Chimpanzee Genome
561
have no offspring whereas others have sev-
eral. Eventually, no ancestral alleles will be
retained and the respective populations
are genetically unique. It is now crucial
for the case of the chimpanzee popula-
tions that the time needed to complete
this process depends on the effective pop-
ulation size and thus varies among the
analyzed loci. Mitochondrial and Y chro-
mosomal loci will have removed ancestral
sequence variants Frst, since their effec-
tive population size is the smallest – only
one sequence variant is passed on to the
next generation. X chromosomal loci will
need on an average triple the time since
they have a threefold higher effective pop-
ulation size – two copies in females, and
one copy in males. And the time for auto-
somal loci to become genetically unique
is about four times that of mitochon-
drial and Y chromosomal loci since on
an average four copies – two copies each
in males and females – are passed on to
the next generation. As a consequence,
the presence of shared sequence variants
at biparentally inherited loci in the three
chimpanzee populations in contrast to
their genetic uniqueness at mitochondrial
and Y chromosomal loci could indicate
that the subspecies started to evolve inde-
pendently – in evolutionary scales – only
recently. Therefore, the time was not suf-
Fcient to establish genetic uniqueness at
all loci.
2
Comparison of the Chimpanzee and
Human Genomes
Early comparative studies of their proteins
and DNA sequences have already provided
evidence that chimpanzees and humans
are genetically highly similar. However,
differences exist that are informative in two
ways. They help understand how genomes
diverge subsequent to reproductive isola-
tion of two populations. And they help
identify those changes that form the ge-
netic basis of the phenotypic properties dif-
ferentiating chimpanzees from humans.
±our categories of differences between
the genomes of chimpanzees and humans
will be discussed in the following text.
2.1
Cytogenetic Differences
The microscopic comparison of chim-
panzee and human chromosomes revealed
a small number of cytogenetic differences,
that is, differences in either chromosome
number or differences in presence, or-
der, and position of large chromosomal
segments. Their detection was greatly
simpliFed by chromosome banding tech-
niques as well as fluorescence
in situ
hybridization (±ISH), methods that al-
low a speciFc and reproducible labeling
of individual chromosomal regions. The
most apparent difference between the
genomes of chimpanzees and humans is
the number of chromosome pairs per cell.
While chimpanzees – as well as gorillas
and orangutans – have 24 pairs, humans
have only 23. A comparison of the chro-
mosome banding patterns between the
species revealed that human chromosome
2 is present as two separate chromosomes
(12 and 13) in chimpanzees (±ig. 4a). A
subsequent molecular characterization re-
vealed that a fusion at the ends (telomeres)
of two ancestral chromosomes gave rise to
human chromosome 2.
Seven additional chromosomes, human
chromosomes 4, 5, 9, 12, 15, 16, and 17
differ between humans and chimpanzees
by an inverted arrangement of chromoso-
mal segments. In all cases, the inversion
breakpoints are located on either side
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