How can hox genes provide evidence of evolution?

Answer 1

Hox genes are a group of related genes that specify the anterior-posterior axis and segment identity of metazoan organisms during early embryonic development. These genes are critical for the proper number and placement of embryonic segment structures (such as legs, antennae, and eyes).

Homeotic genes are defined by a DNA sequence known as the homeobox, which is a sequence of 180 nucleotides that code for a protein domain known as the homeodomain.

The protein products of homeotic genes belong to a class of proteins known as transcription factors, all of which are capable of binding to DNA, thereby regulating the transcription of genes.

Just as homeotic genes regulate realisator genes, they are in turn regulated themselves by gap genes and pair-rule genes, which are in their turn regulated by maternally-supplied mRNA. This results in a transcription factor cascade: maternal turns on gap or pair-rule genes; gap and pair-rule genes turn on homeotic genes; then, finally, homeotic genes turn on realisator genes that cause the segments in the developing embryo to differentiate.

Incorrect expression of homeotic genes can lead to major changes in the morphology of the individual. Homeotic mutations were first identified in 1894, when William Bateson noticed that floral stamens occasionally appeared in the wrong place; he found for example flowers in which the stamens would grow in the place where petals normally grow.

In the late 1940s, Edward Lewis began studying homeotic mutation on Drosophila melanogaster which caused bizarre rearrangements of body parts. Mutations in the genes that code for limb development can cause deformity or lead to death. For an example, mutations in the Antennapedia gene cause legs to develop on the head of a fly instead of the antenna.

Another famous example in the Drosophila melanogaster is the mutation of the Ultrabithorax homeotic gene, which specifies the 3rd thoracic segment. Normally, this segment displays a pair of legs and a pair of halteres (a reduced pair of wings used for balancing). In the mutant lacking functional Ultrabithorax protein, the 3rd thoracic segment now expresses the same structures found on the segment to its immediate anterior, the 2nd thoracic segment, which contains a pair of legs and a pair of (fully developed) wings. These mutants sometimes occur in wild populations of flies, and it was these mutants that led to the discovery of homeotic genes.

Answer 2

The hox genes (actually a subset of the Homeobox genes) were important in evolutionary thought because they provided a basis for saltationism and punctuated equilibrium.

These genes direct morphogenesis, defining the body plan of organisms. Thus, a small mutation in any of them can lead to an enormous change in the physical appearance of an organism.

Answer 3

Small changes in such hox genes or changes in the genes turned on by them could represent evidence of evolutionary change

Answer 4

Hox genes, also known as homeotic genes, control the body plan of an embryo. They are important in establishing the anterior-posterior axis and the identity of each body segment in organisms.

Answer 5

Homologous control genes serve similar functions in animals as different as insects and humans- even though those animals haven't shared a common ancestor in at least 700 million years!