Ectopic testes in the Norway rat

Animals with ectopic testes, some unilateral (on either the right or left side) and some bilateral, have appeared in a colony of King-Holtzman rats. This heritable defect has been given the genetic symbol "ect." Females carrying the gene are unaffected by it. When such females are bred to apparently normal males, they produce no ect offspring. Since 25% of their F2 generation carry the gene, it is assumed it is autosomal recessive. Studies involving selective breeding for males with ectopic testes indicate that some males have genomes resistant to the expression of the ect gene and that the apparent thresholds essential to left side, right side, and bilateral expression appear to be a secondary control of the multifactorial type.