مُجرہ دیکھنے کے لیے نیچے تصویر پر کلک کر دیں۔ دیکھیں کیسے مُجرے کے ساتھ ساتھ سب کُچھ دکھا رہی ہے۔

model that reflects current understanding was presented by Anderson and Sekelsky,[7] and is outlined in the first figure in this article. The figure shows that two of the four chromatids present early in meiosis (prophase I) are paired with each other and able to interact. Recombination, in this version of the model, is initiated by a double-strand break (or gap) shown in the DNA molecule (chromatid) at the top of the first figure in this article. However, other types of DNA damage may also initiate recombination. For instance, an inter-strand cross-link (caused by exposure to a cross-linking agent such as mitomycin C) can be repaired by HRR. As indicated in the first figure, above, two types of recombinant product are produced. Indicated on the right side is a “crossover” (CO) type, where the flanking regions of the chromosomes are exchanged, and on the left side, a “non-crossover” (NCO) type where the flanking regions are not exchanged. The CO type of recombination involves the intermediate formation of two “Holliday junctions” indicated in the lower right of the figure by two X shaped structures in each of which there is an exchange of single strands between the two participating chromatids. This pathway is labeled in the figure as the DHJ (double-Holliday junction) pathway. The NCO recombinants (illustrated on the left in the figure) are produced by a process referred to as “synthesis dependent strand annealing” (SDSA). Recombination events of the NCO/SDSA type appear to be more common than the CO/DHJ type.[4] The NCO/SDSA pathway contributes little to genetic variation since the arms of the chromosomes flanking the recombination event remain in the parental configuration. Thus, explanations for the adaptive function of meiosis that focus exclusively on crossing-over are inadequate to explain the majority of recombination events. Achiasmy and heterochiasmy[edit] Achiasmy is the phenomenon where autosomal recombination is completely absent in one sex of a species. Achiasmatic chromosomal segregation is well documented in male Drosophila melanogaster. Heterochiasmy is the term used to describe recombination rates which differ between the sexes of a species.[8] This sexual dimorphic pattern in recombination rate has been observed in many species. In mammals, females most often have higher rates of recombination. The "Haldane-Huxley rule" states that achiasmy usually occurs in the heterogametic sex.[

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