FIG.8. Mitotic crossing-more and you may homozygosity. A corner-over of a couple of five chromatids regarding an excellent homologous couple of chromosomes leads to an excellent rearrangement regarding alleles between the two chromosomes. During the second mitotic section, the fresh chromosomes normally orientate on their own in 2 you’ll means, causing a couple of you’ll be able to modes out of chromatid separation: (a) step one and you may 4 breaking up off dos and you can 3, or (b) step one and you will step 3 separating out-of 2 and you may cuatro. One among them one or two you can easily settings, (b), leads to homozygosity of one’s facts away from mitosis to have alleles on people locus (B/b) distal to the position regarding mitotic crossing-over.
The process of haploidization throughout the parasexual years appears to be an imprecise you to definitely and to reflect crashes of mitosis causing the production of nuclei having chromosome complements starting within diploid and haploid numbers
throughout haploidization. Those nuclei which have imbalanced chromosomal constitutions is at a severe drawback in contrast to the newest well-balanced haploid nuclei, and this thus predominate between the segregants. Brand new regularity away from haploidization is actually greatly improved because of the cures which have agents such as for instance pfluorophenylalanine, which probably act by the preventing brand new mitotic spindle. Mitotic crossing-more than provides comparable characteristics so you can crossing-over at meiosis, from inside the connected with one chromatid simply of the two contained in for each acting chromosome, nevertheless volume of such transfers try lowest. Therefore
This new parasexual course try off primary benefits on genetic studies out of incomplete fungi (fungus lacking an everyday sexual cycle), whilst will bring a means of gene mapping, one another long-range and you can good
more than one multiple mix-over in the certain chromosome partners is really impractical (but in short menstruation : select afterwards). Mitotic crossing-over, while the label ways, isn’t with the loss in the chromosome amount, therefore the results of good mitosis where a mix-over keeps happened is actually a set of diploid nuclei with some rearrangement away from alleles between homologous chromosomes. New genetic structure of one’s child nuclei relies on this new relative orientation on mitotic metaphase of these two homologous chromosomes you to definitely was in fact involved in crossing-more. There have been two choices (Fig. 8). Among them (Fig. S(a)) contributes to no loss of heterozygosity of the child nuclei, merely reassortment away from markers ranging from homologous chromosomes; additional (Fig. 8b) results in homozygosity when it comes to indicators that have been before heterozygous, provided it rest between the section from crossing-more than and also the prevent of one’s chromosome in a beneficial distal guidance (that is off the centromere). To-do the fresh parasexual stage, haploidization sustained by the all descendant nuclei once mitotic crossing-over provides go up so you’re able to haploids which have the new combos from linked indicators. (b) Density of your parasexual period. nidulans, a fungi which have a consistent sexual period which will be used to confirm the new legitimacy of findings out-of genetic data based on this new parasexual cycle. Using this varieties it was prolonged so you can relevant ascomycetes and no intimate duration, for example Aspergillus niger (Pontecorvo, Roper and you may Forbes, 1953) and you can Penicillium chrysogenurn (Pontecorvo and you may Sermonti, 
1954) in order to almost every other fungus, each other filamentous and you will fungus-such (come across afterwards). (c) Hereditary study by the parasexual stage. Into the bacteria which have a routine sexual course, elements of the fresh new parasexual duration was indispensable for the getting possibilities having performing examination out-of prominence and complementation (by using the heterokaryons and you may diploids). Mitotic crossing-more than brings a means of determining centromere reputation towards a great linkage chart, when you find yourself haploidization provides a way for the expression linkage groups. we. The word linkage organizations because of the haploidization. On detection from genetics borne on the same chromosome, advantage try pulled of your effects out of mitotic haploidization into the recombination regarding linked and you will
unlinked family genes. Suppose i use several haploid challenges of genotype A-b-c and you can abc to help make a great heterozygous diploid filters; allow the loci An excellent l a and you can Blb feel borne on a comparable chromosome and Clc on the a different chromosome.