genetics - How does the colinearity of the HOX genes determine the body plan of an organism?

I was recently reading about colinearity in the HOX genes that give an organism its high-level body plan (where the order of the HOX genes on the chromosome follow the head-to-tail order of body segments, such that the head gene comes before the thorax gene, comes before the abdomen gene, etc).

I'm really just a layman interested in this stuff (only completed A & P I), but I was under the impression that the location of genes on a chromosome has no bearing on the expression of those genes or phenotype of the organism -- in other words, that genes can be anywhere on any chromosome.

Do we understand how the order of the HOX genes ends up being expressed as the order of the body segments? Do we know why the positioning of these genes matters when the order of other genes don't?

Answer

You might be interested in this book

When writing Hox Gene collinearity (or colinearity as it is often misspelled as explain in wiki!) on Google scholar or WebOfKnowledge you will get many results on the subject. It is still today an ongoing debate.

Below are some sentences I pick up from these articles. It is certainly hard to understand as I hardly understood what I wrote! The best for you is to go through this review. It is not easy (at least for me but it is not at all my field) but I think it is the best source of information you can find on the subject.

This and this might also help you to understand the debate on the subject.

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There are several types of collinearity:

Spatial collinearity is the sequential 3' to 5' expression of Hox Genes along a body axis. Spatial collinearity can be associated with time dependence where the most 3' is expressed first. This is defined as temporal collinearity.

There are two main models to explain the mechanisms hidden behind this ordering.

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  1) Collinearity is based on transcriptional regulation and specifically that is limited by the progressive 3' to 5' opening of Hox cluster chromatin and/or mediated by global control regions

  
  


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  2) Collinearity depends on interactions between the Hox genes themselves. These interactions include 'posterior prevalence', a negative interaction among Hox proteins that clearly relates to functional collinearity in Drosophila.

  
  

Evolution:

It is suggested that collinearity evolved by repeated tandem duplication of an ancestral ur-Hox Gene