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.