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EVERGREEN INS 2008 - LECTURE NOTES

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• Genetic determinants are genes (pea color; Y, y)• Alternate versions of genes (yellow, green) are different alleles of that gene• Genotype is the set of alleles in an individual (RrYy)• Phenotype is the outward appearance of an individualTwo alleles of each gene must segregate(separate into different gamete cells during the formation of the egg and sperm in the parents)A monohybrid cross resulted in 3:1Principle of Segregation!Hypothesis:The 2 traits are independent• Dihybrid cross• These were Mendel’s results!• 9:3:3:1Law of Independent Assortmenta) If the mother and father are heterozygous for pea shape, what is the probability that you will get a homozygous dominant offspring?• Rr x Rr• P of R from mother is 1/2 and P of R from father is 1/2• P(R and R) = P(R) x P(R) = (1/2)(1/2) = 1/4b) What is the probability of getting a heterozygote?• Rr x Rr• P of R from mother is 1/2 and P of r from father is 1/2• P (R and r) = P(R) x P(r) = (1/2)(1/2) = 1/4• P (r and R) = P(r) x P(R) = (1/2)(1/2) = 1/4• P (Rr or rR) = P(Rr) + P(rR) = 1/4 + 1/4 = 2/4 = 1/2“Suppose that if a father is blue eyed and a mother is brown eyed then the probability of having a child with blue eyes is 1/4. If such parents have four children what is the probability of getting at least one blue eyed child.”• Walther Flemming; changes during cell division*• Threads appear paired prior to cell division• Paired threads separate just prior to divisionObservations on thread #• Number of threads differ between species– Roundworms = 4; Peas = 14; Humans = 46• Same between individuals within a species– All roundworms = 4• Same between cells within an individual• 1876 - demonstrated in sea urchins – Sperm cell + egg cell = embryo• 1883 - “threads” in gametes (sperm & egg cells) number 1/2 of those in body cells– All roundworm body (somatic) cells = 4– Roundworm gametes = 2• 1887 - proposal: special type of division produces sperm & egg cells = MeiosisTypes of cell division:- One that creates daughter cells with same number of threads as parental cells- One that creates daughter cells with 1/2 that number? Original # restored when gametes fuseThreads are named chromosomes (“colored body”)Walter Sutton:• 12 distinct types (size, shape), but 24 total*a-k = autosomes; X = sex chromosome• Paired threads = homologous chromosomes (homologs)One Maternal, one Paternal12 distinct types (size, shape), but 24 totaln = # of complete chromosome sets (n = 1 set)n = 12 = haploid number2n = 24 = diploid number; 2 versions of each typeKaryotypeChromosome terminologyMitosis• Chromosomes have already replicated• Sister chromatids separate --> equal allocation of chromosomes to daughter cells– Remember, the point is to make a cell that is an exact replica of the parentMeiosis• 2 cell divisions: Meiosis I and Meiosis IIMeiosis I:– Homologs form tetrads– Non-sister chromatids cross over!– Homologs separateMeiosis II:– Sister chromatids separate• Full compliment (diploid #) restored upon fertilizationMeiosis I: StagesMeiosis II: StagesHow is heritable variation produced?1. Homologous chromosomes align independently on the metaphase plate• Varies combination of chromosomes in daughter cells2. Crossing over (recombination) between non-sister chromatids at chiasmata shuffles alleles• Varies combinations of alleles on chromosomes in daughter


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