Question

A donkey and a horse are able to mate and produce an offspring known as a mule. The resultant mule possesses 63 chromosomes in its cells due to the donkey parent having 62 chromosomes and the horse parent having 64. The mule is unable to mate with other mules to produce its own offspring. Which of the following reproductive isolating mechanisms is operating in this situation?
a. Hybrid Inviability
b. Hybrid Mortality
c. Behavioral Isolation
d. Hybrid Sterility
e. Temporal Isolation

Answer 1

Hint: Meiosis is absent in such a reproductive isolating mechanism. For example, a mule is a hybrid between a female horse and a male donkey, could develop into an adult but fails to develop functional gametes, therefore, is sterile.

Complete answer:
Hybrid zygotes at times develop into adults but the adults fall short to develop functional gametes and are sterile.
It refers to the offspring of a successful primary cross; though the secondary cross would fail. It could be due to the consequential condition wherein the offspring of the primary cross contains multiple sets of chromosomes, which in turn could be due to malfunction of chromosome separation (disjunction) during meiosis. For example, a mule is a hybrid between a female horse and a male donkey, could develop into an adult but fails to develop functional gametes, therefore, is sterile.

Hence, Option (D) “Hybrid Sterility”.

Additional Information:
Having extra chromosomes often leads to real problems. But the mule’s overall life is normal.
Mitosis is like the initial step of meiosis. The chromosomes make copies of themselves. But in place of getting matched, they just sort into two new cells. No matching desires to happen. And a single horse chromosome is all right.
The extra genes must not be that extreme challenge for the mule. In simpler words, the extra genes on the horse chromosome do not cause problems for the daily life of a mule. So mules are sterile as horse and donkey chromosomes are just too different. Yet they are alive because horse and donkey chromosomes are alike enough to mate.

Note: Hinnies and other normally sterile interspecific hybrids also cannot produce viable gametes because the extra chromosome cannot make a homologous pair at meiosis, meiosis is disrupted and viable sperm and eggs are not formed. Though, fertility in female mules has been observed seldom with a donkey as the father.