Page 135 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

Problems 127

b. What chromosomal structure(s) cannot be resolved b. What does your answer to part (a) allow you to
in the drawing? infer about the nature of cohesin complexes at the
c. How many chromosomes are present in normal centromere versus those along the chromosome
Tenebrio molitor gametes? arms? Suggest a molecular hypothesis to explain
16. A person is simultaneously heterozygous for two au- your inference.
tosomal genetic traits. One is a recessive condition for
albinism (alleles A and a); this albinism gene is found
near the centromere on the long arm of an acrocentric
autosome. The other trait is the dominantly inherited
+
Huntington disease (alleles HD and HD ). The
Huntington gene is located near the telomere of one
of the arms of a metacentric autosome. Draw all cop-
ies of the two relevant chromosomes in this person as
they would appear during metaphase of (a) mitosis, i ii
(b) meiosis I, and (c) meiosis II. In each figure, label 21. The pseudoautosomal regions (PARs) of the X and Y
the location on every chromatid of the alleles for chromosomes enable the sex chromosomes to pair and
these two genes, assuming that no recombination synapse during meiosis in males. Given the location of
takes place. the SRY gene near PAR1, can you propose a mecha-
17. Assuming (i) that the two chromosomes in every nism for how the mutant X and Y chromosomes in
homologous pair carry different alleles of some Fig. 4.7 (in which part of the X is on the Y, and part of
genes, and (ii) that no crossing-over takes place, how the Y is on the X) may have arisen during meiosis?
many genetically different offspring could any one 22. Remarkably, the platypus has 10 sex chromosomes,
human couple potentially produce? Which of these the largest number found in any mammal. The female
two assumptions (i or ii) is more realistic? platypus has five pairs of different X chromosomes
18. In the moss Polytrichum commune, the haploid (X1–X5), while the male has X1–X5, and also five
chromosome number is 7. A haploid male gamete different Ys (Y1–Y5). During meiosis in the male, the
fuses with a haploid female gamete to form a dip- five Xs always end up together in one gamete, and the
loid cell that divides and develops into the multi- five Ys always end up together in another gamete. To
cellular sporophyte. Cells of the sporophyte then achieve this segregation, during prophase of meiosis I
undergo meiosis to produce haploid cells called the sex chromosomes form a long chain, always in the
spores. What is the probability that an individual order X1 Y1 X2 Y2 X3 Y3 X4 Y4 X5 Y5, in which
spore will contain a set of chromosomes all of the chromosomes are held together through pseudoau-
which came from the male gamete? (Assume that tosomal regions (PARs).
no recombination occurs.) a. How many different PARs must exist to allow the
19. Can you think of anything that would prevent meiosis formation of these chains? (Hint: To answer this
from occurring in an organism whose genome is question, try drawing the chain of chromosomes.)
always haploid? b. In terms of pairing ability of the PARs, explain
20. Sister chromatids are held together through meta- the structural differences between the human and
phase of mitosis by complexes of cohesin proteins platypus sex chromosomes.
that form rubber band–like rings bundling the two
sister chromatids. Cohesin rings are found both at
centromeres and at many locations scattered along Section 4.5
the length of the chromosomes. The rings are 23. Somatic cells of chimpanzees contain 48 chromosomes.
destroyed by protease enzymes at the beginning of How many chromatids and chromosomes are pres-
mitotic anaphase, allowing the sister chromatids to ent at: (a) anaphase of mitosis, (b) anaphase I of
separate. meiosis, (c) anaphase II of meiosis, (d) G 1 prior
a. Cohesin complexes between sister chromatids are to mitosis, (e) G 2 prior to mitosis, (f) G 1 prior to
also responsible for keeping homologous chromo- meiosis I, and (g) prophase of meiosis I?
somes together until anaphase of meiosis I. With How many chromatids or chromosomes are present
this point in mind, which of the two diagrams that in: (h) an oogonial cell prior to S phase, (i) a sperma-
follow (i or ii) properly represents the arrangement tid, (j) a primary oocyte arrested prior to ovulation,
of chromatids during prophase through metaphase (k) a secondary oocyte arrested prior to fertilization,
of meiosis I? Explain. (l) a second polar body, and (m) a chimpanzee sperm?