In wheat and rice the genetic purity of a variety can deteriorates due to

In wheat and rice, the genetic purity of a variety can deteriorate due to Mechanical contamination with seeds of other varieties.
The genetic purity during multiplication stages is prone to contamination due to the presence of pollen shedders as well as physical admixtures during processing.
The deterioration sets in due to natural crossing with undesirable types, diseased plants or off types.