Chromosomes and human GenetiCs 397deletion Loss of part of a
chromosome.
duplication Repetition of a
sequence of nucleotides in
a chromosome.
translocation Movement
of part of a chromosome
to a nonhomologous
chromosome.How can cHange in tHe structure of
cHromosomes lead to genetic disorders?- A viral infection, radiation, and other factors can delete part of
a chromosome. The deletion may not cause a disorder if the
affected chromosome’s homologue is normal. - The shifting of a chromosome segment to a nonhomologous
chromosome is virtually always harmful.
taKe-home messaGename of the disorder, cri-du-chat (French, meaning
“cat cry”). Figure 20.14A shows an affected child.
Figure 20.14B shows an eye in which the colored
iris did not develop properly. This disorder, called
aniridia, is the result of a deletion on chromosome
- People with aniridia usually have other eye abnor-
malities as well, because the deleted gene governs several
aspects of eye development.
Given that humans have diploid cells, it might seem
logical that genes on the affected chromosome’s homologue
would make up for the loss. In fact, this often happens if
a segment deleted from one chromosome is present—and
normal—on the homologous chromosome. However, if the
remaining homologous segment is abnormal or carries a
harmful recessive allele, nothing will mask its effects.
Translocation is another kind of chromosome change.
Here, part of one chromosome exchanges places with a
corresponding part of another chromosome that is not its
homologous partner (Figure 20.15). This sort of change to
a chromosome’s structure is virtually sure to be harmful.
For instance, in some people a region of chromosome 8
has been translocated to chromosome 14—and the result
can be several rare types of cancer. The disease develops
because genes in that region are no longer properly regu-
lated, a topic that we consider again in Chapter 22.
Patients who have a chronic type of leukemia (a blood
cell cancer) have an abnormally long chromosome 9—
called the Philadelphia chromosome after the city where
it was discovered. The extra length is actually a piece
of chromosome 22 (Figure 20.16). By chance, both chromo-
somes break in a stem cell in bone marrow. Then, each bro-
ken piece reattaches to the wrong chromosome—and a gene
located at the end of chromosome 9 becomes fused with
a gene in chromosome 22. Instructions from this altered
gene lead to the synthesis of an abnormal protein. In some
way that researchers do not yet understand, that protein
promotes the runaway multiplication of white blood cells.
Even normal chromosomes contain the changes called
duplications, which are sequences of nucleotides that are
repeated. Often the same sequence
is repeated thousands of times. You
might guess that so much dupli-
cate DNA would be harmful, but no
genetic disorder has yet been linked
to duplication. In the next section
we will look at some genetic effects
that can arise when a gamete or new
embryo receives an abnormal num-
ber of chromosomes—either too few
or too many.F i g u r e 20.15 Animated! A translocation is one kind
of chromosome change. (© Cengage Learning)
F i g u r e 20.16 A karyotype may reveal damage to a chromosome.
Like the karyotype in Figure 18.2, this one displays the 46 chromosomes
in a diploid human cell. The arrow indicates the bit of chromosome 22
fused to the Philadelphia chromosome (chromosome 9). (From “Multicolor
Spectral Karyotyping of Human Chromosomes” by E. Schrock, T. Ried, et al., Science, 26
July 1996, Volume 273, p. 495. Used by permission of E. Schrock and T. Ried and The American
Association for the Advancement of Science)A BC DEFGH IJKLMADEFGH IJBCKLMone chromosomea nonhomologous
chromosome
translocationCopyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).