Some Terms and Notes

Starting from the bottom,

The building blocks of DNA are the Base Pairs:

Adenine and Thymine (A-T)

Guanine and Cytosine (G-C)

All DNA patterns are based on different orderings of these pairs (barring the substitution for Thymine for Uracil [U] in RNA) this simple code is what dictates the order in which the amino acids are placed making up every single protein in the body - enzymes, pigments, cell walls - everything.

A length of Base Pairs is known as either an Exon or and Intron - Exons are the pieces of DNA that code, Introns separate Exons and are thought to 'prepare' the functions the Exons execute.

Groups of Exons and Introns form Genes these genes code for a specific protein or set of proteins that have a defined function, these are given names such as KIT or MAPT. A location on this gene where a specific mutation takes place is known as a Locus (plural Loci) so for example the Cream Locus is on the MAPT gene.

These Genes are parts of Chromosomes, chromosomes are numbered, in the horse ECA1 (Equine Chromosome 1) - ECA31 plus the sex Chromosomes which are Labeled ECAX and ECAY. For example, the Cream Locus is a Mutation point on the MAPT gene which is on ECA21.

Mutation - mutations are changes in the DNA, most of these occur in 'Junk' or non-coding DNA but when they occur in the coding DNA a protein is altered. This alteration can be catastrophic preventing the resulting zygote from even being viable, can be so minimal it passes unnoticed, it can be negative - a shining albino tiger in the grasslands can't blend in to well and is consequently rare as very few survive to breed - or beneficial - the same white tiger in the northern area of Siberia where it is snowy much of the year is at an advantage in terms of camouflage. This is the basis of evolution. Humans also make use of these differing traits in 'unnatural' selection, those cows with a mutation causing them to grow faster and heavier may not live long in the wild where their increased requirement for food is a disadvantage but on the farm will be prized and bred from as they provide more meat faster.

These mutations are what provide the huge array of possible horse colours that I am trying to define and explain here.

There are several types of mutation:

Frameshift Mutation - A frameshift mutation is caused by either an insertion or a deletion in the gene’s coding From the point of the insertion or deletion, the translation is completely different from the original

Nonsense Mutation - Nonsense mutations cause a “stop” in the reading sequence, similar to a faulty computer program. the “stop”is an inability to continue reading the sequence, results in a loss of function of the protein.

Missense Mutation - This type of mutation is the result of one amino acid being replaced by another. The result is generallythat the protein is no longer functional.

Splice Site Mutation - Splice site mutations are insertions or deletions of intron coding as it is being translated into the mature genetic coding of the exon. Some of the intron code is spliced into the exon

Splice Variant - Certain exons are spliced out, altering the expression of the protein

Duplication - A duplication mutation is reasonably self-explanatory, a section of DNA is duplicated.

Inversion Mutation - an Inversion mutation is where a section of DNA code has been inverted - it is still present but back to front.

I have been pretty useless about updating as I have been waiting for replies on requests to use pictures. Unfortunately people are not getting back to me one way or another and much as I hate the idea of stealing photos I think I need to get on with it. The use is purely non-profit and (hopefully) educational so I am pretty sure it comes under fair use and I will link back to the source of any picture I use so I hope no-one will be upset by me using pictures to illustrate the colours.

Sources for this page include:

http://www.thehorse.com/ViewArticle.aspx?ID=4354 - an article from The Horse.com - A Horse of A Different Colour - Stephanie J. Corum

http://www.thesciencedictionary.com/ - A useful on-line dictionary of scientific terms. I use this a lot for when I know something but cannot work out how to say it.