What is "if the black horse is heterozygous?

If the black horse is heterozygous, it means that it carries two different versions of the gene that determines its coat color. In the case of a black horse, the two versions of the gene are likely to be for the black coat color.

Heterozygous black horses have a genotype of Bb, with one dominant allele (B) for black coat color and one recessive allele (b) for another coat color, such as bay or chestnut. Despite carrying both versions of the gene, the dominant black allele will be expressed phenotypically, resulting in a black coat color.

When breeding with another horse, there is a 50% chance that the heterozygous black horse will pass on the dominant black allele to its offspring, resulting in a black coat color. There is also a 50% chance that it will pass on the recessive allele, potentially resulting in a different coat color if the other parent also contributes a recessive allele.