Piebald rats and selection : An experimental test of the effectiveness of selection and of the theory of gametic purity in Mendelian crosses

This work reports a painstaking series of breeding experiments carried out over six years, in which more than twenty‑five thousand rats were raised and examined for their coat patterns. By focusing on the piebald “hooded” trait, the authors test whether the genes transmitted through gametes remain unchanged—or “pure”—after crossing with other stock, a question that lay at the heart of early Mendelian debate. Detailed records of “plus” and “minus” selection lines, as well as crosses with wild and black “Irish” rats, reveal how the visible phenotype shifts even when the underlying genotype is thought to stay constant.

The authors also explore how selective breeding without hybridisation influences the population, juxtaposing their findings against contemporary theories from De Vries, Johannsen, Jennings and Pearl. Their discussion weighs the evidence for genotype‑phenotype separation and considers the broader implications for the emerging field of genetics. Listeners will gain insight into the experimental rigor and intellectual climate that shaped modern evolutionary biology.