Chromosome mapping practice sheet
1. Three Drosophila genes with mutant alleles, which are sc (scute or loss of certain hairs on
thorax), ec (echinus or rough eye surface) and vg (vestigial wings). It was found that
heterogenous females yielded the following phenotypes.
Sc ec vg
235
Sc+ ec+ vg+
241
Sc ec vg+
243
Sc+ ec+ vg
233
Sc ec+ vg
12
Sc+ ec vg+
14
Sc ec+ vg+
14
Sc+ ec vg
16_____
1008
What genes are linked and how? Show map distances. Calculate interference if present.
2. Three Drosophila genes with mutant alleles, which are cn (cinnabar eyes), b (black body),
and cv (curved wings). It was found that heterogeneous females yielded the following
phenotypes. (From actual map of chromosome
Cn+ cv b+
499
cn cv+ b
470
cn+ cv b
2095
cn cv+ b+
1980
cn cv b+
253
cn+ cv+ b
227
cn+ cv+ b+
13
cn cv b
18
Draw a cytogenetic map showing map units between each allele and calculate
interference if present.
3. The allele b gives Drosophila flies a black body, the allele wx of a separate gene gives
waxy wings and a third allele cn gives cinnabar eyes. The female heteroqygous for these
three genes is testcrossed and 1000 progeny are classified as follows: 5 wild type; 6
black, waxy, cinnabar; 69 waxy, cinnabar;67 black;382 cinnabar; 379 black, waxy; 48
waxy; and 44 black, cinnabar. (If the trait is not listed, consider it the wild type.)
a. Explain these numbers.
b. Draw the alleles in their proper positions on the chromosomes of triple
heterozygote.
c. If it is appropriate according to your explanation, calculate the interference.
4. An individual heterozygous for three genes, A/a ∙ B/b ∙ C/c is testcrossed to
a/a ∙ b/b ∙ c/c ∙ d/d, and 1000 progeny are classified by the gametic contribution of the
heterozygous parent as follows:
a B C
42
A b c
43
A B C
140
a b c
145
a B c
6
A b C
9
A B c
305
a b C
310
a. Which genes are linked?
b. If two true-breeding lines had been crossed to produce the heterozygous individual, what
would their genotypes have been?
c. Draw a linkage map of linked genes, showing the order and the distances in map units?
d. Calculate the interference value, if appropriate.
5. If you have the following crosses, calculate the map units between each allele and calculate
the interference by double recombinants.
5.
Two strains of haploid yeast each carrying two non-wild type alleles were crossed to
determine gene linkage. Strain A had asp and gal, which caused it to require aspartate and could
not utilize galactose while strain B had rad and aro, which caused it to be sensitive to radiation
and require aromatic acids. Haploid progeny were obtained and tested for genotype with the
following results (pluses denote wild-type alleles):
asp
asp
asp
asp
+
+
+
+
asp
asp
asp
asp
+
+
+
+
gal
+
gal
+
gal
+
gal
+
gal
+
gal
+
gal
+
gal
+
+
+
+
+
aro
aro
aro
aro
aro
aro
aro
aro
+
+
+
+
+
rad
rad
+
+
rad
rad
+
+
rad
rad
+
+
rad
rad
+
0.136
0.136
0.064
0.064
0.136
0.136
0.064
0.064
0.034
0.034
0.016
0.016
0.034
0.034
0.016
0.016
a. Calculate the six recombinant frequencies.
b. Draw a linkage map to illustrate positions of the four genetic loci and label in map units.
Hint: One of the 0.136 frequencies match each of the parents. The other two show independent
assortment of certain loci. Which ones are different from the parent and they are the ones that
are on different chromosomes.