E X T E N S I O N L I N K
The KIT and Extension loci are on the same chromosome in horses (the tobiano gene is as well), Chromosome 3. These genes are close enough that they become linked to each other. This is interesting in that it means you can actually predict how certain patterns can be inherited if you know the genotype of a horse and can break it down by parentage and progeny. What does this linking actually mean?
Well, a gene has two alleles. Extension has two alleles and so do KIT and tobiano (though as an inversion of KIT, it interferes with the gene's function and acts as if it is actually on the KIT locus). Being neighbors on the same chromosome and being linked means that one allele from one of these genes becomes bound to one allele from the other gene. Let's have an example:
A black tobiano stallion (let's say he is EE aa nTo) is bred to a red homozygous roan mare (ee aa RnRn). The resulting colt is a black that inherited tobiano from dad and roan from mom, so we know he is Ee aa ToRn. Since the sire was the only one with a dominant extension and tobiano, we know that our colt's tobiano pattern is linked to his dominant extension allele. Likewise, since his dam was the only parent with a recessive extension allele and dominant roan, we know that our colt's roan pattern is linked to his recessive extension allele.
This means that whenever he is bred and passes on his dominant E he will always pass on To, and when he passes on his recessive e he will always pass on Rn. This was just one example. It can get a little trickier with horses that are homozygous for extension, whether dominant or recessive because you can't say, "oh it's THIS extension allele that is linked to THIS pattern."
Well, a gene has two alleles. Extension has two alleles and so do KIT and tobiano (though as an inversion of KIT, it interferes with the gene's function and acts as if it is actually on the KIT locus). Being neighbors on the same chromosome and being linked means that one allele from one of these genes becomes bound to one allele from the other gene. Let's have an example:
A black tobiano stallion (let's say he is EE aa nTo) is bred to a red homozygous roan mare (ee aa RnRn). The resulting colt is a black that inherited tobiano from dad and roan from mom, so we know he is Ee aa ToRn. Since the sire was the only one with a dominant extension and tobiano, we know that our colt's tobiano pattern is linked to his dominant extension allele. Likewise, since his dam was the only parent with a recessive extension allele and dominant roan, we know that our colt's roan pattern is linked to his recessive extension allele.
This means that whenever he is bred and passes on his dominant E he will always pass on To, and when he passes on his recessive e he will always pass on Rn. This was just one example. It can get a little trickier with horses that are homozygous for extension, whether dominant or recessive because you can't say, "oh it's THIS extension allele that is linked to THIS pattern."
K I T C O M B I N A T I O N S
As has been stated, all gene loci have room for only two alleles. Generally this isn't confusing because most genes have only two allele forms, a dominant and a recessive. In the case of KIT and tobiano, however, it can get somewhat complicated because there are so many mutations and possibilities. All you have to remember is that there can only be a maximum of two different mutations for one horse.
Sabino 1 nSb1, Sb1Sb1, Sb1W#, Sb1Rn, Sb1To
White Spotting nW#, W#Sb1, W#W#, W#Rn, W#To
Roan nRn, RnSb1, RnW#, RnRn, RnTo
Tobiano nTo, ToSb1, ToW#, ToRn, ToTo
Because tobiano is not actually on the KIT locus, very VERY rarely there can be a genetic break and a horse could be homozygous for tobiano and also have a KIT mutation (there is a red Miniature Horse that has tested as TTW20) or vice versa. This is exceedingly rare and only a small handful of horses have been documented as having such genetics.