A horse and a donkey can breed to produce mule which is an infertile animal. The infertility is because horse and donkey belong to different

A mule is the hybrid offspring of a male donkey (Equus asinus) and a female horse (Equus caballus) [1]. While both animals belong to the same genus, Equus, they are classified as different species. The biological species concept defines a species as a group of organisms that can interbreed to produce fertile offspring. Horses and donkeys have different chromosome counts; horses have 64 chromosomes (32 pairs) and donkeys have 62 (31 pairs). Consequently, a mule inherits 63 chromosomes. This odd number prevents successful meiosis, as the chromosomes cannot pair up correctly to form viable gametes, leading to hybrid sterility [2]. Therefore, the infertility of the mule is a direct consequence of the parents belonging to different species.

Sources

1. [1] History , class XI (Tamilnadu state board 2024 ed.) > Chapter 1: Early India: From the Beginnings to the Indus Civilisation > Hominin and Animal Fossils > p. 3
2. [2] https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/mule