From a cross between e^+ f^+ g^+ and e^- f^- g^- strains of Neurospora, recombin

Question

From a cross between e^+ f^+ g^+ and e^- f^- g^- strains of Neurospora, recombination between these linked genes resulted in a few octads containing the following ordered set of spores: e^+ f^+ g^+ e^+ f^+ g^+ e^+ f^- g^+ e^+ f^- g^+ e^- f^- g^- e^- f^- g^- e^- f^- g^- e^- f^- g^- a. Where was recombination initiated? b. Did crossing-over occur between genes e and g? Explain c. Why do you end up with 2 f^+: 6f^- but 4 e^+: 4 e^- and 4g^+: 4g^-? d. Could you characterize these unusual octads as MI or MII for any of the three genes involved? Explain.

Explanation / Answer

Answer:

(a). The process by which genetic material is broken down and joined with another genetic material is called recombination. Here, recombination must have initiated (it means that the breakdown of first DNA strand occurs) between e and g either between e and f or between f and g.

(b). Crossing-over is the process of exchange of genetic material between genes. Here, crossing over does not takes place because the flanking of gene and the alleles of the genes are in their parental configuration. This leads to the formation of single heteroduplex region. Gene f must have been included in the heteroduplex region to obtain the resulting octad (because gene f does not segregate 4:4).

(c). Mismatching occurs between 4e+: 4 e- and 4 g +: 4 g – hence, it is ended up with 2f+: 6 f-. The chromosome that ended up with f+f-mismatch in the heteroduplex region was corrected by mismatch repair of- f-. As shown the diagram.

(d). The genes e and g have involved in MI octad, while the gene f has neither involved in MI octad, nor in MII octad.

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