Question 1 30 points Gregor Mendel s immortal experiments on plant hybridization made extensive use of variations in seed color and in seed shape For example Parents F1 F2 round seeded plants x wrinkled seeded plants round seeded x round seeded 3 round 1 wrinkled On the basis of these data Mendel deduced that discrete particles of inheritance that we now call alleles are carried by the F1 plants that the particles reach a compromise for the life of the F1 plant and then follow his 1st law equal segregation to generate the F2 progeny This was quite 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 some seeds are more sunlit when in the pod when others and then shrink a little it Maybe the F1 seeds are just a tiny bit wrinkled as per the blending inheritance model and then all of the phenotypic distribution in the F2 could simply be the norm of reaction of the F2 to the amount of 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 strongly against this interpretation The 3 1 phenotypic ratio one sees in the F2 is unequivocally the result of the distinct genetic constitution of the F2 generation and not an environmental effect on a genetically uniform population as Mendel s experimental layout and data clearly show In onetwo sentences each not more please describe those three separate lines of evidence as they relate specifically to seed shape This is a harder question that it seems and many students undoubtedly misunderstood it having not given sufficient thought to what was being asked How did Mendel know that he was NOT working with an environmentally sensitive variably expressive trait acting in a genetically uniform population 1 The phenotyping split was always exactly 3 1 which one would not expect from a variably expressive trait 2 Mendel did further crosses on the F2 plants and found that exacly half of them gave 3 1 progeny while the other half gave uniform progeny when selfed This would not have happened if there was nothing but variable expressivity 3 The biology of the plant argues against an environmental effect the 3 1 split is seen with peas that reside in the same pod Page 1 of 5 Question 2 30 points A series of experiments by Thomas Hunt Morgan and his student Calvin Bridges on eye color inheritance in Drosophila form the centerpiece of the chromosomal fact of inheritance A summary of crosses performed by Morgan is shown below This is hypothesis driven science at its best Write out the central hypothesis that T H Morgan pursued in his studies one short sentence 10 points Morgan hypothesized that the gene that specifies red eye color resides on the X chromosome Answers such as genes reside on chromosomes get 5 points Answers such as eye color is sex linked get 7 points Now pick one cross just one of all the ones shown on the left write it out here do not use genetic notation just verbal descriptions of flies and their pedigrees and explain what specific consequence of his hypothesis Morgan was testing in this particular cross and how the data from the cross supported the hypothesis That part of the answer should take the form if it is in fact true that 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 the daughter 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 females from this cross to be white eyed Page 2 of 5 Question 3 30 points In class we discussed the following 3 point testcross done by Alfted Sturtevant two Drosophila were mated a red eyed fly that lacked a cross vein on the wings and had snipped wing edges to a vermilioneyed normally veined fly with regular wings All the progeny were wild type These were testcrossed to a fly with vermilion eyes no cross vein and snipped wings Sturtevant observed 1448 progeny in 8 phenotypic classes as shown on the left Sturtevant then used these data to map the genes which are of course nicely linked Many other Drosophila species exist and many are the object of investigation You decide to take on genetic mapping in Drosophila virilis which appears to be closely evolutionarily related to D melanogaster in fact you find that the same recessive mutations vermillion no cross vein cut wing edges exist in this species You perform the same cross that Sturtevant did but using Drosophila virilis a red eyed fly that lacked a crossvein on the wings and had snipped wing edges to a vermilion eyed normally veined fly with regular wings Not surprisingly all the progeny are wild type You testcross them to a fly with vermilion eyes no cross vein and snipped wings and are absolutely stunned to discover the proverbial something completely different In analyzing 1 000 progeny from this cross you find the following animals in your vials Red eyes no cross vein cut wing edges Vermillion eyes normal cross vein normal wings Wild type Vermillion eyes no cross vein normal wings 395 405 98 102 Once you ve recovered from the emotional shock of having seemingly disproven a central tenet of 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 between D 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 of statement that communicates the fact that the two genes are directly next to each other gets full credit 2 Map to the best of your ability the genes in the D virilis genome Show your work 20 points v 20 mu ct cv cannot determine order of the latter two from the data Page 3 of 5 Question 4 10 points You have two haploid yeast strains one with a mutation at locus A and one with a mutation at locus B The two loci are on different chromosomes You mate the strains to form a diploid zygote then sporulate force the diploid to undergo meiosis to form haploids You randomly select 1000 of these haploid progeny grow them up into cultures and do a DNA based experiment to test the genotypes You are surprised to find that they do not obey Mendel s second law Instead of seeing 25 of each possible genotype you see that a third of the progeny are wild type a third have only the A
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