7.03 - Genetics - Fall 2004 Massachusetts Institute of Technology Professor Chris Kaiser Professor Gerry Fink Professor Leona Samson 11. Consider an autosomal recessive trait that occurs at a frequency of 10-6 in a specific population that is at Hardy-Weinberg equilibrium (ie. random mating is occurring). When answering the following parts, show all of your calculations. (a, 7 pts) Draw a pedigree below that shows a mating between two relatives that would correspond to an inbreeding coefficient that equals 0.007813. Denote the mating between relatives with a double-bar connecting the two related parents. Start your pedigree with the common pair of ancestors and end your pedigree with the two related parents who are mating. (b, 8 pts) Now say that all matings in Generation X of the given population are either between unrelated individuals, or have the same inbreeding coefficient as the mating described in part (a). If the incidence of the trait in Generation “X+1” increases to a frequency of 2 X10-6, what percentage of matings in Generation X must have been between unrelated individuals? 2(c, 4 pts) Now assume that this autosomal recessive trait causes lethality in childhood. If a constant percentage of matings are between related parents for many generations, would you predict that q would increase OR decrease? (d, 9 pts) A constant percentage of matings has occurred between related parents for many generations, and yet you find experimentally that q has not changed. Of the three choices below, circle ALL that could potentially act against the effect you chose in part (c) in order to keep q constant. Explain in one sentence why you chose or did not choose each option. Choice A: migration Choice B: heterozygote advantage (Aa over AA) Choice C: mutation 2. You are studying regulation of the Wrm1 gene, a yeast gene that is expressed in response to heat. You isolate a wrm1::lacZ strain that expresses β-galactosidase when Wrm1 is normally expressed (which is at 36°C but not at 24°C). You use this wrm1::lacZ strain to perform a genetic screen looking for mutants that do not properly regulate expression of Wrm1. In your screen, you isolate a series of mutant strains that either show constitutive or uninducible expression of wrm1::lacZ. Your results indicate that the following is the correct pathway for regulation of Wrm1 expression. Note that WrmY and WrmX are on the same chromosome, and that WrmX, WrmZ, and Wrm1 are all on different chromosomes. heat WrmY WrmX WrmZ Wrm1 3One of the mutant strains you isolate contains a mutation called WrmX–, which is in the coding region of WrmX. You mate a WrmX– wrm1::lacZ haploid strain to a wrm1::lacZ haploid strain. The resulting diploids are white on X-gal plates that are incubated at 24°C, and are blue on X-gal plates that are incubated at 36°C. (a, 3 pts) Classify the WrmX– mutation as constitutive OR uninducible. (b, 3 pts) Classify the WrmX– mutation as dominant OR recessive. (c, 3 pts) Classify the WrmX locus as cis-acting OR trans-acting with respect to Wrm1. You next isolate a mutant strain containing a mutation called WrmY–, which is in the coding region of WrmY. You mate a WrmY– wrm1::lacZ haploid to a wrm1::lacZ haploid. The resulting diploids are white on X-gal plates, regardless of the temperature at which the plates are incubated. (d, 6 pts) Classify WrmY– by the type(s) of mutation it could be with respect to Wrm1. (Your choices are: repressor –, activator –, UAS–, URS–, super activator, super repressor, dominant negative repressor, dominant negative activator.) 4You create diploid yeast by mating WrmX– WrmY– wrm1::lacZ haploid yeast to wrm1::lacZ haploid yeast. Sporulation of these diploids yields two types of tetrads, and you correctly conclude (given the number of each type of tetrad) that the WrmX and WrmY loci are linked at a distance of 2.22 cM. (e, 14 pts) Depicted below are the two types of tetrads that resulted when you sporulated the above diploids. For each type of tetrad, state how many you found of that tetrad (out of a total of 90 tetrads), classify the tetrad as PD, NPD, or TT, and color in all of the spores that would be blue on each of the following Petri plates. Tetrad Type A Number of these tetrads out of a total of 90: __________________ Classification of these tetrads (PD, NPD, or TT): __________________ Color in the spores that would be blue in color when growing on the following plates: X-gal, 24°C X-gal, 36°CNOTE that the two plates are replicas, so the top spore on the left plate has the same genotype as the top spore on the right plate. Tetrad Type B Number of these tetrads out of a total of 90: __________________ Classification of these tetrads (PD, NPD, or TT): __________________ Color in the spores that would be blue in color when growing on the following plates: X-gal, 24°C X-gal, 36°C 53. You are mapping a certain disorder that is caused by an allele at the N locus. You suspect that the N locus is linked to SSR112 on human chromosome #17. You analyze the following family for these two loci. You will fill in the charts below in subsequent parts of the problem. Individual 1 Individual 2 A alleleB alleleC alleleD alleleSSR 112 maternally inherited allele at SSR112 paternally inherited allele at SSR112 IF the condition is autosomal recessive [parts (a) and (b)] (Individual 2 is “nn”) maternally inherited allele at the N locus paternally inherited allele at the N locus IF the condition is autosomal dominant [parts (c) - (e)] (Individual 2 is “Nn”) maternally inherited allele at the N locus paternally inherited allele at the N locus Answer parts (a) and (b) as if the disorder is autosomal recessive and caused by the “n” allele, so that Individual 2 is “nn.” (a, 9 pts) Fill in the upper four rows of the chart using autosomal recessive inheritance for the disorder. Then answer below: which parent’s alleles will you follow to correctly calculate a LOD score between the N locus and SSR 112 -- Individual 1 or 2? 6(b, 3 pts) Draw all phases of the parent you chose in part (a) with respect to SSR 112 and the N locus that are possible given everything you know about that parent. Make sure to draw the
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