Genes and Body plans
How does an organism become a zygote?
How do the different parts of an organism
know which way is “up”?
Learning Outcomes
Explain that the genes that control development of
body plans are similar in plants, animals and fungi, with
reference to homeobox sequences (HSW1);
What do homeobox genes do?
• Homeobox genes code for the production of
transcription factors
• These proteins can bind to a particular region
of DNA and cause it to be transcribed
– A single homeobox gene can switch on a whole
collection of other genes, regulating gene
expression
Homeobox genes
• Homeobox genes determine how an organism’s body
develops as it grows from a zygote into a complete
organism.
• They determine the organism’s body plan
• These sequences are highly conserved
• Homeobox genes have been discovered in animals,
plants and fungi
Homologous homeobox genes
• These are the sequences of 60 amino acids in the proteins
coded for by the homeobox genes Antp in a fruit fly and
HoxB7 in a mouse.
• All animals have homologous homeobox genes – they are
recognisably similar. Why is this significant?
Key words – homeobox, maternal effect and polarity
genes
What are they? What do they mean?
• Homeobox genes control the development of the body
plan of an organism, including the polarity (head and tail
ends) and positioning of the organs
• Maternal effect genes determine the embryo’s polarity
• Polarity genes Help to define the anterior and posterior
polarities within each embryonic segment of an
organism, such as in the fruit fly
Homeosis - when one part of a developing embryo becomes
anomalously transformed into another (mutation)
Some proteins are expressed and
concentrated in a gradient, highest
in the head
Others are expressed in the head,
or in both the head and the tail of
the body
Another set of proteins is expressed
in a concentration gradient from
ventral to dorsal
Write a summary to
describe what is happening
in this diagram as the
drosophila fly develops from
an embryo.
Use the key words to help
you (you may not need all of
them)
Name the type of gene, if mutated, gives rise to
dramatic changes in body plan
Homeobox gene
Drosophila melanogaster – the fruit fly
A model animal for the study of
genetics and gene regulation
The body of insects is segmented
This is obvious in larvae (e.g. maggots)
In adults these become specialised many develop special appendages
Head Segments - antennae
- labial palps
(mouth appendages)
Thoracic Segments - legs
- wings
- halteres
(balancing organs)
Abdominal Segments - no appendages
Identity of each segment is established in
the embryo
Mutations can destroy the identity of a
segment (more on this later)
Homeosis in Drosophila
Wings for halteres
Legs for antennae
These changes are the
results of mutations in
a set of genes
(homeosis)
In this instance, are
mutations a good thing?
Halteres for wings
Homeobox genes have been found in:
 worms
 snails
 starfish
 fish
 mice
 humans
Summary
• These genes are homeotic (regulatory) containing
_______base pairs forming the
____________sequence that codes for a gene
product which binds to DNA and initiates
______________. In this way they switch genes
that control the development of a _________
______on or off.
• These genes show little mutation because they
are very _____________. Mutation would have
big effects and alter the body plan causing many
other genes to be affected. Any mutation is likely
to be ___________and so selected against
Write a summary to
describe what is happening
in this diagram as the
drosophila fly develops from
an embryo.
Use the key words to help
you (you may not need all of
them)
Model answer
The body plan of the drosophila fly is controlled by the homebox genes.
These contain transcription factors that regulate other genes so that the body
plan develops normally. Transcription factors act like on/off switches for gene
expression.
Some of these genes are maternal-effect genes. They will determine the
polarity of the embryo. This means which end is the front and which is the
back.
Segmentation genes are another group of genes that determine the dorsal
and ventral ends of specific segments of the fly’s body.
Effect of thalidomide in embryo
development
– Homeobox genes HoxA11 and HoxD11 switch on
genes that cause forelimb development.
– The drug thalidomide affected the behaviour of
these homeobox genes at a critical stage in
embryonic development.
Retinoic acid and birth defects
– Retinoic acid
• is a derivative of vitamin A
• activates homeobox genes in
vertebrates
• Is a morphogen (substance governing
pattern of tissue development).
– If a pregnant woman takes too
much Vitamin A, it can interfere
with the expression of homeobox
genes causing birth defects in the
central nervous system and axial
skeleton.
• Write a short note to summarise the effects of
retinoic acid and vitamin A on homebox genes
and development.
Homeobox genes show astonishing similarity across widely different species of animal,
from fruit flies, which are insects, to mice and humans, which are mammals. The
sequences of these genes have remained relatively unchanged throughout evolutionary
history and the same genes control embryonic development in flies and mammals.
(a) State what is meant by a homeobox gene. [2]
(b) Homeobox genes show ‘astonishing similarity across widely different species of
animal’.
Explain why there has been very little change by mutation in these genes.[2]
(c) Frogs reproduce by laying eggs in water. Each egg develops into a tadpole, which
has external gills to extract oxygen from the water, and a tail to help it swim. The tadpole
gradually changes into an adult frog as it grows. During this time its gills and tail
disappear.
List two cellular processes that must occur during the development of a tadpole into a
frog. [2]
(d) Name another kingdom of organisms, other than animals, that have similar homeotic
genes.[1]
Homeobox animation
• http://www.dnaftb.org/dnaftb/37/concept/in
dex.html
– If you have time, sit and watch this animation, as
well as investigating other aspects of this web site.