takes place in the formation of male and female gametes which later fuse to form the embryo called fertilization. During meiosis, there is occurence of two main process which are for responsible for genetic variations :

1) Crossing over - In the meiosis |, during pachytene of prophase |, crossing over occurs bet. ween the non-sister chromatids of the homologous chromosomes. The exchange of genetic material between the non-sister chromatids of the homologous chromosomes is known as crossing over. Crossing over leads to recombination of genetic material which involves a mutual exchange of the corresponding segments of non-sister chromatids of the homologous chromosomes. it takes place by breakage and reunion of chromatids segments. The site where crossing over occurs forms a recombination nodule. The recombination is an enzyme-mediated process. An enzyme called recombinase is involved in this process.

2) Double metaphasic plate - After completing prophase |, chromosomes enter metaphase |. The nuclear envelope disintegrates, hence the chromosomes move into the cytoplasm. The bivalent chromosomes align themselves on the equatorial plate. The centromeres of the two chromosomes are arranged in two rows at the equator. So double metaphasic plate is formed. The distribution of bivalent chromosomes are at random. The two chromosomes can face either of the poles. There is no fixed direction in which paternal or maternal chromosomes would face which lead to the genetic recombination.