Examples of developmental mutants

Many developmental mutations are known in
Drosophila melanogaster. They can be classified
into different hierarchical gene classes. The normal
larva (wild type) consists of three head segments,
three thorax segments, eight abdominal
segments, and the tail end (1). A mutation for
anterior maternal effect, bicoid, leads to the
development of a larva without head or thorax
(2). Amutation of a gene called nanos affects the
posterior end of the early larva. Gap genes establish
the basic pattern of segmental organization.
Mutations of the gap genes lead to omissions
(gaps) in the segmental construction of
the larva. In the Krüppel mutant (3), all thoracic
and the abdominal segements 1–5 aremissing;
in the Knirps mutant (4), abdominal segments
1–6 are absent. The genes for pair-rule determine
the orientation and developmental fate of
the 14 parasegments. Some mutations affect
every second segment. With even-skipped (5),
all even-numbered parasegments are missing.
Mutation of the gene fushi tarazu leads to fewer
than normal segments being formed (fushi
tarazu is Japanese for too few segments). Segment
polarity genes determine the polarity of
each segment (7). There are more than ten segment
polarity genes. Homeotic selector genes (8)
determine the ultimate fate of each segment.
With the mutant antennapedia (Ant), the
antenna normally attached immediately under
the eye is replaced by a leg (homeotic leg).