Could someone explain this to me please?

Question

"Crossing over occurs at the 4-strand stage. A single crossover between two genes at meiosis results in 2 parental-type chromatids and two recombinant chromatids. Double crossovers result in the following proportions of recombinant chromatids: Two-stand double crossover gives 0% recombination. Four strand double crossover gives 100% recombination. (This is equally common as the two strand DCOs). Three stand double crossover gives 50% recombination, and is twice as common as either the 2 stand or 4 strand event. Overall the result is that for genes far from each other, there will be independent assortment (50% recombinant progeny)." http://www.wsu.edu/~thorglab/biol301/lectures/lecture0130.html
I only need help with this: does it mean that two non-sister chromatids will cross over usually, but sometimes they'll cross over again, undoing the first crossover? and sometimes a third chromatid will cross over with one of those two; and that sometimes all four chromatids cross over? Right?

Answer 1

crossing over occurs only at certain specific time i.e. once but at different places on the chromatids. Double cross over means crossing of non sister chromatids on two different places at the same time. after completion of crossing over, some meiotic phases occurs & after this due to meiosis gamete formation takes place. So there is no two times cross over in the same cell.

Answer 2

Good for you! Yes, what you've given is what the paragraph meant. The chromatids don't 'know' which is a sister and which is a non-sister so the breaking and rejoining can affect either one (all four chromatids can play, as you said). But, the sisters are alike genetically so usually you can't tell if they've broken and rejoined. You use the gene variants (alleles) to track which bits of chromatids came from which original chromatids, and the ones with the same alleles can't be told apart (sisters = genetic twins, for chromatids).

Answer 3

no the first crossover remains as is. additional crossovers just increase variation/uniqueness of the chromosome.

all 4 chromatids cannot crossover only the two in the center can.

a 2 strand dc does not result in recom bc it basically undoes it self in the second cross over.

Answer 4

The question is not clear.Crossing over occurs at 4 strand stage only.When there are more crossing overs they do not influence each other.50% recombinants refer to the strands but not the genes.The possibility of all strands participating in crossing over is rare.