lecture outlines for posting7

lecture outlines for posting7 - Lecture 7 EEB 13 29 January...

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Lecture 7 – EEB 13 – 29 January 2008 Dr. Hespenheide Q: How is variation in genotypes, necessary for selection, produced? A: Darwin: “mutation” - incorrect (although Darwin didn’t really know what mutation is) A: now - recombination recombination = new genotypes - produced by sexual reproduction 1. sexual reproduction itself: N mom + N dad = 2N offspring that is, offspring are not identical to either parent, but a combination of both Q: Why and how did sexual reproduction (a property of eukaryotes) evolve? - the question has puzzled evolutionary biologists, but sex is thought to have evolved because it produces a greater variety of genotypes even though it reduces rates of reproduction (because half the population is non-reproductive males). - sources of variability produced during meiosis 2. independent assortment of maternal and paternal chromosomes to gametes – result of reduction division example: - loci A and D are on different chromosomes - individual is diploid with two chromosomes, one of each type, from each parent: from father (paternal chromosomes), with alleles A and D from mother (maternal chromosomes), with alleles a and d - “independent assortment” means that there is no tendency for maternal or paternal chromosomes to be inherited together when the individual produces its gametes; that is, the possible gametes are: AD, Ad, aD, and ad, and they are equally common 3. crossing over between homologous chromosomes example: - loci A and B are on the same chromosome on paternal chromosome, alleles A and B on maternal chromosome, alleles a and b remember that homologous chromosomes are paired during the 4N (tetraploid) stage of meiosis
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chromosomes are long DNA molecules that can become tangled, break and rejoin, but with a portion of the paternal chromosome being joined to a portion of the maternal chromosome (= crossing over); if the break and joining of part paternal with part maternal chromosome occurs between loci A and D, then we can get gametes with the following genotypes: most gametes: AB and ab, and a few gametes: Ab and aB how likely crossing over will take place depends on how far apart the loci are - the farther apart two loci are on a chromosome, the higher the
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This note was uploaded on 04/16/2008 for the course EE BIOL 13 taught by Professor Hespenheide during the Winter '08 term at UCLA.

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lecture outlines for posting7 - Lecture 7 EEB 13 29 January...

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