Reproduction 703so on). These homologous pairs of chromosomes can be pho-
tographed and numbered. Each cell that contains 46 chromo-
somes (that is diploid ) has two number 1 chromosomes, two
number 2 chromosomes, and so on through pair number 22.
The first 22 pairs of chromosomes are called autosomal
chromosomes (chapter 3; see fig. 3.29).
The zygote and all of the cells it forms by mitosis have two
sets of autosomal chromosomes and thus two alleles (forms) of
each gene on these chromosomes. In most cases, both alleles of
a gene are expressed. However, there are presently about 100
known genes (out of the approximately 25,000 genes in the
human genome) that have the two parental alleles expressed dif-
ferently. In most cases, either the maternal or the paternal allele
is silenced. Depending on the gene, only the maternal or paternal
allele may thus be able to function.
Silencing of an allele is accomplished by epigenetic changes
to the chromatin (chapter 3, section 3.5). This involves changes
in chromatin structure produced by methylation of cytosine
bases in DNA and acetylation of the histone proteins in the chro-
matin. Such changes are called epigenetic because they don’t
alter the DNA base sequence, but they are carried forward to the
daughter cells as the cell divides. Genomic imprinting refers to
epigenetic changes in the zygote that result in the silencing of
the allele derived from one parent and the expression of only the
nonimprinted allele of the other parent in the offspring. Because
only the maternal or paternal allele of the gene pair functions in
the tissues of the offspring, these genes are particularly suscep-
tible to mutations that can cause diseases.
The 23rd pair of chromosomes are the sex chromosomes
( fig. 20.2 ). In a female, these consist of two X chromosomes,
whereas in a male there is one X chromosome and one
Y chromosome. The X and Y chromosomes look different and
contain different genes. This is the exceptional pair of homolo-
gous chromosomes mentioned earlier.
The X chromosome has recently been sequenced and shown to
have 1,090 genes; the Y chromosome, by contrast, has only about
80 genes. When meiosis occurs in the testes, the X and Y chromo-
somes cannot undergo recombination (chapter 3; see fig. 3.31), as
can the autosomal chromosomes and the two X chromosomes in
a female. Instead, only the tips of the single X chromosome in a
male, which contain most of the 54 genes that are homologous in
the X and Y chromosome, can recombine with the Y chromosome
during meiosis. Genes outside of the tip regions are restricted to the
solitary X chromosome of the male. A surprisingly large number
of these X-linked genes are responsible for certain diseases—there
are presently 168 diseases known to be caused by mutations in 113
X-linked genes. These diseases are more common in males than in
females because the genes responsible, being unpaired, cannot be
present in a recessive state.
Although the Y chromosome is not much to look at under
the microscope (see fig. 20.2 ), scientists have recently deter-
mined its DNA base sequence and found that it still contains
more than 23 million base pairs of euchromatin. (Euchromatin is
the extended, active form of DNA; chapter 3, section 3.3.) The
DNA of the Y chromosome includes X-transposed sequences
almost identical to regions of the X chromosome (from which
the Y chromosome is believed to have evolved), degenerate
regions, and testis-specific genes. Unusually, most of the testis-
specific genes (expressed by spermatogenic cells) were found
to be located in huge palindromes. Palindromes are regions of
DNA bases that read the same from either direction. The eight
palindromes discovered are quite long, one up to 2.9 million
base pairs! These palindromes may enable the Y chromosome
to have “gene conversions,” where defects in one region of the
palindrome can be corrected by a corresponding region. This
would substitute for the crossing-over (chapter 3; see fig. 3.31)
that normally occurs between homologous chromosomes, help-
ing protect these important genes from genetic changes and con-
serving the genes over evolutionary time.
When a diploid cell (with 46 chromosomes) undergoes
meiotic division, its daughter cells receive only one chromo-
some from each homologous pair of chromosomes. The gam-
etes are therefore said to be haploid (they contain only half
the number of chromosomes in the diploid parent cell). Each
sperm cell, for example, will receive only one chromosome of
homologous pair number 5—either the one originally contrib-
uted by the mother, or the one originally contributed by the
father (modified by the effects of crossing-over). Which of the
two chromosomes—maternal or paternal—ends up in a given
sperm cell is completely random. This is also true for the sex
chromosomes, so that approximately half of the sperm pro-
duced will contain an X and approximately half will contain a
Y chromosome.
The egg cells (ova) in a woman’s ovary will receive a simi-
lar random assortment of maternal and paternal chromosomes.
Because the body cells of females have two X chromosomes,Figure 20.2 The human X and Y chromosomes.
Magnified about 10,000 3 , these images reveal that the X
and Y chromosomes are dramatically different in size and
shape. Despite the small size and simple appearance of the
Y chromosome, recent studies have uncovered its surprising
complexity and sophistication.