Name___________KEY_________________
Genetics C3032 - Examination #2 - November 1, 2005
General instructions: Don't Panic. Be sure your name is on every page and that you write your exam in ink. Answer
the questions in the space provided. Clearly state your reasoning; if I can understand what you are saying during the
grading, there is a greater chance that you will get at least partial credit. The value of each question is indicated.
1. (18 points) (a) (3) Why would the standard Atextbook@ method for obtaining 3-factor data not be useful as a means
to determine map order for an autosomal mutation in gene c relative to mutations in two known genes a and b?
Need to have the triple mutant combination and this cannot be made without learning about the order.
(b) (9) You mate an ab/c females with wild-type males in Drosophila and look at their progeny. Assuming that all three
genes are so close together on the X chromosome that no double crossovers are observed. What relevant progeny
(i.e., you should give their phenotypes) do you expect to be absent in the following situations? Explain your answers.
(i) c is to the left of a and b.
AC males and B males are not seen because they would come from double crossovers.
(ii) c is to the right of a and b.
BC males and A males are not seen because they would come from double crossovers.
(iii) c is between a and b.
ABC males and wild-type males are not seen because they would come from double crossovers.
(c) (6) From the cross in (b) you obtain 13 animals with the A phenotype; 7 of these animals also have the C
phenotype. In addition you find 12 animals with the B phenotype, and 9 of these animals have the C phenotype. If a
and b are 2.0 map units apart, where does c map? Explain.
Because some A and B animals have the C phenotype, c is between a and b. Since 10 of 25
recombinants occurred between a and c (producing 7 AC and 3 B animals), c is 0.4X the distance from
a to b, i.e., if a is at 0.0, c is at 0.8, and b is at 2.0
Name___________KEY_________________
2. (12 points – 4 each) You want to make mosaic clones with a recessive mutation m in Drosophila. A partial genetic
map showing the centromere (Cen) and the relative positions of m and a number of genes represented by dominant
mutations (A, B, C, and D) is:
Cen
A
B
m
C
D
a. Which configuration (cis or trans) would you use to obtain twin spot mosaics using m and any one of the dominant
mutations? Why?
cis; recessive wt alleles of the dominant mutations segregate to one clone; m/m to the other
b. Using the configuration you chose in part (a), what proportion of the mitotic recombinations that could give rise to a
twin spot do you expect will actually do so? Explain your answer.
Half; expect segregation patterns to be equivalent
c. Which marker mutation (A, B, C, or D) would you use with m to optimize the number of twin-spot relative to singlespot mosaics? Why?
C, furthest from the centromere and closest to m
3. (15 points – 5 each) Distinguish briefly between each set of the following pairs
a. Half of all gene conversion events are accompanied by recombination in the Holliday model compared with the
Double Strand Break Repair model.
In the Holliday model cutting and religating the same strands (horizontal) does not lead to recombination,
whereas cutting and religating the other strands (vertical) will. In the DSBR model cut both Holliday junctions
horizontally or vertically will not lead to recombination, where if one is cut horizontally and the other vertically
they will.
Name___________KEY_________________
b. Production of 3:5 octads in the Holliday model compared with the Double Strand Break Repair model.
3:5 octads arise in the Holliday model because only one of two half conversions has been corrected; they arise
in the DSBR because of asymmetric removal of DNA and resynthesis to form the recombination complexes.
c. General vs. specific transduction
Transduction is DNA entry into the cell mediated by phage. If any sequence can be packaged into the phage
particle (e.g., with P22), then the transduction is general; if only certain DNAs can be package (e.g., with
lambda) then transduction is specific.
4. (30 points – 5 each) Explain the following:
a. How you would demonstrate that an Hfr strain arose spontaneously and was not induced by environmental factors.
F+ E. coli defective in some genes and wild type for others are plated on enriched medium. They are replicated
plated onto a plate with minimal medium that is uniform with F - coli that are wild type for the missing genes
but lacking others found in the F+ bacteria. Cells will grow where an Hfr allowed mating. Go back to the
original plate and select bacteria from the equivalent area.
Alternatively, one could do a similar experiment to that of Luria and Delbruck and look at the standard
deviation
b. How polymorphisms can be used to determine linkage of a new mutation m in C. elegans.
Mate to mutant strain in the N2 background to Hawaiian strain with multiple (known) transposon insertions;
reisolate homozygous mutant animals and use PCR to amplify regions around representative insertions on
each chromosome. Separate PCR fragments on an agarose gel. If linkage occurs the PCR fragment will not be
seen; if the fragment is unlinked, then ¾ of the animals will have the band.
Name___________KEY_________________
c. How Hfr strains can be used to determine map order and map distance in E. coli in two ways.
1) Measure positions by the timing of entry of the Hfr chromosome
2) Hfr strains allow entry of DNA for recombination map order can be determined by doing reciprocal crosses.
d. How transposon insertion can be used to identify DNA for a gene?
This is transposon tagging. Insertion of a transposon causes a mutant phenotype. Isolation of a Restriction
fragment containing the transposon will yield adjacent DNA that should be part of the affected gene.
e. How a NPD/PD ratio of 1 indicates that genes are unlinked in yeast
If mutations a and b are on different chromosomes and we start with a trasn heterozygote, then because the
chromosomes will independently assort, half of the tetrads (or octads) will have the PD pattern (a+; a+; +b;+b)
and half will have the NPD pattern (ab;ab;++;++), thus gioving a ration of 1.
f. How association of a DNA polymorphism with an inherited trait in a human pedigree can be shown to be statistically
significant.
The association is compared to nonlinkage by determining a LOD score = log [P(linkage)/P(nonlinkage)]. A
LOD score > 3 show significance at the 95% confidence level.
Name___________KEY_________________
5. (25 points) Describe genetic (not molecular) experiments to do the following
a. (5) Determine if mutations are in the same or different rII cistron.
Do a cis-trans test. If they produce wild-type phage in cis, they are recessive. If they do not produce wild-type
phage in trans, then they fail to complement and are in the same cistron.
b. (5) Determine which chromosomes are joined in a translocation in C. elegans (or Drosophila).
Look for pseudolinkage. Have to test with markers from different chromosomes.
c. (5) Determine how close the eye and antenna are on the fate map of the Drosophila blastoderm.
Use gynandromorphs with cell-autonomous markers that are expressed in the eye and antenna. The distance
between them will be determined by the number of animals in which the eyes and antennae are of different sex
over the total.
d. (10) Obtain mutations in sequenced genes in two of the following: E. coli, C. elegans, Drosophila, mouse
E. coli – homologous recombination; C. elegans – sib selection; Drosophila – double strand break directed
mutation using appropriate P elements (ala Rong and Golic); mouse; transform embryonic stem cells with
constructs with an internal neomycin resistance gene (neo) and an external thymidine kinase (tk) gene; select
with G418 and gancyclovir; cells that survive and implanted into a blastocyst.