NAME: ______STUDENT ID #: ______

MCB 140 1st Midterm Spring 2007

NAME (Please print):______

STUDENT ID #:______

REMINDERS

1. You have 50 minutes for the 150 point exam. Exams will be collected at noon sharp. 2. Print your name and ID# on each page of the exam. You will lose points if you forget to do this. 3. There are 7 pages total, including this cover page. All pages must be turned in. 4. Only the front of each page will be graded. If you use the back of a page, transcribe your answer to the space provided on the front of the page.

The exam is short but the questions are “content-laden,” i.e., well-thought-out answers are expected (note: this does not mean “verbose” – just thoughtful). Think before you start writing about the best way to express the point you wish to make. Personal note from Prof. Urnov: I will grade my portion of the exam myself, and would be grateful for somewhat more lucid prose and intelligible handwriting than one sometimes finds on midterms. – fdu

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TOTAL _____ / 150

Page 1 of 7 NAME: ______STUDENT ID #: ______Question 1 (30 points)

Shortly after the formal founding of the City of Berkeley, on April 1, 1878, you arrive there to study some botany. Interested in the problem of inheritance, you come across a paper in an obscure journal by a certain “Gregor Johann Mendel.” The sweet pea seems too mundane for you, so you pick the California poppy – its bright orange flowers decorate many a hillside in Berkeley. After much searching, however, you realize that all the poppies you can find are orange! Undeterred, you pick it as your model system.

Starting with a wild-type (i.e., available on the hillside) population of uniformly orange-colored California poppies, describe the experimental sequence you would undertake to determine, if petal color inheritance in this plant follows Mendel’s First Law *. Your answer must take the form of a numbered list – for each action (“1. do this and that”), where appropriate, please provide a justification (“… so as to make sure that that and this happens”).

* Herman Muller invented the use of mutagens to induce in the 1920s. You are not ahead of your time in that respect. All you have access to are – some flowers from a hillside, a patch of land, the Sun, a microscope, and maybe some forceps, brushes, etc.

Page 2 of 7 NAME: ______STUDENT ID #: ______Question 2 (30 points)

A series of experiments by , and his student, Calvin Bridges, on eye color inheritance in Drosophila provided strong experimental evidence that Mendel’s “particles of inheritance” – the – lie on . A summary of crosses performed by Morgan is shown below, and two questions about these experiments are on the next page.

See questions on next page, please.

Page 3 of 7 NAME: ______STUDENT ID #: ______Morgan himself wrote the following: “[if my interpretation of the data is correct, and the for white is on the X ], there should be two classes of females in the F2 generation, namely …”

What two “classes” of females is Morgan referring to? (5 points)

Describe, using modern genetic notation, the experiment Morgan did to confirm this specific hypothesis, and his data. (10 points)

In apparent contradiction to his mentor’s data on “criss-cross inheritance” of eye color in Drosophila, Calvin Bridges found a rare white-eyed daughter offspring from a mating between a white-eyed mother and a red-eyed father. He correctly interpreted this finding as evidence of nondisjunction in meiosis to yield an “exceptional” XXY female. Describe, in narrative form, the two distinct forms of evidence Bridges provided in support of this hypothesis.

You do NOT have to describe every cross he did – you are being graded on your ability to able to describe in a general sense, and in a clear sentence or two, Bridges’s overall experimental approach to the problem (15 points).

Line of evidence #1

Line of evidence #2

Page 4 of 7 NAME: ______STUDENT ID #: ______Question 3 (30 points)

Alfred Sturtevant’s name is rightly inscribed in the history of for his drafting of the first ever genetic map. His famous paper describing the data is entitled “The Linear Arrangement of Six Sex-Linked Factors in Drosophila, as Shown By Their Mode of Association.”

“Linear” – that’s a strong word. Think about this for a couple of minutes and write a few well- worded sentences describing, in general terms, what evidence Sturtevant provided in support of the fact that genes are arranged in linear (as opposed to some other) order. Yes, he’s right, genes lie on chromosomes, and a chromosome is a linear piece of DNA – but that wasn’t discovered until ~1953. How did Sturtevant convince the reviewers that genes are in linear order? (20 points)

The garden pea, Pisum sativum, has a diploid karyotype of 14, i.e., has only 7 linkage groups. In his study of dihybrid crosses, Mendel, therefore was very likely to have picked two linked loci. In fact, for some of his two-factor crosses, he DID pick two linked loci – which, nonetheless, yielded a perfect 9:3:3:1 ratio in a AaBb self-cross. How can this be? (10 points)

Page 5 of 7 NAME: ______STUDENT ID #: ______Question 4 (10 points)

A and B are yeast genes immediately next to each other on chromosome II, close to the telomere—that is, they are separated by a distance of 0 cM and far from the chromosome II centromere. Consider a cross between two haploid yeast strains, one A/b and one a/B. What proportion of parental ditypes, nonparental ditypes, and tetratypes do you expect?

Question 5 (20 points)

George Beadle and Edward Tatum mutagenized Neurospora strains and identified a haploid which harbored a single making its growth dependent on vitamin B6. You get interested in this finding and repeat the experiment. Surprisingly, you identify a second haploid strain with an identical phenotype to that of Beadle and Tatum’s mutant. Your new Neurospora strain will not grow without vitamin B6; it accumulates the same chemical precursors to B6 as Beadle and Tatum’s did; and a cross between your mutant and a wild-type strain indicates that a single locus is responsible for the phenotype in your strain.

You cross your strain to Beadle and Tatum’s original mutant (still available from the Stanford freezers), to form a diploid. You induce the diploid to undergo meiosis but, rather than bothering with octad analysis, you simply collect and grow 100 random haploid progeny. All are viable on rich media. You test the progeny for growth on media lacking B6 and find that 75 cannot grow under these conditions, accumulating the B6 precursors; the other 25 progeny can grow without B6. What do you conclude about your new mutant and about the enzyme controlling this step in B6 synthesis?

Page 6 of 7 NAME: ______STUDENT ID #: ______Question 6 (30 points)

The schematic on the right describes a passage from your book on experimentally distinguishing between two modes of inheritance for coat color in mice.

In general terms, what set of circumstances could impel a geneticist to begin considering the segregation of differences at more than one locus as underlying the inheritance of the trait under study? (10 points)

The example above offers the following “rule-of-thumb” to distinguish between two possible modes of inheritance: if the F2 albino mouse crossed to a pure-breeding brown mouse yields only black mice, then it’s a single locus with incomplete dominance; if some F2 albino mice yield only black mice in that cross, but other F2 albino mice yield a 50:50 black:brown split, and yet other F2 albino mice yield only brown mice – then we’re dealing with two loci and a recessive epistatic relationship between them. “Wait a minute” – says a doubting Thomas in the audience about your last set of data – “What if it’s not epistasis at all, but simply environment- dependent incomplete penetrance, you know, like coat color in Siamese cats – their fur color is a function of the temperature they’re grown at!” Fair enough, you say.

For the example above, describe two distinct experimental approaches that would directly address the issue of whether or not one is dealing with temperature-of-environment-dependent incomplete penetrance, or true epistasis? (20 points)

Approach 1

Approach 2

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