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KEY TO HOMEWORK QUESTIONS, LECTURE 5
CHAPTER 8:
C1.
Duplications and deficiencies involve a change in the total amount of genetic
material, while inversions and translocations involve only a change in location
of the genetic material.
Duplication: a repeat of some genetic material
Deficiency: a shortage of some genetic material
Inversion: a segment of genetic material in the opposite orientation
Translocation: a segment of genetic material attached to the wrong
chromosome
C7.
There are four products from meiosis. One would be a normal chromosome
and one would contain the inversion. These are the two “outside” chromatids
of each homologue that were not involved in the crossover event:
A
AB
B
C centromere D
J I
H G F
E F G H
E D centromere
I J K L M (normal)
C
K
L M (inversion)
The other two chromosomes would have the both duplications and deletions:
A
B
M
L
C centromere D
K
J I H G F
E FGH
E
I J B
A
D centromere C K L
M
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Individuals who carry inversions and reciprocal translocations have the same
amount of genetic material (i.e., the same number of genes) as do normal
individuals. Therefore, they commonly have a normal phenotype. In some
cases, however, the breakpoint in an inversion or translocation disrupts an
important gene and thereby has a phenotypic consequence. In other cases, the
chromosomal rearrangement may have a position effect that alters the
expression of an important gene.
C13. It is expected to be rare because the normal driving force for segregation is
the segregation of centromeres. For example, the centromeres on
chromosome 2 normally align during meiosis and segregate from each other
whether or not the chromosome contains a translocation. On rare occasions, a
misalignment of centromeres can lead to adjacent-2 segregation in which two
centromeres from one chromosome travel to one pole and two centromeres
from another chromosome travel to the opposite pole. Such an event is called
a non-disjunction.
C15. It is because the homologous chromosomes are trying to synapse with each
other. As shown in Figure 8.15, the formation of a translocation cross allows
the homologous parts of the chromosomes to line up (i.e., synapse) with each
other, and sequence homology is the only thing that counts during synapsis.
The proteins that mediate synapsis will twist the chromosomes into every
possible yoga posture to achieve it!
C17.
A.
B.
C.
D.
E.
16
9
7
12
17
C19. One parent is probably normal while the other parent has one normal copy of
chromosomes 14 and 21 and one chromosome 14, which has most of
chromosome 21 attached to it (a Robertsonian translocation). When the
individual carrying the Robertsonian translocation undergoes meiosis, all three
chromosomes line up and synapsis because they all share some areas of
sequence homology. There are three possible outcomes, depending on which
centromeres go to which sides of the cell during anaphase:
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The results can be summarized as follows:
1/6
balanced with translocation 14/21 (viable)
1/6
normal gametes with one copy of 14 and one copy of 21
1/6
nullisomy 21 (14 only in gamete; non-viable)
1/6
disomy 21 (14/21 translocation plus 21; viable but leads to DS)
1/6
nullisomy 14 (21 only in gamete; non-viable)
1/6
disomy 14 (14/21 translocation plus 14; non-viable because trisomy 14
is non-vaible)
C21. Imbalances in aneuploidy, deletions, and duplications are related to the copy
number of genes. For many genes, the level of gene expression is directly
related to the number of genes per cell. If there are too many copies, as in
trisomy, or too few, as in monosomy, the level of gene expression will be too
high or too low, respectively. It is difficult to say why deletions and
monosomies are more detrimental although one could speculate that having too
little of a gene product causes more cellular problems than having too much of
a gene product. Also, the loss of one copy of the gene could “expose” a
recessive mutation that is present on the other chromosome that would not
otherwise be expressed in the phenotype.
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