Biology of Disease

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15.9 Variations in Chromosome Numbers


Eukaryotic organisms are normally diploid and produce haploid gametes
(Section 15.3). However, chromosomal mutations with numerical aberrations
in the number of chromosomes present occur. These can be divided into two
major types. Aneuploidyoccurs when the number of chromosomes differs
in having more or fewer than an exact multiple of the haploid number of
chromosomes. Table 15.7 lists a number of human aneuploid abnormalities
of autosomes and sex chromosomes. In contrast, euploidy is the presence of
an exact multiple of the haploid number of chromosomes.


Chromosomes Syndrome

Autosomes

Trisomy 13 Patau syndrome

Trisomy 18 Edward syndrome

Trisomy 21 Down syndrome

Sex chromosomes, female

X0 Turner syndrome

XXX triple X syndrome (trisomy X)

XXXX tetrasomy

Sex chromosomes, male

Y0 nonviable

XYY XYY syndrome

XXY Klinefelter syndrome

XXXY Klinefelter syndrome

XXYY Klinefelter syndrome

Table 15.7Aneuploid abnormalities of human chromosomes


Aneuploidy


Aneuploidy is usually caused by the nondisjunction of paired chromosomes
at meiosis I or of sister chromatids at meiosis II or by delayed movement of a
chromosome at anaphase. Nondisjunction is caused by the failure of pairs of
homologues to separate or disjoin during segregation. Figure 15.31 (A) and (B)
illustrates the consequences of nondisjunction during first meiosis and second
meiosis for a single chromosome. Thus gametes are formed that either lack
the chromosome or contain two copies of it. If these are fertilized by a normal
haploid gamete, then zygotes are produced with one or three chromosomes.
Thus nondisjunction can lead to a variety of aneuploid conditions.


The loss of a single chromosome from an otherwise diploid genome is
called monosomy (2N – 1). Nullisomy results from the loss of one pair of
homologous chromosomes (2N – 2). The gain of one chromosome results in
trisomy (2N + 1). Tetrasomy describes the presence of four copies of a specific
chromosome rather than the normal two (2N + 2). Aneuploidy can also involve
the loss or the addition of more than one particular chromosome or pair of
chromosomes. Thus a double monosomy involves the loss of two separate
nonhomologous chromosomes (2N – 1 – 1), while a double tetrasomy would
describe the presence of four copies of two chromosomes (2N + 2 + 2). Both


VARIATIONS IN CHROMOSOME NUMBERS

CZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY )(,


Nondisjunction

A)

B)

2N 2N O O

Nondisjunction

2N O N N

Figure 15.31 The consequences of nondisjunction
of a single chromosome at (A) meiosis I and (B)
meiosis II.
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