1. Genes and Body plans

2. Ubx (Ultrabithorax) gene mutant
Normal fly
Extra set of wings!
Ubx (Ultrabithorax) gene mutant
Antennae are transformed into legs!
Antp (Antennapedia) gene mutant
Carroll S.B. et al. From DNA to Diversity (2001) Blackwell Science

4. What caused those body parts to be in the WRONG places?
What causes body parts to grow in the CORRECT places?
Remember that
EVERY CELL of your body contains ALL your GENES

5.
Evolution: Library: Genetic Tool Kit

6. Homeotic and Homeobox Genes
Control how an organism’s body develops as it grows from a zygote into a complete organism.
They determine the body plan including the polarity (front and back part) and positioning of organs.
Homeotic genes define a region or position in the embryo and code for transcription factors that regulate the activity of other genes.
Genes in this group contain a conserved stretch of DNA called the homeobox and are nearly identical in all species.

7. The homeobox is only a portion of each gene
e.g. if the words below were homeotic genes, the capital letters would represent the homeobox:
togeTHEr / THEoretical / gaTHEring / boTHEr
The homeobox is a 180-basepair sequence of DNA that has been found in many regulatory genes.

8. A particular set of genes, the Hox genes, are responsible for assigning the head to tail body pattern in very early embryos.
Hox genes are a subset of homeotic genes that contain a homeobox
Homeobox genes code for the production of polypeptides about 60 aa long (the homeodomain) and act as transcription factors.
These factors bind to a particular region of DNA and cause it to be transcribed.
So, a single homeobox gene can switch on a whole collection of other genes.

9. On your worksheet What do you notice?
Choose 3 colours and assign a colour each to a, g and i
Locate the regions labelled a, g and i in the diagrams of the Drosophila Homeobox gene, the embryo and the adult
Use your colour key to colour in these sections in the Drosophila Homeobox gene, the embryo and the adult
What do you notice?

10. The pattern of Homeobox expression
Colinearity between the order of the homeobox genes and the expression pattern
Eight homeobox genes regulate the identity of regions within the adult and embryo.
The order of the genes determines the body plan
Adult
Embryo
Carroll S.B. et al. From DNA to Diversity (2001) Blackwell Science

11. All animals have homeobox genes that are similar (homologous) and highly conserved.

12. Evidence that there was colinearity in homeobox genes between species
Evolution: Library: Genetic Tool Kit

13. Hox Genes

14. Hox genes are a specific family of homeobox genes
that regulate other genes
They are a group of related genes that control the body plan of an embryo along the anterior-posterior (head-tail) axis. 
They are highly conserved
Every Hox gene is a homeobox gene, but not every homeobox gene is a Hox gene

15. This conservation of genes implies that their activity is fundamental to the development of an animal.
Homeosis is a mutation of a homeobox gene that causes transformation of one area of the body into another area.
A mutation in a homeobox gene is so disastrous that the organism usually doesn’t survive.

16. Drosophila (fruit fly)
Body plan of typical insect – head thorax and abdomen.
Thorax is made up of segments (T1, T2, T3).
A pair of legs grows from each segment.
A pair of wings grows from T2.
A pair of halteres (used for balance when flying) grows from T3.

17. It is turned off in the head.
A homeobox gene called Antp is usually turned on in the legs where it causes legs to develop.
It is turned off in the head.
In some mutant flies the Antp gene is switched on in the head producing legs
instead of antennae.
Mutants
Wild-type

18. Mutant fly with 2 wings
Normal adult fly
Drosophila has a homeobox gene called Ubx – this stops wings forming in T3.
If the fly has a mutation in both copies of Ubx, wings grow in T3 instead of halteres.
Bithorax mutant

19. Hox clusters Hox genes are arranged in clusters.
their order on the chromosome is the same as the order in which they appear along the body.

20. What this means is that if you look at the chromosome that contains the Hox genes they are all lined up in order on the strand of DNA.
This shown in the drosophila fruit fly below:

21. Hox genes are universal in animals.
Vertebrates have Hox genes, and they have the same properties: each gene contains a homeobox, and they are organized on the chromosome in the order of their expression from front to back

22. Nematodes (round worms) = 1 Hox cluster
Larger Hox gene number
More complicated body pattern
Nematodes (round worms) = 1 Hox cluster
Drosophila = 2
Vertebrates = 4

23. Drosophila and human Hox genes
Mammals
In mammals Hox genes are found In 4 clusters
Hox A: Chr. 16
Hox B: Chr. 11
Hox C: Chr 15
Hox D: Chr. 2

24. Evolution of Hox gene clusters
Hypothetical common ancestor
Amphioxus

25. Haeckel's 1874 version of vertebrate embryonic development.
The top row shows an early stage common to all groups, the second row shows a middle stage of development, and the bottom row shows a late stage embryo.

26. The Role of retinoic acid
Retinoic acid is derived from vitamin A
It is an activator of HOX genes in vertebrates
It activates the genes in the correct sequence running from head to tail
It is called a morphogen because it regulates the pattern of tissue development
If it is in xs it can however start to switch on the genes in wrong order resulting in major birth defects
Pregnant women should be very careful about their vitamin A intake

27. Homework
Make notes and diagrams on the development of drosophila and its genetic control p114