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What is Homozygosity?
Genetic Testing Information is at the bottom of the page
What is a Homozygous Tobiano?
Genes exist in pairs and the offspring receive one gene from its sire and one gene from its dam. Genes are either dominant (T=Tobiano) and can be seen in the horse's color pattern or a gene can be recessive (t=non-tobiano or solid color). The dominant gene (T) rules over the recessive gene (t) when both genes (Tt) are present in the same horse. A homozygous tobiano must have two identical dominant (TT) tobiano genes to produce 100% tobiano paint foals. If a horse has two identical recessive genes (tt) it is a solid colored horse and does not have the tobiano color pattern. If a horse has one dominant (T) tobiano gene and one recessive (t) gene it is called a heterozygous tobiano (Tt) and it will have the tobiano color pattern. Heterozygous (Tt) tobianos represent the majority of the tobiano horse population.
How do you produce a homozygous tobiano?
A homozygous tobiano (TT) is the result of mating two horses which express (or have) the tobiano color pattern. A homozygous tobiano can be produced from one of the following breeding programs; 1) two tobiano parents or 2) a tobiano bred to a Tob/overo parent or 3) two Tob/overo parents. A tobiano bred to an overo can result in a horse that has both tobiano and overo color patterns and are called Tob/overo or Tovero. The overo gene is not related to the tobiano gene and the overo gene does not effect the expression of the tobiano gene. Regardless of your breeding program to produce a homozygous tobiano, the sire and the dam must first have the tobiano gene present and then give one of its dominant (T) tobiano genes to its offspring.
Homozygous tobiano (TT) bred to a homozygous tobiano (TT):
TT x TT=all of the offspring will be colored and homozygous (TT) tobianos
Homozygous tobiano (TT) bred to a heterozygous tobiano (Tt):
TT x Tt=all of the offspring will be colored foals, however, the offspring have a 50% chance of being homozygous (TT) and 50% chance of being heterozygous (Tt).
Homozygous tobiano (TT) bred to solid or non-tobiano (tt) paint horse
TT x tt=all of the offspring will have the tobiano color pattern and will be heterozygous for the tobiano gene (Tt).
Heterozygous tobiano (Tt) bred to heterozygous tobiano (Tt)
Tt x Tt=75% chance of producing a tobiano paint foal and 25% chance of producing a solid colored foal. The statistics break out as follows: 25% chance the foal will be Homozygous (TT) and a 50% chance the foal will be heterozygous (Tt) and 25% chance the foal will be solid (tt).
Heterozygous tobiano (Tt) bred to a solid color or non-tobiano (tt) horse
Tt x tt=50% chance of producing a tobiano (Tt) foal and a 50% chance of producing a solid (tt) color foal.
Solid Colored horse bred to a non-tobiano paint horse
No chance of producing a tobiano foal
Genetic Testing is available to determine if a horse is homozygous for the tobiano gene. Below are the links to 4 well respected laboratories
Blood Marker Tobiano homozygosity Testing only
University of California at Davis Veterinary Genetics Laboratory very informative with applications/information on testing on many types of colors, patterns, homozygous, genetic disorders, etc. A must see for anyone who breeds colored horses.
DNA and Blood Marker analysis for Tobiano homozygosity
Shelterwood Laboratories, a full service genotyping laboratory, is committed to serve the horse industry with the finest in equine genetic testing services. Since 1985, Shelterwood Laboratories has provided genetic testing and disease diagnostic services to the equine industry.
University of Kentucky The Equine Parentage Verification and Research Lab offers a variety of services to horse owners and individual breed registries. The laboratory participates in international comparison tests to assess its accuracy and ability to detect rare blood type variants; this assures the quality of its results. Since its establishment in 1986, the lab has consistently maintained its status as one of the top three blood typing & DNA labs in the world. http://www.ca.uky.edu/gluck/EPTRL.asp#color