The following is the pathway for synthesis of E, an essential metabolite of yeast:
Which compound or compounds (amongst A - E) will allow growth of yeast lacking--
Genetics 371B Practice problems--Autumn 2000 week 8
(More questions may be added, time permitting.)
| 1. | Two true-breeding, deaf strains of rats were crossed to each other. The F1 rats all showed normal hearing. When these F1 rats were crossed to each, the resulting progeny consisted of 92 rats with normal hearing and 69 deaf rats. | |
| (a) | Are the two original parental strains mutated in the same gene? If not, what is the mimimum number of genes involved? | |
| (b) | Explain the results (why the F1 rats could hear, and why this ratio of F2 phenotypes was obtained). What progeny ratio would you expect if the F1 rats were crossed to fully homozygous recessive rats? | |
| (c) | What fraction of the F2 would you expect to be true-breeding? | |
| 2. | E. coli cells that have a normal lacZ gene are able to grow on medium containing lactose as the sole source of carbon, and furthermore, can cleave a colorless, artificial substrate called X-gal to make a blue-colored product. Cells lacking a functional copy of this gene cannot grow on lactose and are unable to cleave X-gal. Given a strain of E.coli that has a frameshift mutation in the lacZ gene (rendering the gene non-functional), outline a selection and a screen to identify revertants that have a functional lacZ. |
| 3. | A lab in Seattle is interested in understanding the genetics of flower petal development in Arabidopsis thaliana. After doing mutagenesis on the plant and screening the mutant products, they identify three homozygous recessive mutant strains that all fail to form petals; they call these mutants strains s1 through s3. Meanwhile, they hear from their friends in Paw Paw, Michigan, who had done a similar screen and identified four recessive mutant alleles p1 through p4 that also failed to form flower petals. The labs exchanged strains and performed crosses between the various strains; the ability (+) or inability (-) of the progeny to form petals is indicated: | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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How many genes are represented in this collection of mutant strains? Which mutants represent alleles of the same genes? Why is half the table above left blank? |
| 4. | Blood types A and B are the result of glycosylation of "substance H" (made by the H gene) on red blood cells. (Blood type O is the result of failure to glycosylate.) The product of the h allele fails to make substance H, thereby preventing glycosylation. Predict the phenotypes ratios (for blood type) from the following crosses: | |
| (a) | AOHh x AOHh | |
| (b) | AOHh x BOHh | |
| (c) | ABHh x ABHh | |
| 5. | In a certain breed of dogs, B and b determine black (dominant) and brown (recessive) coat color, respectively. The recessive allele e of a separate (independently assorting) gene blocks expression of both B and b alleles, giving a yellow coat. (The dominant E allele does not affect coat color.) Determine the parental genotypes (giving reasons) for each of these crosses: | |
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| (b) |
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| 6. |
The following is the pathway for synthesis of E, an essential metabolite of yeast:
Which compound or compounds (amongst A - E) will allow growth of yeast lacking-- |
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| (a) | enzyme E3? | |
| (b) | enzyme E4? | |
| (c) | enzymes E2 and E3? | |
| Which compound do you think would accumulate in each of the above mutants? | ||
| 7. |
The following branched pathway depicts the synthesis of E and F, two essential amino acids in a certain mold.
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| (a) | Name two compounds (amongst A-F) that will allow growth of a mutant lacking enzyme E3. | |
| (b) | Name one compund that will allow growth of a mutant lacking enzyme E1. | |
| 8. |
Purple flower color in a plant species requires the conversion of a white precursor to red pigment by enzyme E1 and to blue pigment by enzyme E2. The combination of the two pigments gives purple color, as indicated:
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| (a) | What will be the phenotype of a plant homozygous for a null allele of the gene for E1? | |
| (b) | What will be the phenotype of a plant homozygous for a null allele of the gene for E2? | |
| (c) | What will be the F1 phenotype(s) if the plant in (a) is crossed to the plant in (b)? | |
| (d) | What will be the F2 phenoptypes and ratio if the F1 plants are crossed to each other? | |
| 9. |
Yeast that were capable of synthesizing the amino acid histidine were mutagenized and mutants incapable of histidine biosynthesis were isolated. [Aside: the original, normal strain that can grow in the absence of added histidine is said to be prototrophic for histidine, while the mutants are auxotrophs.] The mutations fell in four complementation groups, M1-M4. The ability of various compounds to rescue growth of the mutants when added to minimal growth medium is shown (+ indicates growth, - indicates lack of growth):
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| Propose a pathway for the biosynthesis of histidine. (Use M1 - M4 to denote the genes that are represented by these mutations.) | |||||||||||||||||||||||||||||||
| 10. |
Homework problem from 1998 Fuchsia plants usually produce fuchsia-colored flowers. However, red-flowered and white-flowered variants, both true-breeding, also occur. A florist crosses these variants with fuchsia-color plants and with each other, and selfs the F1 plants. The results are shown.
Having just taken Genetics 371, the florist concludes (correctly) that two genes are involved. |
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| (a) | Which cross, and which outcome, led the florist to this conclusion, and why? | |||||||||||||
| (b) | If a mutation in a gene called H gives white flowers and a mutation in gene R gives red flowers, deduce the genotypes involved in each cross, indicating the phenotypes as well. | |||||||||||||
| (c) | Propose a pathway for pigment production in these plants, clearly indicating the role of each gene. | |||||||||||||
| 11. |
Homework problem from 1998 A certain protein in yeast can either be targeted to the lysosome (a cytoplasmic organelle) or be exported from the cell. The two pathways are normally regulated so that half the protein molecules on average end up in lysosomes and half are exported. The phenotypes of various null mutations in export pathway genes are as follows:
Suggest a pathway for targeting of the protein, indicating the gene(s) involved in each step and the intermediate locations along the pathway. Be sure to indicate the role of each gene as well. NOTE: In any of these genetic pathway analyses, you may be able to come up with more than one pathway that is consistent with the data. That is often the case with real experiments. The experimenter then has to devise additional experiments to try and distinguish between the various possibilities. |
Also do:
1999 Problem Set 5, #s 1 and 3.
