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31 Cards in this Set
- Front
- Back
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genes that are found on the same chromosome are said to be
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linked
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crossing over refers to the exchange of genetic informtion via
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nonsister chromatids
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The amount of crossing over is primarily determined by
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distance between linked genes
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complete linkage results from the production of only
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parental gametes
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For two genes located on the same chromosome and exhibiting complete linkage, what is the characteristic phenotype ratio?
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1:2:1
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For independent assortment of two pairs of non-homologous chromosomes, each containing one heterozygous gene pair, the number of genetically different gametes that are formed is __.
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4
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Morgan proposed that recombinant genes that he observed in Drosophila were associated with __.
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chiasmata in prophase I of meiosis
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Single crossover events are only detectable if __
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they occur between the linked genes under consideration
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What is the largest fraction of recombinants in the final gametes for single crossover events?
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1/2
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Genes A, B, and C are linked. Crossover gametes between A and B are recovered 15% of the time and crossovers between B and C 10% of the time. What percentage of double crossovers is expected among A, B, and C?
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1.5%
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One map unit __.
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is equal to 1% recombination between two genes
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Their probabilities are always less than individual probabilities for corresponding single crossovers.
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double crossover events
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A double crossover experiment involving genes A, B, and C finds 1% crossover gametes for A and B and also for B and C. How many offspring would have to be examined (on average) before you would expect one double crossover?
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The double crossover probability is the product of the two single-crossovers: 0.01 x 0.01 = 0.0001 = 1/10,000. Therefore, only one event in 10,000 is a double crossover and on average we must examine 10,000 offspring before finding one with recombinant genes. This is of course a statistical statement only. It is possible that the very first offspring examined exhibits crossover.
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The correct order of the genes in a three-point mapping experiment may generally be determined by examining __.
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the phenotypes that can result from double crossovers for each possible gene order
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For linked genes A, B, and C, if the A-B map distance is 1.56 map units and the B-C map distance is 4.06 map units and the number of double crossover events out of 10,000 offspring is 6, what is the coefficient of coincidence?
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0.95
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If the observed number of double crossovers is 75, the expected number is 85, and the total number of noncrossover events is 907, what is the value of the interference?
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0.12
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The non-crossover phenotypes in a three-point crossing experiment are generally easily identified because they correspond to __.
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the most numerous events
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In order to make a successful three-point mapping cross: 1) all the genotypes under consideration must be observable phenotypically, 2) a large number of offspring must be generated for all classes, and 3) _______.
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the genotype of the parent being studied for crossovers must be heterozygous at the loci being studied
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For linked genes A, B, and C, if the A-B map distance is 2.33 map units and the B-C map distance is 3.25 map units and the number of double crossover events out of 10,000 offspring is 4, what is the coefficient of coincidence?
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The expected number of double crossovers is 0.0233 x 0.0325 x 10,000 = 7.6 and the observed number is 4. Thus, C = 4.0 / 7.6 = 0.53.
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For linked genes A, B, and C, if the A-B map distance is 1.44 map units and the B-C map distance is 2.91 map units and the number of double crossover events out of 10,000 offspring is 2, what
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0.41
The expected number of double crossovers is 0.0144 x 0.0291 x 10,000 = 4.19 and the observed number is 2. Thus, C = 2.0 / 4.19 = 0.48. |
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If the observed number of double crossovers is 62, the expected number is 78, what is the value of the interference?
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0.20
The interference is 1.0 -62/78 = 0.12. |
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If the observed number of double crossovers is 50, the expected number is 62, what is the value of the interference?
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The interference is 1.0 -50/62 = 0.19.
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The probability of a double crossover between the genes A and C along a chromosomal segment containing three genes A, B, and C is equal to:
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The probability of single crossovers between A and B times the probability of the single crossovers between B and C.
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Why didn't Mendel find linkage?
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Some genes were linked, but they were too far apart for crossing over to be distinguished from independent assortment, or linked genes were never tested for at the same time in the same cross.
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Given what is known about single, double, and noncrossovers in a three-point map, triple crossovers in a four-point map can be predicted to be __________.
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less frequent than all of the above
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The mutations blistered (bs) and clot (cl) are located on opposite ends of chromosome II in Drosophila. A blistered female is crossed with a clot male. The F1 female is then backcrossed to a blistered clot male. What ratio will be expected in the offspring of the backcross?
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1/4 wild: 1/4 clot: 1/4 blistered: 1/4 blistered, clot
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Genes A and B are farther apart than are genes A and C, and all three are linked. What CANNOT be concluded?
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C might be between A and B.
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Interference in genetics is __________.
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the phenomenon observed when one recombination event on a chromosome inhibits other closely linked recombination events
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LOD scores are used to predict __________.
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gene linkage
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If you have three genes (A, B, and C) linked on the same chromosome, how can you determine the gene order?
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Look for double crossover phenotypes involving the wild type and mutant alleles of genes A, B, and C.
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members of the same family expressing the same trait
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are not directly related to linkage groups
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