Sometimes solid-colored horses throw cropouts because some spotting patterns are recessive genes that are not necessarily expressed unless the relevant allele is inherited from both parents.
The term originally came about because excess white in certain breeds was considered an undesirable trait and such foals were deemed ineligible for registration, making them grade horses of little economic value.
[1] In recent years, as DNA testing has become available to verify parentage, breed registries that once excluded cropouts are now accepting them.
[4] Cropouts also occur when two solid-colored Quarter Horses produce a foal with Appaloosa coloring.
[2] In the case of most pinto and some Paint cropouts, another cause is the sabino gene, which can exhibit its presence with anything from tall white stockings or bold face markings, known as "high white," to significant spotting patterns.