Download Problem Set V - Biology 2970

Problem Set V - Biology 2970
The following text problems are assigned:
Chapter 14: 35
Chapter 15: 23, 24, 26, 28, 29, 34, 36, 39, 40, 42, 43, 44, 47, 49, 50, 51, 53.
In addition, the following problems should be worked:
1)
A strain of Neurospora that contains an inversion is crossed to a normal strain. The
breakpoints of the inversion are very close to two genes that are about 10 map units apart.
a. Calculate the frequencies of meioses that have
i. no crossover in the inversion loop
ii. one crossover in the inversion loop
iii. two crossovers in the inversion loop
(Remember that interference is complete within 10 map units.)
b. Ascospores that contain deficient chromosome complements do not darken in
Neurospora. How many light and dark spores would you find in asci resulting from the
types of meioses listed above?
2)
Humans have 46 chromosomes whereas chimpanzees, gorillas, and orangutans have 48.
These apes possess two pairs of acrocentric chromosomes that are not present in humans
but that are morphologically very similar to the long and short arms of the large
submetacentric human chromosome 2. Suggest a mechanism that could account for the
different chromosome numbers present in humans and apes.
3)
If X-chromosome inactivation occurs as described in class, then an individual with a single
X chromosome and no Y should be a normal female, rather than an abnormal female with
Turner's syndrome. Suggest an explanation.
4)
In female kangaroos (which are XX), the X chromosome inactivated in somatic cells is
always of paternal origin. Is criss-cross inheritance seen for X-linked genes in the
kangaroo?
5)
Coat color in cats is determined in part by a gene that is on the X chromosome. The
genotype oo is black, Oo is tortoiseshell, and OO is orange. A tortoiseshell female is
crossed with a black male. What offspring would be expected? Rarely, a tortoiseshell
male is produced from such a mating. Suggest an explanation.
6)
H. Charat Chandra recovered triploids of the mealybug Planococcus (n=5). He found that
daughter cells always had 15 chromosomes at the end of meiosis I, but had variable
numbers of chromosomes ranging from 5 to 10 at the end of meiosis II. Is this what you
would expect? Suggest an explanation.
7)
The British wild plant Lythrum salicaria (purple loosestrife) is a tetraploid species. One
variety has pink rather than the usual purple flowers. A cross between purple and pinkflowered plants produced only purple-flowered plants in the F1. When F1 plants were
backcrossed to pink, a ratio of approximately five purple to one pink was obtained in the
F2. Explain these observations.
8)
In general, individuals with Down's syndrome are trisomic for a small acrocentric
chromosome that is designated chromosome 21. Such trisomic individuals have 47
chromosomes rather than the normal 46. Down's syndrome patients that have 46
chromosomes are occasionally found, however. Almost always in such cases the long arm
of chromosome 21 has been translocated to another chromosome. Most translocations of
this type involve the joining of the long arm of chromosome 21 to the short arm of
chromosome 14, another acrocentric chromosome. The chromosome resulting from this
translocation can be designated T(14;21). Down's syndrome children that carry this
chromosome also carry one normal chromosome 14 and two normal chromosomes 21.
Usually the complementary translocation chromosome, which would carry the short arms
of 14 and 21, is not present. When the phenotypically normal parents of translocation
Down's children are examined, generally one parent is found to be karyotypically normal
and one carries the T(14;21) chromosome as well as one normal 21 and one normal 14.
a) Assuming that the T(14;21) chromosome and the normal 14 and 21 form a trivalent in
prophase I and that two-from-one segregations always occur in anaphase I, determine what
chromosomal types of progeny would be produced by the parents described above.
b) Since the only children born to parents of the type described are either phenotypically
normal or show Down's syndrome, which classes must spontaneously abort?
c) Of the phenotypically normal children produced, what proportion will carry the
T(14;21) chromosome?
d) Would maternal age be expected to have any major influence upon the frequency of
Down's syndrome children from such parents?
e) It has been found that when the mother carries the T(14;21) chromosome the frequency
of Down's children is about 11%, whereas when the father carries this chromosome the
frequency is only 2%. Suggest an explanation.
9)
G. Leblon and J.-L. Rossignol made the following observations in Ascobolus (a fungus
similar to Neurospora). Single-nucleotide-pair insertion or deletion mutations show gene
conversions of the 6:2 and 2:6 type and only rarely of the 5:3 or 3:5 type. Base-pair
transition mutations (changes of AT to GC or vice versa) show gene conversions of the 5:3
and 3:5 type, but rarely of the 6:2 or 2:6 type.
a. Propose an explanation for these results.
b. Leblon and Rossignol also showed that there are far fewer 6:2 than 2:6 conversions for
nucleotide pair insertions and far more 6:2 and 2:6 conversions for nucleotide pair deletions
(where the ratios are + to mutant). Suggest an explanation.
10) How could a quintuplication of the Bar region be derived?