W20/W20 and Tobiano TOGETHER

Ever hear someone say that a horse can be W20/W20 OR W20/N, TO/N (Tobiano), but never homozygous for one AND heterozygous for the other? Recent discoveries made by our team of researchers at EtalonDx have debunked this myth and proved that it is possible for horses to have W20/W20 AND Tobiano! Due to a genetic phenomenon known as linkage it is rare, but it does happen. Amazing, right? Meet this adorable pair - Dam is Midnight Diamond Babe and foal is Awesome Midnight Snow, owned by Flo Belliere. Both horses have W20/W20 AND Tobiano.

Are you wondering how this is possible? W20 is within the KIT gene of the equine genome. The sequence change responsible for Tobiano is located just outside of KIT about 100kb downstream. Tobiano is a different kind of variation called an "inversion" whereby a section of the genome has been completely "flipped" and reinserted into the chromosome. Once in a great while, a chromosome carrying that inverted region will swap in a portion of the KIT region that contains a W20 mutation from another ECA3 by a process known as recombination. Therefore, a horse with W20/W20 and Tobiano can pass either the single, "normal" W20 alone or the linked combination of W20/Tobiano to its offspring as one unit.

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