Question

Why mule is sterile in nature?

Answer 1

They are sterile because they can't make sperm or eggs. They have trouble making sperm or eggs because their chromosomes don't match up well. And, to a lesser extent, because of their chromosome number.

Answer 2

Mule is sterile in nature because it cannot produce male or female gametes (sperm or eggs).  

•   Mule is a hybrid organism that is made between a horse which has "64 chromosomes" and a donkey that has 62 chromosomes.  
•   This ends up in a mule which results in "63 chromosomes", this odd number of chromosomes ends up in a chromosomal defect ( the chromosomes cannot divide equally ) in the mule makes it sterile.