Page 1 of 5Question 1 (30 points)Gregor Mendel’s immortal experiments on plant hybridization made extensive use of variationsin seed color and in seed shape.For example:Parents: round seeded plants x wrinkled seeded plants↓F1: round-seeded x round-seeded↓F2: 3 round : 1 wrinkledOn the basis of these data, Mendel deduced that discrete particles of inheritance (that we nowcall alleles) are carried by the F1 plants, that the particles reach a compromise for the life of theF1 plant, and then follow his 1st law – equal segregation – to generate the F2 progeny. This wasquite revolutionary – for a period of science when people believed in “blending inheritance.”Come to think of it, “seed wrinkling vs not” could be in the eye of the beholder. Surely someseeds are more sunlit when in the pod when others, and then shrink a little it? Maybe the F1seeds are just a tiny bit wrinkled (as per the “blending inheritance” model), and then all of thephenotypic distribution in the F2 could simply be the “norm of reaction” of the F2 to the amountof sun the seeds are getting? Maybe seed shape exhibits variable expressivity?There are, in fact, 3 distinct (i.e., separate) lines of evidence in Mendel’s data that argue stronglyagainst this interpretation. The 3:1 phenotypic ratio one sees in the F2 is unequivocally the resultof the distinct genetic constitution of the F2 generation, and not an environmental effect on agenetically uniform population – as Mendel’s experimental layout and data clearly show. In one-two sentences each, not more, please describe those three separate lines of evidence – as theyrelate specifically to seed shape.This is a harder question that it seems, and many students undoubtedly misunderstood it, havingnot given sufficient thought to what was being asked. How did Mendel know, that he was NOTworking with an environmentally sensitive, variably expressive trait, acting in a geneticallyuniform population?1. The phenotyping split was always exactly 3:1, which one would not expect from a variablyexpressive trait.2. Mendel did further crosses on the F2 plants, and found that exacly half of them gave 3:1progeny, while the other half gave uniform progeny when selfed. This would not havehappened if there was nothing but variable expressivity3. The biology of the plant argues against an environmental effect – the 3:1 split is seen withpeas that reside in the same pod!Page 2 of 5Question 2 (30 points)A series of experiments by Thomas Hunt Morgan, and his student, Calvin Bridges, on eye colorinheritance in Drosophila, form the centerpiece of the “chromosomal fact of inheritance.” Asummary of crosses performed by Morgan is shown below.This is hypothesis-driven science at its best. Write outthe central hypothesis that T.H. Morgan pursued in hisstudies (one short sentence) – 10 pointsMorgan hypothesized that the gene that specifies redeye color resides on the X chromosomeAnswers such as “genes reside on chromosomes” get 5points. Answers such as “eye color is sex-linked” get 7points.Now, pick one cross (just one) of all the ones shown onthe left, write it out here (do not use genetic notation,just verbal descriptions of flies and their pedigrees), andexplain, what specific consequence of his hypothesisMorgan was testing in this particular cross, and how thedata from the cross supported the hypothesis. That partof the answer should take the form “if it is, in fact, truethat …, then one would expect that …” (20 points)A sample answer (several such answers are possible,and will be correct):Cross C was between a red-eyed female, herself thedaughter of a white-eyed male and a red-eyed female,and a white-eyed male. If Morgan’s hypothesis is true,then one expects half the males and half the femalesfrom this cross to be white-eyed.Page 3 of 5Question 3 (30 points)In class, we discussed the following 3-point testcross done by AlftedSturtevant: two Drosophila were mated: a red-eyed fly that lacked across-vein on the wings and had snipped wing edges to a vermilion-eyed, normally veined fly with regular wings. All the progeny werewild type. These were testcrossed to a fly with vermilion eyes, nocross-vein and snipped wings. Sturtevant observed 1448 progeny in8 phenotypic classes, as shown on the left. Sturtevant then usedthese data to map the genes, which are, of course, nicely linked.Many other Drosophila species exist, and many are the object ofinvestigation. You decide to take on genetic mapping in Drosophila virilis, which appears to beclosely evolutionarily related to D. melanogaster – in fact, you find that the same recessivemutations (vermillion, no cross-vein, cut wing edges) exist in this species. You perform thesame cross that Sturtevant did, but using Drosophila virilis: a red-eyed fly that lacked a cross-vein on the wings and had snipped wing edges to a vermilion-eyed, normally veined fly withregular wings. Not surprisingly, all the progeny are wild type. You testcross them to a fly withvermilion eyes, no cross-vein and snipped wings, and are absolutely stunned to discover theproverbial “something completely different.” In analyzing 1,000 progeny from this cross, youfind the following animals in your vials:Red eyes, no cross-vein, cut wing edges 395Vermillion eyes, normal cross-vein, normal wings 405Wild-type 98Vermillion eyes, no cross-vein, normal wings 102Once you’ve recovered from the emotional shock of having seemingly disproven a central tenetof genetics, you do two things (not necessarily in that order):1. Come up with a hypothesis that explains the striking difference in the mapping data betweenD. melanogaster and D. virilis (10 points)The ct and cv genes appear to have relocated very closely to each other in D. virilis (any form ofstatement that communicates the fact that the two genes are directly next to each other gets fullcredit)2. Map – to the best of your ability – the genes in the D. virilis genome. Show your work. (20points)v – 20 mu – ct/cv(cannot determine order of the latter two from the data)Page 4 of 5Question 4 (10 points)You have two haploid yeast strains, one with a mutation at locus A and one with a mutation atlocus B. The two loci are on different chromosomes. You mate the strains to form a diploidzygote, then sporulate (force the diploid to undergo meiosis) to form haploids. You randomlyselect 1000 of these haploid progeny, grow them up into cultures, and do a
View Full Document
Unlocking...