Chapter 9 - Chromosome Variation
These notes are provided to help direct your study from the textbook. They are not designed to explain all aspects of the material in great detail; that is what class time and the textbook is for. If you were to study only these notes, you would not learn enough genetics to do well in the course.
Changes in chromosome number
Most organisms are diploid, or have two sets of chromosomes.
Many others are haploid, or have one set of chromosomes.
euploidy - a general term that refers to any number of sets
aneuploidy - a general term that refers to at least one chromosome
more or less than the diploid number
polyploidy - refers to a cell having more than two sets of chromosomes
1 set = haploid
2 sets = diploid (the "normal" condition for most eukaryotic cells)
3 sets or more = polyploid
3 sets = triploid,
4 sets = tetraploid
As it turns out, many species of plants are polyploid descendants
of diploid ancestors. Polyploidy is tolerated rather well in many
species of plants.
In animals, polyploidy is not tolerated and very few polyploid species
are known to exist. Those that do exist are usually asexual,
parthenogenetic, or hermaphroditic.
Most of the problems resulting from polyploidy occur during synapsis of
homologues during prophase I.
As plants do not have a chromosomal mechanism for sex determination,
synapsis and subsequent disjunction is not as great a problem.
In fact, most plants are monoecious.
Also, organisms that have an odd number of sets, for example triploid
or pentaploid, are usually sterile because during prophase I, three
homologues may synapsis. Disjunction at anaphase can result in
two one way and one the other way. This leads to variable numbers of
chromosomes in the gametes. Another possibility is two of the three
homologues synapse and the third does not synapse at all. This also leads
to unbalanced gametes (Figure 9.28).
Those organisms that have an even number of sets, for example
tetraploid or hexaploid, will have an even number of homologues
synapsis, and have a better chance of getting the same number of
chromosomes to each gamete, though still not very likely.
Any individual that has multiple sets from the same genome. . .
2N ----> 4N is called an autopolyploid. This individual will
have four homologues synapsis during prophase I of meiosis.
This is better than three or five, but there are still some problems.
Polyploids can be created artificially by treatment with colchicine, which
blocks spindle formation so that the chromosomes do not separate. Eventually, the nucleus reforms but with double the number of chromosomes.
Within somatic cells, for example liver cells, many individual cells may
be tetraploid, with no apparent ill effects for the individual.
If, however, the sets represent separate genomes, things are better in terms of how meiosis proceeds and ultimately in terms of fertility.
With two sets from two different genomes (allopolyploidy), the synapsis
involves only two chromosomes, and works well (Figure 9.29).
Allopolyploidy can come about in several ways.
1.a) Haploid gametes of two distinct species combine to form a hybrid.
The hybrid reproduces vegetatively until somatic doubling occurs in
a cell of the floral meristem, which produces a flower stucture that
has all chromosomes existing as homologous pairs.
At this point, meiosis is normal and sexual reproduction can occur
2. Two distinct species produce unreduced (dipliod) gametes, which fuse
to produce an allopolyploid, which is fertile if it can self-fertilize.
Aneuploidy refers to at least one more or one less chromosome than the
diploid number. If an individual has 2N+1, in which one of the
chromosomes has three copies, the individual is trisomic. Two extras
(2N+2) is tetrasomic and only one copy (2N-1) is monosomic.
Aneuploidy results from nondisjunction or failure of either homologous
chromosomes in anaphase I or sister chromatids in anaphase II to
separate at some stage of meiosis. This produces a gamete with one
fewer or one extra chromosome than the normal haploid number
Upon fertilization, a monosomic or trisomic results. In humans, the
most common viable aneuploids involve the sex chromosomes, giving rise
to XXX, XO, or XXY individuals.
There are no clear deleterious effects associated with the karyotype
XYY. However, XYY men tend to be taller. Interestingly, there is a
twenty-fold higher incidence of XYY males among the prison population
than in the population as a whole, though XYY males in the non-prison population do not show an increased tendency for criminal behavior.
Within the autosomal complement, the only aberrations to have survived to birth are:
trisomy-21 = Down's syndrome (1/1000)
trisomy-13 = Patau syndrome (1/15,000)
trisomy-18 = Edward syndrome (1/7500, mostly female)
Apparently, the deleterious effects of trisomic conditions are due to
overproduction of certain proteins that lead to a number of developmental
problems. The only ones to have been detected in live births involve
the smaller chromosomes in the human complement.
The only cases of monosomy found in live births involves the X chromosome,
where XO individuals are viable, but are compromised in certain mental and
Monosomy in the autosomal complement is always inviable in humans,
probably due to the unmasking of lethal mutations that exist in a
recessive state, as well as disruption of developmental processes because
of an underproduction of regulatory proteins.
30% of all fertilizations spontaneously abort within the first three months.
50% of spontaneous abortions in the United States have some sort of
obvious chromosomal rearrangement, such as monosomic or trisomic conditions.
