DNA Testing for the Tobiano gene; a quick lesson.

Coloured horses and ponies, often referred to as Piebald, Skewbald, or Tri-coloured have long been amongst the most desirable of all equine specimens. Despite their rising popularity, little thought has been given to the genetics involved in creating their unique appearance. This article briefly explains the Tobiano gene and the testing available for this common coat colour pattern.

The Tobiano gene is a dominant gene expressing in both horses that are heterozygous and homozygous for Tobiano.  The gene is associated with the loss of pigmentation, leading to a horse with large white markings in certain defined areas. These markings can include some or all of the following:

• White oval-shaped patches crossing over the back.
• White Legs, extending from the hocks and knees down.
• Neck, face, and front of forelegs are generally unaffected.
  

Inheritance from Parents

• A horse that does not carry a Tobiano gene is Tobiano negative. In science, we give this a code and refer to a Tobiano negative simply as ‘nn’. A nn horse displays no Tobiano markings and will not pass the Tobiano gene on to its foals.
  
  
• A horse that carries a single Tobiano gene is referred to as ‘Tobiano heterozygous’ –the word heterozygous in reference to genes means ‘two different genes’. Tobiano heterozygous horses are referred to as ‘nT’. A nT horse displays Tobiano markings and will statistically pass the Tobiano gene on to 50% of its foals.
  
  
• A horse that carries TWO Tobiano genes is referred to as ‘Tobiano homozygous’ –the word homozygous in reference to genes means ‘two of the same’. Tobiano homozygous horses are referred to as ‘TT’. A TT horse not only displays Tobiano markings, but will pass the gene to 100% of its foals.
  

So, a Tobiano horse can carry either one or two Tobiano genes. The number of Tobiano genes the dam and sire carry will ultimately determine whether its offspring can potentially be heterozygous or homozygous for Tobiano.

The following points simplify how carrying one or two copies of a certain gene affect the inheritance of that gene to the offspring.

• The offspring inherits 50% of its genetic makeup from the dam and 50% from the sire.
  
  
• If a horse has two copies of a certain gene (homozygous), it is guaranteed to pass one copy on to its offspring.
  
  
• If a horse has one copy of a certain gene (heterozygous), it is 50/50 as to whether or not it will pass the gene on to its offspring.

Here is an example of the above points, using the Tobiano gene as mentioned before

The offspring in the chart above is heterozygous, 50% of the time it will pass on the Tobiano gene to its own offspring and 50% of the time it will not.

With some basic information of the Tobiano pattern and the genetics involved explained above, the following is a description of the possible results obtained via genetic testing:

• Tobiano Negative (nn): Solid, non-Tobiano horse.
• Tobiano Heterozygous (nT): Positive for the Tobiano gene and will produce both coloured foals and solid foals on a 50% likelihood basis for each.
• Tobiano Homozygous (TT): Will always produce coloured foals regardless of the mate.

By utilising Animal Genetics’ testing services, you have the ability to predict the ratio of Tobiano foals you can expect, as well as other equine genetic tests.  Animal Genetics has developed a program to assist horse breeders in predicting possible offspring coat colours. 
The ‘Coat Colour Calculator’ can be reached by following the link below:
http://animalgenetics.us/CCalculator1.asp

DNA Tests Available

At present, genetic testing for the Tobiano pattern is carried out by testing for a mutation found in the KIT gene, which is closely linked to the Tobiano gene. Currently, there is not a direct test for the Tobiano gene, but the outstanding accuracy of this test makes it more than a viable option for any serious enthusiast or breeder.

If you wish to carry out the genetic test with Animal Genetics UK and determine if your coloured horse is homozygous or heterozygous, you can download the submission form through our web-site: www.avianbiotech.co.uk

Sample Collection

Collect sample by pulling (not cutting) 30-40 mane or tail hairs with roots attached. It is important that you pull the hairs and confirm that the actual root of the hair is being collected. The root contains the genetic material of your horse that is needed for DNA testing. Therefore, cut hairs do not provide an adequate sample. Place the collected hairs of each horse in a separate zip-lock bag labelling the bags accordingly with the horse’s name or identification number. Complete a submission form for each sample and send the form along with hair sample(s) and payment to Animal Genetics.

Please note, that because this is an ‘indirect’ genetic test for Tobiano, there is a small percentage of Tobiano horses which we have found to test as ‘Negative’ using this technique. This has occurred in less than a dozen horses, from 3,000 tests.

Animal Genetics also offers a number of other Equine genetic tests, some of which are relevant to Tobiano horses.


The Red/Black Factor Test determines your horse’s basic pigment colouring, or “Base colour” either Black or Red. This will help determine the non-white colouring of the foal your Tobiano horse will produce.

We also test for:

• Agouti (Bay)
• Cream Dilution
• Silver Dilution
• Sabino1
• Lethal White (Frame) Overo
• HYPP (Muscle Twitch)

If you have any queries about the nature of our tests, or wish to order kits, please feel free to contact us.

Animal Genetics UK
PO BOX 107
Truro
Cornwall
TR1 2YR