(209) 477-1536, J. Sharon Batteate ( firstname.lastname@example.org), PO Box 8535, Stockton, CA 95208 USA.
A distinction must be made between an animal's phenotype (outward appearance) and genotype (actual genetic composition). A similar outward appearance between animals can often be the result of a variety of unrelated factors. For example, in Pinto/ Paint horses, all are outwardly white spotted (phenotype). However, the various white spotting patterns are now known to result from several different unrelated genes (genotypes). Since the genes are located at different loci, it is only coincidental that the genes produce a similar white-spotted outward appearance. Occasionally, spotting even occurs due to a coat developmental abnormality, a mosaic or chimera condition, or a displaced pigment patch, and is thus not the result of any genes at all.
Some phenotypical (outwardly appearing) brindle patterns can result from mosaicism or a chimera condition. Since a specific gene is not involved in producing a mosaic or chimera pattern, these types of brindle patterns usually do not reproduce themselves, unless the original chimera condition can be duplicated. A number of Brindle horses have never reproduced their brindle pattern, pointing to a possible chimera origin for some brindle patterns. For a discussion of chimera see the article on Catch-A-Bird in the stallion section. On the other hand, there is evidence for an inheritable gene responsible for producing some of the other phenotypical brindle patterns based on the production records of some brindle horses.
Below is a summary of the two family lines I have been working with since 1990. Each family line started with a single mare. I have designated the brindle pattern from Brenda Batty Atty's line as Type 1, and I have designated the brindle pattern from Im A Star Moon Bar's line as Type 2.
Even though both family lines are outwardly brindled in appearance (phenotypically brindle), I believe they are different genetically. Foals from Brenda's line are born with the pattern and exhibit the pattern year-round. Foals from Star's line are generally not born with the pattern, but tend to develop it later, and are only seasonally brindle. In addition, one of Star's colts, even though not obviously brindled himself, has passed the pattern when breed to non-brindled mares, indicating it may be possible for an animal to be a carrier of brindle without showing obvious signs of brindle striping. This is also seen in some of the paint/pinto patterns, when a minimally marked "carrier" produces "crop-outs."
I am now in the process of crossing between the two family lines. Both family lines have shown an approximate 50% inheritability of their type of brindle pattern from crosses on non-brindles, and an even higher percentage on crosses between the two family lines. Brenda (type 1) has had 100% brindle foals when bred to Star's colt (type 2). This pattern of inheritability indicates their patterns are likely to be the result of a gene, rather than a chimera origin, which can also cause an outwardly brindled appearance.
I have now added my foals from 2003 that should provide more insights into the brindle pattern. There is also an article on each mare with more details than in the summary table below. Note that some names appear on both sides as a result of the crossing between the two family lines. Duplicate names appear in Purple. Click on Names in Blue for a pedigree and additional information.
Family Line (Type 1) Patterns
Family Line (Type 2) Patterns
Im A Star Moon Bar (source) Article