These two facts indicate that nondisjuction during gametogenesis or
possibly during early mitotic division is a quite common occurrence and
that most aneuploid fetuses fail to survive to birth.
These are also referred to as Robertsonian events. It occurs
when two acrocentrics fuse to produce one metacentric. The process can
also go backwards. The metacentric chromosome created by a centric
fusion may retain both centromeres.
These fusion/fission events change the chromosome number, but do not
change the number of arms. The number of arms in the autosomal complement is referred to as the
fundamental number. A centric fusion will change the number of chromosomes but will not change the fundamental number.
As a rule, an individual that is heterozygous for a centric fusion will
still be fertile. Disjunction will proceed normally. Thus, some
species will have individuals with different diploid numbers,
but the same fundamental number.
Change in chromosomal structure
Deletions can result from one or two breaks from a single chromosome.
If a single break occurs across both chromatids after replication, a
dicentric fragment can result (Figure 8.1). The fragment
will be lost due to the lack of a centromere. The chromosome will not
move properly during anaphase and will either break or be totally lost.
If two breaks occur, the ends may rejoin and the interior fragment will
be lost (Figure 9.10).
The deletion can be detected due to mismatched pairs in a standard
karyotype. It can be confirmed by the appearance of a bulge during
synapsis at prophase I. Also, similar structures in the polytene
chromosomes of fruit flies can be seen, and the absence of certain
bands can be detected (figure 9.10.
G- and C-banding
G-banding - dark stains of AT rich regions
C-banding - dark stain for heterochromatin
G- and C-banding are quite useful in detecting chromosomal changes that
occur within a species, as well as chromosomal changes that characterize
differences between species (figure 9.4).
Deletions are almost always detrimental. Most species cannot tolerate
the loss of any chromosomal material. The deletion of a piece of one
chromosome probably allows the unmasking of lethal recessive genes
present on the homologue, as seen in a monosomic individual. In
additions, underproduction of regulatory proteins probably disrupts
fetal development. An example in humans is cri-du-chat, a deletion in the
short arm of chromosome 5 (table 9.1). The infant makes a meowing sound
like a cat, and is severely retarded.
Duplications are usually detected by repeated band sequences, especially
in polytene chromosomes. They can be tandem repeats such as ABCABC, or
inverted repeats such as ABCCBA.
Besides being caused by chromosomal breaks, they are usually caused by
unequal crossing-over between chromatids. This results in one chromatid
with an excess and the other with a deletion (figure 9.8).
Serially duplicated genes are thought to have given rise to the moderately
and highly repetitive DNA found in eukaryotes.
The genes alpha-globin and beta-globin of hemoglobin represent a
duplication that occurred in the genome leading to the vertebrates. The
alpha-globin gene duplicated itself and the repeated sequence became the
beta-globin gene. Both are very similar to each other in the
placement of introns and exons and both genes share many amino acid
Inversions involve two breaks in a chromosome, where a piece is cut out,
flipped end over end and reinserted. Crossing-over occurs between
chromatids within the inversion loop (figure 9.12). The dicentric and acentric
pieces are either lost totally or give rise to abnormal gametes that
produce inviable offspring. An individual heterozygous for an inversion
has a 50% reduction in fertility because of inviable gametes. An
inversion represents a strong barrier to interspecies reproduction, a
post-mating isolating mechanism. We will examine this in more detail
when we get to population genetics.
If the centromere is included within the inverted sequence, it is called
a pericentric inversion (figure 9.14). If the centromere is not included
within the inverted sequence, it is called a paracentric inversion (figures 9.12 and 9.13).
An inversion can be detected either by a change in banding pattern or
by the presence of inversion loops formed during synapsis in the
If the species has a mechanism to suppress crossing-over within the
loop (as in Peromyscus), or a mechanism to sequester malformed
chromosomes into polar bodies (as in Drosophila), the heterozygous state
of the inversion loop is not detrimental. However, because the inverted
segment of the chromosome has suppressed crossing-over, this region of
the chromosome does not recombine and behaves as one large gene, called a
Translocation is the transfer of a part of one chromosome to another
nonhomologous chromosome. If it involves one break and a transfer to
another chromosome, it is called a simple translocation. If the
chromosomes exchange segments, the exchange is called a reciprocal
An individual that is heterozygous for a reciprocal translocation, such as
the hybrid that may result from the mating of two different species, will
have a reduced fertility because of problems during meiosis (figure 9.17).
Some chromosomal rearrangements (such as translocations) may change the
position of a gene such that is finds itself in a highly active region of
a chromosome. This results in increased expression of the gene. This
change in expression is know as position effect. An example is in
Burkitt's lymphoma, in which a translocation between chromosome 8 and
14 moves a growth factor gene close to the antibody genes, which are
very active in a lymphocyte. This leads to the cell becoming cancerous
(figures 9.32 and 9.31).
Last update on 22 October 2004
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