Joane Karohl, PhD, Biochemistry/Biophysics
I’m pretty sure that most everyone who owns dairy goats will agree that disbudding kids is one of those onerous tasks that we all wish we didn’t have to do. That may be one reason that the siren song of the entirely polled (hornless) herd is so difficult to escape—oh, the joy of never having to disbud another kid! Sadly, it is not something that can ever be a reality.
The Polled Intersex (PIS) locus has been the subject of considerable scientific study. Both the wild type and mutated sequences have been determined, and it is known that the polled trait is due to a large deletion in an area of goat chromosome 1q43. The mutation is homologous to one in a similar region in humans responsible for a condition called blepharophimosis-ptosis-epicanthus inversus syndrome. It is also now known that the mutations disrupt the transcription of at least 3 genes, including the FOXL2 gene, which encodes a transcription factor required for correct ovarian differentiation.
The polled trait in goats is an autosomal dominant trait in both sexes; in other words, one copy of the gene results in the polled phenotype. However, if a goat is homozygous for (having 2 copies of) the polled allele, the result is either a pseudohermaphrodite (in XX, or what would normally be female, goats) and generally a reduction in fertility in XY, or male goats. Homozygous males usually suffer from a condition called sperm granulomas at a young age, which will generally render them infertile. The absence of correct production of the protein encoded by FOXL2 makes it impossible for a female goat to properly develop a functional reproductive system, and results in various degrees of masculinization of the fetus during development. A goat that is homozygous for the PIS allele cannot produce transcripts from the FOXL2 gene, and cannot develop as a normal female.
The question of whether it is a good idea to breed polled to polled comes up on social media fairly frequently. Inevitably, one or more people will volunteer that they have bred polled to polled with no ill effects, even multiple times. I usually respond that it is pretty much a matter of the devil being in the statistics. Assuming that both polled parents are heterozygous (having only 1 copy each of the polled allele), statistically 50% of their offspring will be polled, and also carry a copy of the normal horned allele; 25% will be horned, and 25% will be homozygous for the polled trait. Of these, half will be female and be pseudohermaphrodites, and the other half will be XY males. So right away we see that the intersex trait only occurs on average in 12.5% of births, a rate that can easily result in the luck of the draw allowing multiple litters to be born with no intersex kids. In addition, to complicate things even further, the XX PIS goat can have a significant degree of female to male sex reversal, which can result in misidentifying PIS animals as males at birth. I usually point out to people that as long as the reality of how the PIS allele works is understood, the choice of whether to breed polled to polled or not is fairly easy. When breeding polled to polled over time you can expect 50% fertile polled offspring, 25% fertile horned offspring, and 25% non-breeding animals. When breeding polled to horned, you get 50% polled, 50% horned, and 100% healthy fertile offspring. To me that’s an easy choice. Of course, the math would be different if one of the parents were actually both homozygous for polled AND fertile—as might be possible, for example, if the polled buck were still young and fertile even though homozygous polled. I have challenged many individuals over the years to present to me a fertile doe who might be presumed to be homozygous polled, as shown via many breedings with horned bucks that produce nothing but polled offspring. I have not ever been presented with such a creature, and honestly, I don’t believe it is possible.
There is a lot of scientific literature available on this topic, but much of it is behind academic journal paywalls. One such that is not and is pretty readable (and includes some nice illustrations) is available here: