Chapter 12 Chromosomal Rearrangements and Changes in
Chromosome Number Reshape Eukaryotic Genomes
Outline
I.
Rearrangements of DNA sequences within chromosomes
A. Deletions remove material from the genome
1. Homozygosity for a deletion is often, but not always, lethal
2. Heterozygosity for a deletion is often detrimental
3. Heterozygosity for deletions affects mapping distances
4. Deletions in heterozygotes can “uncover” genes
5. Using deletions to locate genes
B. Duplications add material to the genome
1. Duplications can affect phenotype
2. Unequal crossing over between duplications increases or decreases gene copy number
3. How duplications and deletions affect phenotype and evolution
C. Inversions reorganize the DNA sequence of a chromosome inversion heterozygotes reduce the
number of recombinant progeny
D. Translocations attach part of one chromosome to another chromosome
1. Heterozygosity for germline translocations diminishes fertility and results in pseudolinkage
2. Translocations can sometimes help map important genes
3. Translocations and inversions have several effects in common: a summary
E. Transposable elements move from place to lace in the genome
1. Retroposons: transcription generates an RNA that encodes a reverse-transcriptase-like enzyme
2. Transposons encode transposase enzymes that catalyze the events of transposition
3. Genomes contain defective copies of transposable elements
4. Transposable elements can generate mutations in adjacent genes
5. Transposable genetic elements can also generate chromosomal rearrangements
6. Transposition can relocate genes
F. Rearrangements and evolution: a speculative comprehensive example
II.
Changes in chromosome number
A. The loss or gain of one or more chromosomes results in aneuploidy
1. Autosomal aneuploidy is harmful to the organism
2. Aneuploidy for the X chromosome is often not lethal to the individual because of dosage
compensation resulting from X inactivation
3. Aneuploidy results from meiotic nondisjunction
4. Aneuploid mosaics can result from mitotic nondisjunction or chromosome loss
B. Euploid species contain complete but nondiploid sets of chromosomes
1. Monoploid organisms contain a single copy of each chromosome and are usually infertile
2. Polyploidy has accompanied the evolution of many cultivated plant species
3. Triploids are almost always sterile
4. Tetraploids are often the source of new species
5. Some polyploids have agriculturally desirable traits derived from two species
Essential Concepts

Rearrangements reorganize the DNA sequences within genomes. Like point mutations,
rearrangements are subject to natural selection and thus serve as instruments of evolution.
a. Deletions remove DNA from a chromosome. Homozygosity for a large deletion is usually
lethal, but even heterozygosity for a large deletion can create a deleterious genetic imbalance.
Deletions may uncover recessive mutations on the homologous chromosome, and are thus
useful for gene mapping at the cytological and molecular levels.
b. Duplications add DNA to a chromosome. The additional copies of genes contained in a
duplication are a major source of new genetic functions. Small duplications sometimes
produce changes in phenotype that allow their detection. Homozygosity or heterozygosity for
duplications causes departures from normal gene dosage that are often harmful to the
organism. Unequal crossing over between duplicated regions expands or contracts the copy
number of a gene.
c. Inversions alter the order, but not the number, of genes on a chromosome. They may
produce novel phenotypes by modifying the activity of genes near the rearrangement
breakpoints or by disrupting genes at the breakpoints. Inversion heterozygotes act as
crossover suppressors because progeny formed from recombinant gametes are genetically
imbalanced.
d. In reciprocal translocations, parts of two chromosomes trade places without the loss or gain
of chromosomal material. By relocating pieces of DNA, translocations may modify the
function of genes at or near the translocation breakpoints. Heterozygosity for translocations in
the germline results in semisterility and pseudolinkage.
e. Transposable genetic elements are short, mobile segments of DNA that reshape genomes
by generating mutations, causing chromosomal rearrangements, and relocating genes.

Changes in chromosome number also reshape genomes.
a. Aneuploidy, the loss or gain of one or more chromosomes, creates a genetic imbalance.
Aneuploidy can result from mistakes in meiosis, which result in aneuploid gametes, or from
mistakes in mitosis, which generate aneuploid clones of cells. Autosomal aneuploidy is
usually lethal to the organism. Aneuploidy for the X chromosome is better tolerated because
of dosage compensation mechanisms; nevertheless, it can have significant consequences.
b. Euploid organisms contain complete sets of chromosomes. Organisms with three or more
sets of chromosomes are polyploids. Autopolyploids derive all their chromosome sets from
the same species; allopolyploids are hybrids in which the chromosome sets come from two or
more distinct, though related, species.
c. Organisms containing odd numbers of complete chromosome sets are sterile because the
chromosomes cannot pair properly during the first meiotic division.
d. Polyploids with even numbers of chromosome sets can be fertile if proper chromosome
segregation occurs sufficiently frequently. Amphidiploids, produced by chromosome
doubling of two genomes derived from different diploid parental species, are often fertile and
are sometimes useful in agriculture.