Welcome to the fascinating world of dilute horse colour genetics ....When we talk about dilute horse colour genetics we are referring to the colour genetics that create the palomino, buckskin, cremello, perlino, silver, dun, champagne, pearl and mushroom coat colours.
Dilute horse colour genetics - the brief version
Basically, genetics - including dilute horse colour genetics - works like this….
living creatures inherit one half of their DNA material from one parent and the other half from the other parent.
The DNA inherited from each parent contains one half of all the genetic codes (genes and alleles) that will ultimately determine the various genetic characteristics of the new individual at conception. Each of these various genetic codes ‘match up’ with the relevant code on the other half ie the code for eye colour from the female parent will match up with the code for eye colour from the male parent. Every characteristic is determined by this pairing of genetic codes.
When a pair of genetic codes are the same that individual is homozygous for that genetic characteristic ie both codes for eye colour are blue so the eyes will be blue.
When a pair of genetic codes are not the same that individual is heterozygous for that genetic characteristic ie one code for eye colour is blue and the other is brown.
A genetic code is either dominant or recessive - a dominant genetic code will hide or mask a recessive one....
....if one of the codes is recessive and the other is dominant the characteristic controlled by that pair of codes will take on the attributes of the dominant code ie in the case of horse coat colour genetics, black is dominant over red. A horse that carries one copy of the code enabling production of black pigment (black) and one copy of the code that lacks the ability to produce black pigment (red) will be black in appearance. It is also heterozygous for this particular pair of genetic codes – one copy is black and the other copy is red.
If a recessive code is to remain 'in control' the code it matches up with from the other parent has to be similarly recessive. Again, using horse coat colour codes, and remembering that black is dominant over red, this means that for a horse to have a red or chestnut coat it must have two genetic codes for a red coat. If it carried one code for black and one code for red the black would dominate the red and it would have a black coat.
The base coat colours of black and red in horses can be modified by other codes (alleles or modifiers) such as agouti and the pigment diluting codes (cream, silver, dun, champagne, pearl & mushroom).
These various ‘modifying’ codes, technically called 'alleles', are inherited in exactly the same way as all other genetic codes – one from each parent.
A horse that inherits the code for cream from one parent and a non-cream code from the other parent is said to carry ONE copy of the code for cream ie palomino, buckskin and dilute black (smoky black / black buckskin)
A horse that inherits the code for cream from both parents is said to carry TWO copies of the code for cream (cremello, perlino and smoky perlino).The Extension gene is what controls the ability to produce black pigment. The 'norm' is for horses to carry a normal black pigment producing copy of this gene. At some point in evolution though a mutation occurred in this gene in some individuals which basically resulted in the loss of the ability to produce black pigment.
When an individual carries either two 'normal' copies of this gene or one 'normal' copy and one 'modified' copy they produce black pigment.
When an individual carries two modified copies of this gene they lack the ability to produce black pigment. They produce yellow/red pigment instead.
Agouti is what controls the distribution of black pigment in the coat.
A black horse does not carry agouti so it has a totally black coat.
Bay and brown are variations of black caused by the presence of either bay or brown agouti
When a black horse carries a copy of bay agouti the black pigment is removed from its body leaving it with a red coat and black points ie it becomes a bay horse.
When a black horse carries a copy of brown agouti (and no bay agouti) black pigment is removed from various part of its body - generally the soft spot areas like the flanks, muzzle and so on to create a brown horse. I’ve also noticed that brown agouti has a tendency to ‘kill’ white markings – think of most of the genuinely brown horses you know and I bet most of them have minimal white markings.
Red (chestnut) horses can also carry agouti but because it only controls black pigment it doesn’t affect them
Genes and Alleles....
Genes control a particular characteristic and alleles modify the appearance of that characteristic.
For instance, base coat colour (red or black) is controlled by the extension gene but its appearance or phenotype is modified by various alleles or modifiers such as agouti, cream, silver, dun and so on.
Genotype and Phenotype....
Genotype refers to the genetic makeup of an individual...what they carry in their DNA. This includes all the various recessive and dominant genes.
Phenotype refers to the external appearance or visible expression of the genes carried by an individual....what they look like on the outside. Recessive genes are masked by dominant genes so remain invisible on the outside.
Dominant or not ....?
When it comes to dilute horse colour genetics there are currently 3 different 'types' of modifiers or alleles:
Simple dominant
Incomplete dominant
Incomplete recessive
Simple dominant
Simple dominant modifiers are easy to understand. When present a simple dominant allele will always modify what it's supposed to modify.
There is also no difference in phenotype (external appearance) between individuals that carry one copy of a simple dominant allele or two copies of a simple dominant allele.
Examples for the purposes of dilute horse colour genetics are silver, dun and champagne.
Incomplete dominant
Incomplete dominant modifiers are a bit more tricky. Whilst an incomplete dominant allele, being dominant, will always modify the characteristic it controls, the extent of modification is determined by the number of copies present....
If one copy is carried there is only partial or incomplete modification of that characteristic.
The full effects are not seen until there are two copies of the modifier present.
The prime example of this as far as dilute horse colour genetics goes is of course cream. One copy of cream only dilutes red hair pigment - skin, eye and black pigment remains undiluted.Two copies of cream dilutes both red and black pigment in the hair, skin and eyes.
Incomplete recessive
Incomplete recessive modifiers, being recessive, will not modify anything when carried in single copy. Two copies are required before any modifications at all are seen.
The only incomplete recessive currently identified in the world of dilute horse colour genetics is pearl. One copy of pearl carried by a base coat coloured horse has no visible effect. Two copies dilutes both black and red pigment in the coat, skin and eyes. Pearl is also cream activated!
Well, that's the basics of dilute horse colour genetics 'sort of' in a nutshell....
More information about dilute horse colour genetics....
No comments:
Post a Comment