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A
B
FIGURE 2.4 Elimination of the Hox gene cluster in the Tribolium beetle results in all segments develop-
ing an identical morphology. A shows the normal appearance of the beetle, and B shows an animal without
a Hox gene cluster. The normal number of segments develop, but all of the segments acquire the morphol-
ogy of the antennal segment, showing the importance of the Hox genes in the development of positional
identity in animals. (Reproduced from Stuart et al., 1993, with permission)
A
P
Drosophila
3'
lab
pb
Antennapedia complex
dfd
scr
Antp
Ubx
Abd-A
Abd-B
5'
Bithorax complex
Mouse
Hoxa, chromosome 6
3'
a1
a2
a3
a4
a5
a6
a7
a9
a10
5'
Hoxa, chromosome 11
b1
b2
b3
b4
b5
b6
b7
b8
b9
Hoxa, chromosome 15
c4
c5
c6
c7
c8
c9
c10
Hoxa, chromosome 2
d1
d3
d4
d8
d9
d10
A
P
FIGURE 2.5 Hox gene clusters in arthropods (Drosophila) and vertebrates (mouse embryo) have a similar
spatial organization and similar order along the chromosomes. In Drosophila , the Hox gene cluster is aligned
on the chromosome such that the anterior most expressed gene is 3¢ and the posterior most gene is 5¢. In the
mouse, there are four separate Hox gene clusters on four different chromosomes, but the overall order is
similar to that in arthropods: the anterior to posterior order of gene expression is ordered in a 3¢ to 5¢ order
on the chromosomes.
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