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Duplications of chromosomal segments can occur, as we have just seen, by the breakage-fusion-bridge cycle or by crossovers within the loop of an inversion. There is another way that duplications arise in small adjacent regions of a chromosome. We illustrate this with a particularly interesting example, the Bar eye phenotype in Drosophila ( g. 8.12). The wild-type fruit y has about 800 facets in each eye. The Bar (B) homozygote has about 70 (a range of 20 120 facets). Another allele, Doublebar (BB: sometimes referred to as Ultrabar, BU), brings the facet number of the eye down to about 45 when heterozygous and to about 25 when homozygous. Around 1920, researchers
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Another interesting variant of the simple reciprocal translocation occurs when two acrocentric chromosomes join at or very near their centromeres. The process, called a Robertsonian fusion after cytologist W. Robertson, produces a decrease in the number of chromosomes, although virtually the same amount of genetic material is maintained. Often, closely related species undergo Robertsonian fusions and end up with
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Tamarin: Principles of Genetics, Seventh Edition
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II. Mendelism and the Chromosomal Theory
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8. Cytogenetics
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The McGraw Hill Companies, 2001
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Cytogenetics
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Synapses occur
h g f a b c d e
h g f
4 5 6
3 2 1 3 2 1
A reciprocal translocation heterozygote forms after breaks occur in nonhomologous chromosomes. Synapsis at meiosis forms a cross-shaped gure.
showed that about one progeny in 1,600 from homozygous Bar females is Doublebar. This is much more frequent than we expect from mutation. Alfred Sturtevant found that in every Doublebar y, a crossover had occurred between loci on either side of the Bar locus. He suggested that the change to Doublebar was due to unequal crossing over rather than to a simple mutation of one allele to another ( g. 8.13). If the homologous chromosomes do not line up exactly during synapsis, a crossover produces an unequal distribution of chromosomal material. Later, an analysis of the banding pattern of the salivary glands con rmed Sturtevant s hypothesis. It was found that Bar is a duplication of several bands in the 16A region of the X chromosome ( g. 8.14). Doublebar is a triplication of the segment. A position effect also occurs in the Bar system. A Bar homozygote (B/B) and a Doublebar/wild-type heterozygote (BB/B ) both have four copies of the 16A region. It would therefore be reasonable to expect that both genotypes would produce the same phenotype. However, the Bar homozygote has about seventy facets in each eye, whereas the heterozygote only has about forty- ve.Thus, not only the amount of genetic material, but also its con-
guration, determines the extent of the phenotype. Bar eye was the rst position effect discovered.
Chromosomal Rearrangements in Human Beings
Several human syndromes and abnormalities are the result of chromosomal rearrangements, including deletions and translocations. The most common are described here. Keep three points in mind as you read. First, all of these disorders are rare. Second, the deletion syndromes are often caused by a balanced translocation in one of the parents. And third, about one in ve hundred live births contains a balanced rearrangement of some kind, either a reciprocal translocation or inversion.
Fragile-X Syndrome
The most common cause of inherited mental retardation is the fragile-X syndrome. It occurs in about one in every 1,250 males and about one in every 2,000 females. Symptoms include mental retardation, altered speech patterns, and other physical attributes. The condition is
Tamarin: Principles of Genetics, Seventh Edition
II. Mendelism and the Chromosomal Theory
8. Cytogenetics
The McGraw Hill Companies, 2001
Variation in Chromosomal Structure
h g f a
First Second
h g f
4 5 6 Third Fourth
3 2 1 3 2 1
Alternate segregation First with fourth Second with third
Adjacent-1 segregation First with third Second with fourth
Adjacent-2 segregation First with second Third with fourth
a b c
a b c
a b c
a b c
a b c
a b c
d e f g h
6 5 4 3 2 1
3 2 1
6 5 4
d e f g h
6 5 4
3 2 1
6 5 4 3 2 1
d e f g h
3 2 1
6 5 4
6 5 4 3 2 1
f g h
f g h
f g h
Normal
Reciprocal translocation
Duplication deficiency
Duplication deficiency
Duplication deficiency
Duplication deficiency
Three possible results of chromatid separation during meiosis in a reciprocal translocation heterozygote.
called the fragile-X syndrome because it is related to a region at the X chromosome tip that breaks more frequently than other chromosomal regions. However, the break is not required for the syndrome to occur, and the fragile-X chromosome is usually identi ed by the lack of chromatin condensation at the site; in fact, under the microscope, it appears that the tip of the chromosome is being held in place by a thread ( g. 8.15). The gene responsible for the syndrome is called FMR-1, for fragile-X mental retardation-1.
Fragile-X syndrome has a highly unusual pattern of inheritance: the chance of inheriting the disease increases through generations. This is so unusual a pattern that it was termed the Sherman Paradox. Approximately 20% of males with the fragile-X chromosome do not have symptoms but have grandchildren who do have the symptoms. The daughters of the symptomatic males also don t have symptoms, but obviously, they have another X chromosome to mask the symptoms. As generations proceed, the percentage of affected sons of
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