Influence of stochastic events on the phenotypic variation of common white leg markings in the Arabian horse: implications for various genetic disorders in humans.

One method of assessing the influence of stochastic events on phenotypic variation is to study morphological differences in paired limbs of the same individual. These limbs have identical genotypes and similar intra-uterine environments and are analogous to monozygotic twins. Common white leg markings have a multifactorial mode of inheritance in the Arabian horse. Asymmetry occurs frequently for these markings. Using computerized registration records obtained from the Arabian Horse Registry of America, Inc., the types of markings were quantified in the left foreleg and left hind leg of bay and chestnut horses when a specific marking occurred in (1) a paired right leg, (2) both right legs, and (3) both right legs and the other left leg. The variation in the markings in the left legs of these horses is evidence of the role of stochastic events during development. The relatively high frequency of a specific type of asymmetry, with one leg being completely pigmented, is of special biological interest and has implications for various human disorders that are characterized by reduced penetrance and variable expressivity.