In a recently published paper out of the Netherlands, the research group set out to estimate the genetic parameters of cryptorchidism and testis size in Friesian colts. From the years of 2009-2012, data was collected by a veterinarian during inspections on Friesian colts from 1-7 months of age. Data included age of the colt in months, location, month of inspection, unilateral or bilateral cases, right side vs. left side descended in the unilateral cases and size of the testis.
Their results were reported as such:
most affected colts were unilateral (only one testicle was descended).
the right testicle was most often retained in the unilateral cases.
at the age of the investigated Friesian colts, cryptorchidism genetically coincides with smaller testicular size.
the estimated prevalence of cryptorchidism in Friesian colts was 14.2%.
heritability of cryptorchidism (absent/present) was 0.13 +/- 0.06.
genetically, cryptorchidism coincides with smaller testis size.
the development of the left and right testis might differ, which could be hereditary.
stallions should not be inspected for cryptorchidism at a very young age.
In their conclusion, they stated that "more precise phenotyping, like recording position and size (and side) of the retained testis and age of the stallion, might contribute additionally to disentangling the genetic background of equine cryptorchidism and identifying the gene(s) affecting this disorder. For now, the continuation of the data recording as described in our study will enable the studbook to estimate breeding values and thereby select against cryptorchidism."
Scientific Article: Anouk Schurink, Adrianne de Jong, Hans R. de Nooij, Ids Hellinga, Bart J. Ducro. Genetic parameters of cryptorchidism and testis size in Friesian colts. Livestock Science Aug. 2016, vol. 190: 136-140, doi: 10:1016/j.livsci.2016.06.012.