BIOLOGY 207 - Dr. McDermid
Lecture#24
Chromosome Rearrangements
Readings: Griffiths et al, 7th Edition: Ch. 17 pp 523-539
Problems: Griffiths et al, 7th Edition: Ch. 17, Tier 1: #1,2,4,9,13,15,16 Tier 2: #3,7,8,10,18
Concepts:
How can chromosomes be altered?
1. Chromosomes can undergo physical rearrangements of their DNA, which include deletions,
duplications, inversions, and/or translocations of DNA segments.
2. Rearranged chromosomes may pair improperly at meiosis and alter the distribution of
chromosomes, thereby affecting fertility.
3. Rearrangements can break genes and produce abnormal gametes.
Chromosome Rearrangements
Involve breaks in the DNA duplex followed by the rejoining of the broken ends
The result is a reorganized chromosome that usually can be transmitted to subsequent
generations.
Types of chromosome changes:
Normal chromosome
abcdef • ghij
(with • = centromere)
Deletions
- the loss of a region of DNA
abcdef • ghij
abef • ghij
Duplications
- the gain of a region of DNA
abcdef • ghij
abcdcdef • ghij
Inversions
- involve the inversion of a segment of a chromosome
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abcdef • ghij
abchg • fedij
Translocations
- involve breaks on non-homologous chromosomes with an exchange of parts
abcdef • ghij
abcdef • gvw
and
and
qrst • uvw
qrst • uhij
Mechanisms
Deletions
interstitial terminal -
Genes in a deleted region can show Pseudodominance
Deletions remove many consecutive genes
The recessive allele is pseudo-dominant when the
homologue is paired with the deletion.
Deletions permit mapping the location of genes on a cytogenetic map
Deletion loop- Figure 17-3
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The location of the deletion maps genes cytogenetically.
To identify a deletion mutation:
Deletions can be recognized by
1) Pseudo dominance
2) Cytologically
3) Usually recessive-lethal
4) Lack of reverse mutation
Duplications
Duplications come in two major forms:
Tandem
- abcde cdef • ghij
Reverse (inverse)
- abcde edcf • ghij
Result is extra genes -
Duplications can be recognized by:
1) Cytologically
2) Usually not recessive lethal
3) Can revert and at relatively high rate (crossing over)
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Inversions
Inversions have two major types
Paracentric Inversions (| = break)
ab | cde | f • ghij -> ab edc f • ghij
Pericentric Inversions
ab | cdef • gh | ij -> ab hg • fedc ij
Pairing at meiosis
Paracentric inversions
Figure 17-16, 17-17.
When inversion homologue pairs with normal sequence homologue, an inversion loop results.
Consequence is that
1) "recombinants" (vs. parentals) will be reduced in
frequency
2) Markers within the loop will have an RF of ~ 0 - absolute linkage.
3) Also inversions inhibit the actual pairing of regions near, or in-between the break points, then
crossing-over can not take place.
Pericentric inversions are similar
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Translocations
Reciprocal translocation
Consequences for the pairing at
meiosis.
Figure 17-23
Homologous regions of each
chromosome synapse
Result is a pairing configuration that
involves two pairs of
homologues.
Such a configuration can have several types of segregation depending upon how it lines up on
the equatorial plate
Translocations and Inversions result in reduced fertility due to inviable gametes.
Inviable gametes
Inversions and translocations can also affect linkage
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Lecture notes: Copyright © 2002 Heather McDermid and the Department of Biological Sciences, University of Alberta
Images are Copyright©2000 by W.H. Freeman & Co. in Griffiths et al, Introduction to Genetic Analysi
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