The scoop? There are two leading grey types: a fast and a slow. Fast greying horses will show signs of greying soon after birth and can be expected to turn fully white; fast greying horses are at the highest risk of developing melanoma. Slow greying horses may not show signs of greying until maturation, may not fully white out, and are at a lower risk of melanoma.
The following is an excerpt from the study below:
“The speed of greying varies considerably among Grey horses. Here we demonstrate the presence of two different Grey alleles, G2 carrying two tandem copies of the duplicated sequence and G3 carrying three. The latter is by far the most common allele, probably due to strong selection for the striking white phenotype. Our results reveal a remarkable dosage effect where the G3 allele is associated with fast greying and high incidence of melanoma whereas G2 is associated with slow greying and low incidence of melanoma.”
Direct link to published study:
https://www.nature.com/articles/s41467-024-51898-2
UC Davis article on study: