Final answer:
The ancestry of the wheat stem rust strain Ug99 can be traced back to the genetics of both the fungus and the wheat plant. The plant's resistance involves the ability to detect and respond to the fungus. However, Ug99 has evolved to overcome the plant's defenses.
Step-by-step explanation:
The ancestry of the wheat stem rust strain Ug99 can be traced back to the genetics of both the fungus and the wheat plant. Resistance to the disease involves the ability of the plant to recognize invasion by the fungus and respond to it. The plant produces its own proteins that detect the injected proteins of wheat stem rust and trigger a physiological response to prevent the fungus from growing and causing disease. However, the Ug99 strain of wheat stem rust is able to cause disease on previously resistant wheat plants, indicating that it has evolved to evade the plant's defenses.