Genetics 466 - midterm 2 outline

Genetics 466 - midterm 2 outline - I Genetic Linkage and...

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I. Genetic Linkage and Recombination A. Independent segregation gives equal ratios of gamete types a. 1:1:1:1 for AB:Ab:aB:ab B. Recombination and Linkage gives unequal ratios of gamete types a. 1:9:9:1 for example for AB:Ab:aB:ab in a dihybrid cross of AAbb x aaBB b. genes are positioned close together spatially on a chromosome C. Parental vs. Recombinant gametes a. Parental gametes in F 1 have the same arrangement as P generation i. AAbb x aaBB AaBb (x) parental gametes are Ab, aB b. Recombinant gametes in F 1 have a switched representation from P generation i. AAbb x aaBB AaBb (x) recombinants are AB and ab D. Due to spatial proximity it is more likely that chiasmata does NOT separate alleles on a chromosome thus parental gametes occur in greater proportion a. Remember chiasmata begins in Prophase I and is resolved in Anaphase I E. Recombination Frequency a. Genetic distance between two genes is the average number of crossovers between them b. Recombination Frequency (RF) = total # of recomb offspring ÷ total # offspring c. RF × 100 = Map distance in map units or CentiMorgans F. Three-Factor Cross – test cross to determine gene order a. Double Crossovers must be accounted for in total map distance so in a three point cross we find this formula for map distance: b. Map Distance = [# recombinants + 2×(#double recombinants)] ÷ total c. Double recombinant class is the smallest in number because it is least likely for two chiasmata to occur and is almost always the middle in the gene order G. Interference: extent to which crossovers occur independently of each other a. P(expected double crossover) = P(single a-b) × P(single b-c) b. P(observed double crossover) = # double recomb ÷ total # offspring c. Coefficient of Coincidence = P(observed) ÷ P(expected) d. Interference = 1 – coefficient of coincidence e. Proportion of Double Recombinant Offspring = (1-I) × P(expected) f. Low interference = 0, High interference = 1 H. Double crossovers are rare within shorter intervals (~20cM) and increase in frequency as the interval length increases II. Chromosome Mapping A. There is a correspondence between the physical and genetic map of a chromosome. a. Physical map is measured in DNA base pairs b. Genetic map is measure in m.u. or cM, it is a function of recombination and offspring ratios B. The two are different because frequency is different in different regions a. Recombination near chromosome ends and near centromeres is lower III. Deletions A. There is no recombination within deleted region no homologous material that corresponds to deletion loop. a. Markers flanking deletions appear closer than they are because of decreased recombination 1
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B. Gametes with chromatids that undergo recombination do not produce viable offspring a. Paracentric inversions reduce the number of recombinant progeny the recombinants die crossover suppressors C. Balancer Chromosomes – contain multiple inversions so that they effectively
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Genetics 466 - midterm 2 outline - I Genetic Linkage and...

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