3 Multiple Choice 1 point What piece of evidence suggested that the loci studied by Morgan and Sturtevant were on the same chromosome, but separated by some genetic distance? O The genotype frequencies were somewhere between completely independent (as expected for separate chromosomes) and tightly coupled (as expected by adjacent on a single chromosome) O None of the above O The b+b/vg+vg genotype was found in the cross (impossible without recombination) O None of the expected genotypes were found in the cross
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In Section $4.6 .1,$ we briefly described Sturtevant's analysis of mutant flies that culminated in the generation of the first chromosome map. In Table $4.2,$ we show the crossover data associated with the different mutations that he used to draw the map. A crossover refers to a chromosomal rearrangement in which parts of two chromosomes exchange DNA. An illustration of the process is shown in Figure $4.26 .$ The six factors looked at by Sturtevant are $\mathrm{B}$, $\mathrm{C}$, O, $P, R,$ and $M .$ Flies recessive in $B$, the black factor, have a yellow body color. Factors $C$ and $O$ are completely linked, they always go together and flies recessive in both of these factors have white eyes. A fly recessive in factor P has vermilion eyes instead of the ordinary red eyes. Finally, flies recessive in $\mathrm{R}$ have rudimentary wings and those recessive in M have miniature wings. For example, the fraction of flies that presented a crossover of the $\mathrm{B}$ and $\mathrm{P}$ factors is denoted as BP. Assume that the frequency of recombination is proportional to the distance between loci on the chromosome. Reproduce Sturtevant's conclusions by drawing your own map using the first seven data points from Table 4.2 Keep in mind that shorter "distances" are more reliable than longer ones because the latter are more prone to double crossings. Are distances additive? For example, can you predict the distance between $\mathrm{B}$ and $\mathrm{P}$ from looking at the distances $\mathrm{B}(\mathrm{C}, \mathrm{O})$ and $(\mathrm{C}, \mathrm{O}) \mathrm{P} ?$ What is the interpretation of the two last data points from Table $4.2 ?$
One of the standard techniques to determine whether two loci are physically close to one another on the same chromosome is to conduct a "dihybrid cross", a cross of two individuals that are heterozygous at both loci. For example, if the two loci have alleles A and a at the first locus and B and b at the second locus then crossing two AaBb individuals (where the capital alleles are known to be on the same chromosome) is performed. If the loci are not close to one another (or are on different chromosomes) then the expected frequencies of the gametes would be 1/4 AB, 1/4 Ab, 1/4 aB and 1/4 ab - leading to expected frequencies of 1/16 for each of the possible gamete combinations in the offspring. Consider the following situation. At the first locus AA homozygotes are Black while aa homozygotes are white and the A allele is dominant to the a allele. At the second locus BB individuals are large while bb individuals are small and Bb individuals are intermediate in size. An experiment is conducted in which two AaBb heterozygotes are crossed to produce numerous offspring with phenotypes in the following numbers. Observed Black large 297 Black intermediate 649 Black small 288 White large 119 White intermediate 244 While small 103 Perform a chi-square test to determine whether the loci are likely to be physically close to one another. (1, 4 pts) What X² value do you obtain? (report to nearest 0.001) X² = ? (2, 2 pts) How many degrees of freedom do you have for this X² statistical test? df = ? (3, 2 pts) Choose the statement that best matches the conclusion of your t test. Your choices will be: The observed frequencies of the phenotypes are not significantly different from those predicted from a model of independent assortment ( 0.05 < p ). The observed frequencies of the phenotypes are not significantly different from those predicted from a model of independent assortment ( 0.025 < p < 0.05 ). The observed frequencies of the phenotypes are not significantly different from those predicted from a model of independent assortment ( 0.01 < p < 0.025). The observed frequencies of the phenotypes are not significantly different from those predicted from a model of independent assortment ( 0.001 < p < 0.01 ). The observed frequencies of the phenotypes are not significantly different from those predicted from a model of independent assortment ( p < 0.001 ). The observed frequencies of the phenotypes are significantly different from those predicted from a model of independent assortment ( 0.05 < p ). The observed frequencies of the phenotypes are significantly different from those predicted from a model of independent assortment ( 0.025 < p < 0.05 ). The observed frequencies of the phenotypes are significantly different from those predicted from a model of independent assortment ( 0.01 < p < 0.025). The observed frequencies of the phenotypes are significantly different from those predicted from a model of independent assortment ( 0.001 < p < 0.01 ). The observed frequencies of the phenotypes are significantly different from those predicted from a model of independent assortment ( p < 0.001 ). (4, 2 pts) Choose the statement that best matches your biological conclusion. Your choices will be: The color and size loci are physically close (linked) to each other on the same chromosome. The color and size loci are not physically close (linked) to each other on the same chromosome.
Adi S.
Map the above loci along a chromosome. The frequency of crossing over is proportional the distance separating two genes on a chromosome. The greater the distance, the more likely for crossing over to recombine those alleles. Scientists use a simple calculation to estimate the distance separating two genes on a chromosome. The percent of offspring showing recombinant phenotypes equals the distance between the two chromosomes. The unit of this distance is centimorgans (cM) or map units (MU). a. Calculate the separation distance of the A and B genes using the data on the frequency of offspring phenotypes above. [Number of recombinant phenotypes__________/total offspring__________] X 100 = __________ 5. Another gene in flies that is on the same chromosome as the A and B genes is body. The gray allele is dominant over yellow. A breeding study similar to the one above determined that recombination rates between the G locus and the B locus were 3% and the recombination rate between the A locus and the G locus were 28%. Using this information, draw a map of the chromosome indicating the relative positions of the A, B, and G loci.
Sri K.
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