The Haldane Rule

The Haldane Rule states that when two different species or populations mate and produce hybrid offspring, the first generation of hybrids is often healthier and more viable in males (or the sex that has two of the same sex chromosomes, depending on the species) than in females. This pattern is observed in many animals and helps scientists understand how species stay separate and why some hybrids are less successful. Essentially, it highlights that hybrid problems often disproportionately affect one sex, usually the one with two identical sex chromosomes, revealing underlying genetic incompatibilities.