The bread wheat genome sequence and gene expression analyses reveal that this hexaploid exhibits a similar gene order on each of its three related (homoeologous) chromosomes, and mostly balanced meiotic expression between the three genes on the homoeologues.
How is this major human staple so stable, when these homoeologues could pair and exchange (crossover) during meiosis leading to sterility?
In late 1950s, Riley and Sears reported that the deletion of the 5B chromosome leads to some crossover between wheat homoeologues, and between wheat homoeologues and wild relative chromosomes. This suggested that the 5B chromosome carried a locus (later termed Ph1) which stabilised wheat meiosis by controlling pairing and crossover between the homoeologues.
Early attempts by Sears to define Ph1 revealed that it lay in a 70Mb deletion of wheat chromosome 5B. This Sears mutant has been used for the last 40 years in breeding to introgress useful traits of agronomic importance from wild relatives into wheat. However, the mutant now exhibits extensive rearrangements making it less useful for breeding and confirms that bread wheat, a major human staple would not exist in the long term without the presence of Ph1.
Therefore, Ph1 is defined as a deletion induced phenotype.