Question

When a male donkey is bred with a female horse, the offspring produced is a mule, which is healthy but sterile. Karyotyping experiments have revealed that horses and donkeys have different numbers of chromosomes, 64 for horses and 62 for donkeys. The mule, therefore, has 63 chromosomes. Knowing this, what is the best explanation for the inability of mules to produce offspring?

Answer 1

In an organism with even number of chromosomes, the chromosomes become half in number during gamete formation . For example: A horse has 64 chromosomes. Its gametes will have 32 chromosomes. The gamete formation process in mules will fail because of its odd number of chromosomes.

63 chromosomes would not be able to divide perfectly for gamete formation. This will happen because proper pairing of homologous chromosomes will not occur during meiosis I. Synapsis is the pairing of homologous chromosomes before they segregate and chromosomal crossover also occurs during this stage. In mules, some gametes will be formed with missing chromosomes and some will have extra chromosomes because synapsis formation will be abnormal. Hence, mules are not able to produce viable gametes and are sterile.