Page 181 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

Problems 173

DNA marker in a known chromosomal arrangement wild-type female was crossed to a reduced, cinnabar
of alleles, and his or her mate does not have the same male, and the F 1 males were then crossed to the F 1
alleles of the DNA variant), the reliability of such a females to obtain the F 2 . Of the 400 F 2 offspring ob-
marker as a predictor of the disease in a fetus is re- tained, 292 were wild type, 9 were cinnabar, 7 were
lated to the map distance between the DNA marker reduced, and 92 were reduced and cinnabar. Explain
and the gene causing the disease. these results and estimate the distance between the
Imagine that a man affected with the disease cn and rd loci.
(genotype Dd) is heterozygous for the V and V 19. In Drosophila, the autosomal recessive dp allele of
1
2
1
forms of the DNA variant, with form V on the same the dumpy gene produces short, curved wings, while
2
chromosome as the D allele and form V on the same the autosomal recessive allele bw of the brown gene
3
3
chromosome as d. His wife is V V dd, where V is causes brown eyes. In a testcross using females
3
another allele of the DNA marker. Typing of the fetus heterozygous for both of these genes, the following
by amniocentesis reveals that the fetus has the V and results were obtained:
2
3
V variants of the DNA marker. How likely is it that
the fetus has inherited the disease allele D if the wild-type wings, wild-type eyes 178
distance measured in a two-point cross between the wild-type wings, brown eyes 185
D locus and the marker locus is (a) 0 m.u., (b) 1 m.u., dumpy wings, wild-type eyes 172
(c) 5 m.u., (d) 10 m.u., (e) 50 m.u.? dumpy wings, brown eyes 181
16. Figure 5.7a shows chromosomes during prophase of In a testcross using males heterozygous for both of
meiosis I in a mouse primary spermatocyte. these genes, a different set of results was obtained:
a. How would you know immediately that this figure
shows male meiosis in a mouse and not in a wild-type wings, wild-type eyes 247
human? (Hint: In mice, n = 20.) dumpy wings, brown eyes 242
b. Are most mouse chromosomes metacentric or a. What can you conclude from the first testcross?
acrocentric? Explain. b. What can you conclude from the second testcross?
c. How many chromatids in total are represented in c. How can you reconcile the data shown in parts (a)
Fig. 5.7a? and (b)? Can you exploit the difference between
d. Where is the X-Y bivalent in Fig. 5.7a? (Note: these two sets of data to devise a general test for
Mouse sex chromosomes have only a single pseu- synteny in Drosophila?
doautosomal region, instead of two as in humans.) d. The genetic distance between dumpy and brown is
Diagram this bivalent, showing the X and Y chro- 91.5 m.u. How could this value be measured?
mosomes, the locations of the centromeres of these 20. From a series of two-point crosses, the following map
chromosomes, and the pseudoautosomal region. distances were obtained for the syntenic genes A, B,
e. Explain the importance of the pseudoautosomal C, and D in peas:
region(s) of the sex chromosomes of mammals
for ensuring proper sex chromosome segregation B ↔ C 23 m.u.
during meiosis. A ↔ C 15 m.u.
C ↔ D 14 m.u.
17. Figure 5.7b shows bivalents in mouse primary A ↔ B 12 m.u.
spermatocytes that have previously undergone recombi- B ↔ D 11 m.u.
nation events, as indicated by the presence of a single A ↔ D 1 m.u.
chiasma. The artist’s image in Fig. 5.7c depicts how
cohesin complexes are involved in keeping the homolo- Chi-square analysis cannot reject the null hypo-
gous chromosomes together within the bivalent. Explain thesis of no linkage for gene E with any of the
using diagrams why the key cohesin complexes that other four genes.
connect the homologous chromosomes are those located a. Draw a cross scheme that would allow you to
distal to a chiasma (that is, farther away from the determine the B ↔ C map distance.
centromere) rather than those located proximal to a chi- b. Diagram the best genetic map that can be assem-
asma (that is, between the centromere and the chiasma). bled from this data set.

c. Explain any inconsistencies or unknown features in
Section 5.3 your map.
18. Cinnabar eyes (cn) and reduced bristles (rd) are autoso- d. What additional experiments would allow you to
mal recessive characters in Drosophila. A homozygous resolve these inconsistencies or ambiguities?