Chimpanzee Genome
567
3.2
From Genotype to Phenotype
Maybe the most widely noticed feld in
evolutionary research is concerned with
the emergence oF new phenotypes that
contribute to the uniqueness oF individual
species.
Mainly
three
aspects
are
in
the Focus oF interest: (1) When did the
new phenotype arise? (2) Was it chance
or selective advantage that caused its
f
xa
t
ioninthespe
c
ie
s
?(3
)Wha
ta
rethe
genetic changes that underly the particular
phenotype? While the answers to the frst
two questions are mainly oF academic
interest, the correlation between genotype
and phenotype is – as will be discussed in
Sect. 3.3 – oF immediate practical use.
When comparing humans and chim-
panzees, the impression is that both
species are strikingly alike, yet show a
certain set oF well-defned diFFerences.
Meanwhile, an extensive catalog oF phe-
notypic diFFerences between humans and
chimpanzees has been collected. Among
the most well-known examples are bipedal
walking, enlarged brain size, and spoken
language, which distinguish humans From
chimpanzees and all other primates. This
set oF phenotypic diFFerences is opposed
by a considerably low amount oF genetic
diFFerences (see Sect. 2), which provides
an excellent starting point For the corre-
lation oF genotype and phenotype. Two
approaches can be chosen For such a
venture. In a bottom-to-top strategy, the
genomes oF humans and chimpanzees are
screened For diFFerences that aFFect gene
Function. This can be either changes in
the coding region oF genes resulting in a
structural change oF the gene product, or in
regions that control the expression pattern
oF a gene. While the identifcation oF diFFer-
ences potentially aFFecting gene Function
is considerably simple, the subsequent
determination oF the likely eFFect on the
phenotype is experimentally extremely de-
manding. In the frst step, the Function oF
the aFFected gene must be determined, and
in the next step, the influence oF the genetic
change on the respective Function oF the
gene product must be determined. This
latter step will be especially problematic
For such changes that aFFect the expression
pattern oF genes rather than the structure
oF the gene product. Eventually, inFerences
oF the likely phenotypic consequence oF the
genetic change can be made. The reverse
approach, a top-to-bottom strategy, starts
From a recognized phenotypic diFFerence
between humans and chimpanzees. Cer-
tain genes that are known or suspected
to contribute to the phenotype oF inter-
est are assigned as candidates whose DNA
sequence can be screened For diFFerences
between humans and chimpanzees. Once
species-specifc diFFerences are Found For
a candidate gene, the subsequent proce-
dure resembles that oF the bottom-to-top
strategy with the subtle but essential diFFer-
ence that a particular phenotype has been
already correlated with the gene.
To date, the study oF candidate genes
has been successFully applied at least in
two studies, one oF which originated From
the observation oF a structural diFFerence
between the cell surFaces oF humans
and great apes. It could be shown that
this is due to an inactivation oF an
enzyme on the human lineage caused by
a deletion in the corresponding gene (see
Sect. 2.4). Even though this comprises a
marked biochemical diFFerence between
human and chimpanzee cells with possible
implications For the intercellular cross
talk,
the
relevance
oF
this
diFFerence
For human evolution is still unclear. A
second study Focused on a gene
FOXP2
,
which when mutated, causes a distinct
type oF speech impairment in humans.
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