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Lecture 5

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McGill University
Biology (Sci)
BIOL 202
Daniel Schoen

Multiple gene inheritence part 2 • Chromosomal basis of independant assortment • Recombination (a facet of multiple gene inheritance) • Polygenic inheritance • Organelle genes (cytoplasmic inheritance) Why Sex? It's quite inefficient, when you think about it. Sexual female needs male to reproduce (meiosis) =99.9% of organisms do this Clonal female, [meiosis has shut down]...These should replace sexual populations, unless theres something about meiosis thats important for life, and there is. Dandelions and the Rotifer don't need meiosis, theyre the .1%, but the vast majority do need meiosis. So why is this? Big AB is favored in population. In clonal population,AB can be produced by mutation, but won't occur simultaneously. Eventually, the population may evolve towards the favored genotype, but it will take aw hile. Sexual populations will get there a lot faster due to independant assortment\ From Lecture 4: We were discussing about more complex crosses, which can be very cumbersome ifyou use a punnet square, resolved by using other tools ex: trihybrid cross, can use a branching diagram, keep in mind, each gene is assorting independantly, Multiply the individual probabilities of the phenotypes. Ex: 5 hybrid cross, can use product rule (from probability theory) in the example in the last lecture: 1/4 * 1/2*1/4*1/2*1/4=1/256 Chromosomal basis of independant assortment (Estrella Eleanor Carothers, 1883-1957) need to follow chromosomes around Carothers==> worked wt grasshopper chromosomes. (because grasshoppers have big chromosomes, easy to see wt microscope) Long form of the heteromorphic chrm (brown) short form of the heteromorphic chrm, also brown The consequences of independant assortment: Used heteromorphic chromosomes as a benchmark, Chromosomes are unpaired --> Interphase Chromosomes and centromeres have replicated, but centromeres have not split-->Prophase Homologs synapse -->Prophase, we don't know how they find each other in the nucleus (continued) centromeres attach to spindle and are pulled to the poles of cell -->Anaphase (THIS CAN GO EITHER WAY, HALFTHE TIME, BIGAGOES UPTO THE TOPWITH BIG B, OTHER HALFTHE TIME, IF GOES WITH LITTLE b) The other equally frequent alignment two cells form --> telophase (50:50 probability, from the last step) new spindles form and centromeres finally divide -->Second Phase Four cells produced from each meiosis -->END OF MEIOSIS (have 5 gametes, 1/4 of the time AB, 1/4 ab, 1/4 Ab, 1/4 aB) N. Crassa (can be used to study meiosis unlike any other organism, it's a Haploid organism. Like yeast or moth....Advantage to working with a haploid organism is that the Genotype=Phenotype) Life cycle: Haploid organism
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