During meiotic division involved in the formation of the gametes in organisms. The meiotic division reduces the number of chromosomes by half during gamete formation.
In the given question, Horse contains 32 pairs of chromosomes or 64 chromosomes and the donkey contains 31 pairs of chromosomes or 62 chromosomes.
The gametes of a horse through meiotic division will contain 32 chromosomes and gametes of a donkey through meiotic division will contain 31 chromosomes.
When these gametes will fuse then zygote will be formed with 32+ 31 = 63 chromosomes. Since the mule contains 63 chromosomes, therefore they are sterile organisms.
