🚀 go-pugleaf

Greg Dressler (greg.dressler) wrote:
: The position of genes are not necessarily important for their function. 
: There is no obvious reason why genes are where they are.   Alleles are on
: the same genetic locus by definition.  An allele is really the same gene! 
: It is a slight variation of a gene product.  For example, at a single hair
: color locus the genes for blonde or brown hair are allelic.  You have one
: or the other, but not both.  The actual proteins that these genes encode
: are probably slight variations of each other.  Crossing over has nothing
: to do with alleles.  Just think of it this way.  If genes did not have
: alleles, we would all be identical, as in twins.  Do you understand? 
: Humans have all the same genes at the same loci, that is what makes us
: human.  However, we have many different combinations of alleles at most
: loci, thus we are all a bit different.  Alleles generally are regulated
: the same way, by the same promoter sequences.  The differences between two
: alleles are really quite small and may only represent a single base-pair
: change that may change one amino acid.

I'd like to point out one level of complexity you have left out. While it 
is true that many genes can be shuffled to different positions in the 
genome without any great change in expression, there are a decent number 
of genes that have dramatic change of expression when they are in the 
wrong genetic "neighborhood". It is called "position effects". There is 
something called the locus control region (LCR) which is apart from 
and upstream of the globin locus but is necessary for proper globin 
expression. There may be "reasons" for genes to be located at specific 
physical locations on the chromosome. The basis of this is studies of 
matrix attachment regions (MARs) which have to do with the anchoring of 
chromatin in the nucleus and the higher order packing of DNA. Genes that 
need to be regulated coordinately are sometimes localized together within 
a stretch of DNA that is bounded by MARs. Anyhow, my main point is that 
the position of a gene CAN but does not necessarily impact on the 
expression of the gene.

Gregory S. Turenchalk