F13 lecture 4 - today meio*c recombina*on Thursday Hardy...

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today meio*c recombina*on Thursday Hardy-­‐Weinberg with exercise Homework will be assigned DUE NEXT THURSDAY 19-­‐SEP Next Tuesday – molecular markers
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the heterogame*c (XY) sex and new muta*ons most new muta*ons, if recessive, and on autosomes, are lost even advantageous ones ini*al frequency = 1/2N (N = pop size) rare alleles almost completely in heterozygotes recessive phenotype not expressed driS determines their fate but if on the X are expressed in males most muta*ons are deleterious => stronger purifying selec*on on X in males faster evolu*on of X linked genes
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recombina*on what is it? also called crossing over occurs at metaphase of first meio*c division physical exchange of material between homologous chromosomes simplest scenario: single exchange between two markers much more complicated events are possible
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recombina*on is in*mately involved with meiosis, sexual reproduc*on DNA repair recombina*on machinery = DNA repair machinery advantages of DNA repair may have helped spur the evolu*on of meiosis/sexual reproduc*on
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details of meiosis vary widely between species Drosophila (and insects in general) no recombina*on in males Drosophila on average, one exchange per chromosome per meiosis seems to be required for proper segrega*on Mammals, including humans recombina*on rates differ by sex and by chromosome males generally have slightly lower rates
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where on the chromosome does recombina*on occur? some species: specific DNA sequences other species: random with hot spots and cold spots recombina*on landscape (see next 2 slides for examples)
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True, J.R., Mercer, J.M. and Laurie, C.C. 1996 Differences in crossover frequency and distribu*on among three sibling species of Drosophila . Gene.cs 142:507-­‐523.
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Fig. 5 A gene*c map of the X chromosome. The grey plot shows cumula*ve gene*c distance on a 100<ce:hsp sp="0.25"/> kb-­‐step scale with an iden*cal ra*o of cM and Mb scales to those of <ce:cross-­‐ref refid="fig0010"> Fig. 2</ce:cross-­‐ref> maps... C. Phillips , D. Ballard , P. Gill , D. Syndercombe Court , ?. Carracedo , M.V. Lareu The recombina*on landscape around forensic STRs: Accurate measurement of gene*c distances between syntenic STR pairs using HapMap high density SNP data Forensic Science Interna*onal: Gene*cs Volume 6, Issue 3 2012 354 -­‐ 365 hkp://dx.doi.org/10.1016/j.fsigen.2011.07.012
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recombina*on / sexual reproduc*on advantages DNA repair recombina*on = varia*on among offspring survival in varying environment survival of epidemic diseases, parasites Avoiding Muller’s ratchet (see later slide) disadvantages several costs (1) cost of males = males do not produce offspring => asexual species = all females = all produce offspring (John Maynard Smith) (2) costs of ma*ng = finding males, sexual behavior (risky), sexually transmiked diseases (3) cost of recombina*on breaks up advantageous mul*-­‐locus genotypes (4) “cost of meiosis” – sexual
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