To help explain it you should first understand the fact that all genes come in PAIRS, one comes from the mother and one comes from the father. So when a horse is "homozygous" for a gene what you're saying is that BOTH genes that it got from it's parents are identical. Whether it be a recessive gene, or a dominant gene. For instance a red horse is homozygous for red (ee) which is a recessive gene. A horse that is heterzygous has two DIFFERENT genes...one black gene and one red gene (Ee). Then a horse that is Homozygous for BLACK has two black genes (EE).
In the case of a homozygous tobiano it means the horse has two tobiano genes (ToTo) as opposed to a regular (heterozygous) tobiano that has one tobiano gene and one non-tobiano gene (To to). A homozygous tobiano will pass one tobiano gene to each and every one of it's foals. Does this help?
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