Colchicine is a chemical used to “double” chromosome numbers.

The chemical interferes with spindle fiber formation. Consider a 2n plant cell. It undergoes a normal S phase prior to mitosis, leading to two sister chromatids per cell. The failure of spindle fibers to form at Anaphase leaves the replicated chromosomes in the same cell, resulting in a nucleus with twice the number of chromosomes. 

Since the two chromosomes are identical, the resulting cell contains two copies of the genome and is completely homozygous.

Plants derived from successful colchicine doubling are, therefore, completely homozygous.

Colchicine - the product