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In graphics I've seen, crossing over occurs between the "inner" two chromatids in a side-by-side arrangement of two duplicated chromosomes:enter image description here

This suggests that only two of the four meiotic daughter chromosomes of a homologous pair can be recombinant via crossingover. Is this true? Or can you have more complex patterns, like the one below, in which all four daughters are recombinant? Does the X-shaped geometry of the doubled chromosomes play a role in models of crossover phenomena? enter image description here

ps. Sorry if my terminology is strange. I'm a beginner! Experts: Edits to improve clarity welcome!

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  • $\begingroup$ You appear to be assuming that your organism only has one chromosome. Since that is a highly unusual situation and may not be what you meant I encourage you to edit your question to make it clearer what you are trying to ask. One possibility is that you didn't mean gamete and you are actually trying to ask about chromosomes .... ———— Please also specify an organism or at least group of organisms — there are many variations on meiosis including some where no crossing over takes place! $\endgroup$ – tyersome Jun 12 at 23:39
  • $\begingroup$ Thanks for the advice, @tyersome! I hope it makes more sense now. I'm focusing on one pair of homologous chromosomes in a cell undergoing meiosis. I have drosophila in mind because that's what I've been reading about. $\endgroup$ – fmg Jun 13 at 1:37
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There can be and often is more than one crossover per chromosome in meiosis, but how many crossovers occur can depend on the species, sex, age, environment, and which particular chromosome is involved1,2,3. For example, humans typically show 2-3 crossovers per chromosome, but females often show higher recombination rates than males.

However, the structure you drew seems unlikely due to a phenomenon known as crossover interference, which suppresses the occurrence of closely spaced crossovers4,5. See also this historical introduction from Nature.

In Drosophila melanogaster, males do not make any crossovers6. In contrast, females show an average of around 1.2 crossovers per chromosome7. This means that most tetrads have just one crossover, but some will have more and this second crossover can occur between any pair of chromatids. Thus, there can be from two to four recombinant chromosomes, but most commonly there will be two.

(Note that this last reference (7) is a 2018 review article on this subject and could be a good place for you to start learning more once you've mastered the basics of this subject — figure 1 in particular seems directly relevant to your question.)

References:

  1. Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Meiosis. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26840/
  2. Fledel-Alon, A., Wilson, D. J., Broman, K., Wen, X., Ober, C., Coop, G., & Przeworski, M. (2009). Broad-scale recombination patterns underlying proper disjunction in humans. PLoS genetics, 5(9).
  3. Wang, Z., Shen, B., Jiang, J., Li, J., & Ma, L. (2016). Effect of sex, age and genetics on crossover interference in cattle. Scientific reports, 6, 37698.
  4. Berchowitz, L. E., & Copenhaver, G. P. (2010). Genetic interference: don't stand so close to me. Current genomics, 11(2), 91-102.
  5. Otto, S. P., & Payseur, B. A. (2019). Crossover Interference: Shedding Light on the Evolution of Recombination. Annual review of genetics, 53, 19-44.
  6. John, A., Vinayan, K., & Varghese, J. (2016). Achiasmy: male fruit flies are not ready to mix. Frontiers in cell and developmental biology, 4, 75.
  7. Hughes, S. E., Miller, D. E., Miller, A. L., & Hawley, R. S. (2018). Female meiosis: synapsis, recombination, and segregation in Drosophila melanogaster. Genetics, 208(3), 875-908.
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