Why do HOX genes occur in clusters?
1 Answer
Because of the way they evolved.
Explanation:
This matter has actually not been resolved completely. Why Hox genes occur in clusters is most likely because they evolved from the duplication of a homeobox gene in a distant ancestor. See this answer for more information on the evolution of Hox genes.
Because this replication the genes ended up next to each other and developed further to code for specific different cell types. This type of evolution resulted in two interesting phenomena:
- spatial colinearity: genes at one end of the chromosome define the head of the embryo and genes at the other end define the 'tail' end.
- temporal colinearity: the genes that define the head side of the embryo are expressed before the genes that define the tail end.
It seems plausible that it is convenient for a cell to have the genes in clusters to time their expression. However, there is also research that shows that clustering is not essential. So, there is more to it and still enough to discover!
If you would like to read more on the regulation of Hox-genes I can recommend the article by Mallo & Alonso 2013 .
