Mules are usually sterile because their horse and donkey chromosomes cannot pair and segregate normally during meiosis. This prevents the production of balanced, functional gametes and acts as a postzygotic reproductive barrier between the parent species.
The mechanism
A mule is a hybrid produced by mating a female horse with a male donkey. Horses have 64 chromosomes in diploid cells, whereas donkeys have 62.
| Organism | Diploid chromosome number |
|---|---|
| Horse | 64 |
| Donkey | 62 |
| Mule | 63 |
The horse contributes a haploid gamete containing 32 chromosomes, while the donkey contributes one containing 31. The resulting mule therefore has 63 chromosomes.
Before meiosis I, homologous chromosomes normally pair to form bivalents. Each chromosome must have a sufficiently similar homolog so that the pair can orient and segregate correctly. In a mule, the horse-derived and donkey-derived chromosomes differ in number and structure, so pairing is incomplete or irregular.
Consequently, chromosomes segregate unevenly during meiosis. Most resulting gametes contain missing or extra chromosomes and are therefore aneuploid and non-functional. The mule can develop into a healthy adult because ordinary growth uses mitosis, which does not require homologous chromosomes to pair. However, successful sexual reproduction requires meiosis, so fertility is disrupted.
A common misconception is that mules are sterile simply because they have an odd chromosome number. The odd total contributes to the problem, but the more precise explanation is the failure of sufficiently homologous horse and donkey chromosomes to pair and segregate normally. Rare female mules have produced offspring, so sterility is usual rather than absolute.
Exam technique
This is AHL content in A4.1 Evolution and speciation. For an “explain” question, link the full sequence: different parental chromosome numbers → hybrid has 63 chromosomes → disrupted homologous pairing and segregation in meiosis → unbalanced gametes → sterility. Then identify hybrid sterility as a postzygotic barrier that prevents gene flow between species.
