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Recombination

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Meosis overview.GIF

Recombination is the exchange of DNA segments between the two copies of a chromosome (one maternally inherited and one paternally inherited). This occurs during the creation of an egg or sperm for the next generation and the mechanism that enacts it, crossing over, happens during Prophase I of Meiosis I.[1]

Once each pair of chromosomes, called homologous chromosomes, are tightly aligned along their lengths, sections of DNA are split at the same locations by what’s termed a double-strand break and the pieces of DNA are swapped between the two homologous chromosomes.[1] The locations of the breaks (crossover points) are somewhat random, though they are driven by other biological factors which, to differing degrees, either heighten or suppress the likelihood of a double-strand break at certain positions on a given chromosome.[2][3][4]

The newly formed chromosome in the egg will be a patchwork of contributions from the maternal grandparents, and the newly formed chromosome in the sperm will be a patchwork of contributions from the paternal grandparents, but there is still one copy of every gene on each individual chromosome. Some eggs and sperm will retain a complete copy of one grandparent’s chromosome without recombination. The creation of each egg and sperm is an independent event, so siblings will inherit different portions of their grandparents’ DNA.

Males and females have different rates of recombination. Studies that have used direct SNP (Single-nucleotide_polymorphism) test comparisons to identify crossover points have arrived at generally similar frequencies. For example, Chowdhury, et al., in 2009 showed an average of about 27 crossovers for the male genome and 41 crossovers for the female.[5] A methodologically similar 2008 study led by U.C. Davis evolutionary geneticist Graham Coop found a mean of about 26 crossovers for the male and 40 for the female.[6] David Reich, of Harvard’s Reich Lab, defaults to a generalized average of 26 for the male and 45 for the female genome; of note is that includes the X chromosome where the two previously cited studies looked only at the autosomes.[7]

Research using certain types of markers at a molecular, chemical level that are associated with double-strand breaks, as contrasted with SNP-based parental comparisons, typically arrives at larger numbers of per-generation recombination points. One such study by Gruhn, et al., in 2013 showed frequency ranges of 43 to 54 in males, and 53 to 88 in females.[8]

In addition to differences in recombination rates between males and females, there are also variations in recombination rates among populations.[9]

These crossover or recombination points are what determine the segment matching used in genetic genealogy for autosomal DNA and the X chromosome, which also undergoes recombination. The Y chromosome does not go through meiosis and thus does not recombine.[10]

A recent study by Veller, et al., from 2019 performed analyses of both segmental crossovers and the mechanism of independent assortment as biological processes affecting the “shuffling” of genetic material. They found that independent assortment’s contribution to the genetic shuffling diversity was “about 30 times greater than that of crossovers.”[11]

There is still much that is not yet understood about the totality of genetic recombination. This, in part, is why it can be difficult or even impossible for genetics alone to reliably determine a generational origin for an autosomal segment of modest or small size. For example, a study by Speed and Balding in 2014 indicated that fewer than 40% of segments 10 million base pairs (Mbp) in length originated in the inheritance chain within the last 10 generations, and that about 40% of segments even 20 Mbp in length dated back beyond 10 generations.[12]

Further reading

Blog posts

Scientific papers

See also

References

  1. 1.0 1.1 Scitable, by Nature Education. “Replication and Distribution of DNA during Meiosis” (2014). See also a permalink capture at Archive.org.
  2. Li, Haosheng, Erica Berent, Savannah Hadjipanteli, Miranda Galey, Nigel Muhammad-Lahbabi, Danny E. Miller, and K. Nicole Crown. “Heterozygous Inversion Breakpoints Suppress Meiotic Crossovers by Altering Recombination Repair Outcomes.” PLOS Genetics 19(4) (13 April 2023): e1010702. DOI: 10.1371/journal.pgen.1010702.
  3. Pazhayam, Nila M., Carolyn A. Turcotte, and Jeff Sekelsky. “Meiotic Crossover Patterning.” Frontiers in Cell and Developmental Biology 9 (2021). DOI: 10.3389/fcell.2021.681123.
  4. Nesta, Alex V., Denisse Tafur, and Christine R. Beck. “Hotspots of Human Mutation.” Trends in Genetics 37(8) (1 August 2021): 717–29. DOI: 10.1016/j.tig.2020.10.003.
  5. Chowdhury, Reshmi, Philippe R. J. Bois, Eleanor Feingold, Stephanie L. Sherman, and Vivian G. Cheung. “Genetic Analysis of Variation in Human Meiotic Recombination.” PLOS Genetics 5(9) (18 September 2009): e1000648. DOI: 10.1371/journal.pgen.1000648.
  6. Coop, Graham, Xiaoquan Wen, Carole Ober, Jonathan K. Pritchard, and Molly Przeworski. “https://www.science.org/doi/10.1126/science.1151851 High-Resolution Mapping of Crossovers Reveals Extensive Variation in Fine-Scale Recombination Patterns Among Humans].” Science 319(5868) 1395-1398 (7 March 2008). DOI: 10.1126/science.1151851. Note that this article is paywalled.
  7. Reich, David. Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past (London, England: Oxford University Press, 2018), 11. ISBN 978–0–19–882125–0.
  8. Gruhn, Jennifer R., Carmen Rubio, Karl W. Broman, Patricia A. Hunt, and Terry Hassold. “Cytological Studies of Human Meiosis: Sex-Specific Differences in Recombination Originate at, or Prior to, Establishment of Double-Strand Breaks.” PLOS ONE 8(12) (20 December 2013): e85075. DOI: 10.1371/journal.pone.0085075.
  9. Graffelman, Jan, David J. Balding, Anna Gonzalez-Neira, and Jaume Bertranpetit. “Variation in estimated recombination rates across human populations.” Human Genetics 122, 301–310 (July 2007). DOI: 10.1007/s00439-007-0391-6. Note that this article is paywalled.
  10. Brons, Mercedes. “All About DNA Segments.” Who Are You Made Of?; blog article posted April 2021. See also a permalink capture of the article at Archive.org.
  11. Veller, Carl, Nancy Kleckner, and Martin A. Nowak. “A Rigorous Measure of Genome-Wide Genetic Shuffling That Takes into Account Crossover Positions and Mendel’s Second Law.” Proceedings of the National Academy of Sciences 116(5): 1659–68 (29 January 2019). DOI: 10.1073/pnas.1817482116.
  12. Speed, Doug, and David J. Balding. “Relatedness in the Post-Genomic Era: Is It Still Useful?Nature Reviews Genetics 16(1):33-44 (November 2014). DOI: 10.1038/nrg3821. Note that this article is paywalled. Co-author Doug Speed reprinted the article as a PDF file on his own website.