Amphidiploidy (also known as allotetraploidy) is a specific type of polyploidy where a new species or variety is formed by combining the entire chromosome sets of two different ancestral species, followed by a doubling of this combined set. This results in an organism that is essentially a diploid for each of the two original parental genomes.

The process typically involves:

1. Hybridization: Two distinct diploid species (e.g., Species A with 2nA chromosomes and Species B with 2nB chromosomes) cross to form an F1 hybrid. This F1 hybrid is often sterile because the homologous chromosomes from different species cannot pair properly during meiosis.

2. Chromosome Doubling: The chromosome number of this sterile F1 hybrid is then spontaneously or artificially doubled (often using colchicine). This creates an amphidiploid with 2nA + 2nB chromosomes.

The doubling restores fertility because each chromosome now has a homologous partner from the same ancestral species, allowing normal pairing and segregation during meiosis. Amphidiploidy is a significant evolutionary mechanism for the origin of new plant species (e.g., Triticale from wheat and rye, or Brassica napus from B. rapa and B. oleracea). It combines desirable traits from both parent species.