Horse of a Different Color? Part II
Predicting foal coat colors in paint horses...
Well this is Part II in the coat color genetics thread and we are going to talk specifically about paint horses (or pinto) coat color genetics. Paint horses have the same 'base' coat colors as solid colored horses, you can read about those in 'Part I' of 'Horse of a Different Color?'. We are now going to discuss white markings that modify the basic coat colors from Part I.
Even solid horses can have white markings that range from a subtle star to a flashy white bald face and high stockings. These markings are a result of genetics and possibly conditions in the womb and are usual on almost horse breeds. The next level of 'white markings' are the paint or pinto horse patterns which have some predictability although further research is still being done to understand all the patterns.
A dominant and common paint marking pattern is the tobiano pattern represented as T when present or t when not present. Again, genes come in pairs, meaning that a horse can carry a variety of genes based on the parent's genetics and also based somewhat on luck or 'genetic variation'. A horse with at least one tobiano gene will be marked with a specific white pattern that looks as if white paint has been thrown over the top of them. They will often have 'usual solid horse' facial markings and multi-colored manes or tails. Many tobianos are 50% their base coat color and 50% white, though there is a range of color that is considered 'normal' and still within the tobiano pattern. Tobiano horses often have white legs and striped hooves. The tobiano pattern, when homozygous or TT is expressed 100% of the time and the foal will be guaranteed to carry at least one copy of the tobiano gene. This is a desirable trait in breeding stock, but should not be bred for over ability, conformation or temperament.
Another paint pattern is the overo pattern. This pattern, once thought to be just one gene is now understood to be many genes that can co-exist or may exist alone to create different variations of a smiliar pattern.
If a horse carries a certain overo gene called 'frame overo' then a 'lethal white overo' can occur if the foal receives two copies they will have a fatal condition that is caused by this gene being 'double dominant' or homozygous. In genetic testing this gene is referred to as LWO and if someone is going to be breeding a horse that is known to be carrying this gene it is in their best interests to ensure that they do not breed to a horse that may carry LWO or they have a 25% chance of the foal dying before or just after birth. The 'frame' overo gene is recessive, meaning it can be masked and does not always display and can still be carried by solid looking horses. When expressed, his 'frame' overo pattern usually results in white facial markings, irregular spots along the neck, belly and sometimes hindquarters and white, irregular leg markings. The horses can have just one or two small spots or a whole range of wild, lacy spots. The horses white markings are sometimes described as having paint thrown up under them and it dripping down.
Another 'overo' pattern is the splash overo. This is not a lethal condition and in fact is considered to be a 'dominant' gene, similar to tobiano. The horses are distinctive with bald faces, often with blue eyes, large under-belly spots and white legs. This gene can be minimally expressed, and thus appear to be 'recessive' but in fact, the minimal white markings on the horses that appear to be 'normal' solid horse markings are in fact a minimal expression of the spalsh overo gene. This gene is often the cause of American Quarter Horse 'crop outs' being double regsitered as American Paint Horses due to excessive white markings.
Two more rare overo genes which express in a way that makes them look similar to the roan gene is the sabino and the rubicano genes. These genes are still being studied so they are better understood but they are carried in some well known lines of American Quarter Horses and Paint Horses and express as a roaning along the belly and ribs with 'white rib lines' often showing due to the minimal roaning these genes create. As more than one overo gene can exist on one horse, some express several having large amounts of white and producing more than the expected 50% color that most overos would produce.
There are other combinations, such as the 'tovero' that are produced when at least one tobiano and one overo gene exist in a horse's genetic make-up creating a white horse with minimal base color who will produce more than the expected 50% paint color rate of offspring due to two or more paint genes existing that will produce patterned color in their foals. These horses can produce tobiano, overo, tovero and soild foals if they are not homozygous for the tobiano gene. They usualy have close to an 80% color producing rate but this can go up if more than one overo gene is present.
These pattern genetics that make up the basis for the paint and pinto coat patterns are separate from the basic coat colors that are discussed in Part I of the article. A horse can be a black roan tovero. Many different coat colors and paint patterns can exist on one horse, and as breeders are discovering the different combinations, breeding for specific coat color is becoming more popular. The risk of course to focusing on any color, be it coat modifiers or paint patterns is that the overall quality of the horse can be overlooked. It is best to create a breeding program based on excellent individuals who excell in confirmation, talent and temperament and then add color as a secondary desired trait. It's not that wanting a specific color is bad, and we can try to breed for specific colors, but we should always remember the most important goal when breeding horses is to produce offspring that would be excellent in ANY color.
If you want to know more about base color coat genetics and modifying genes please refer to Part I of the article.