Step 1: Understanding the Concept:
This question is about the causes of allergies, as discussed in the prose piece 'Allergy'. An allergy is an exaggerated response by the immune system to substances that are normally harmless.
Step 2: Detailed Explanation:
The development of allergies is multifactorial, meaning it depends on several factors.
The text explains that these factors can be broadly categorized into two types:
1. Genetic predisposition: A person is more likely to develop allergies if their parents have allergies. This hereditary component is genetic.
2. Environmental factors: Exposure to allergens like pollen, dust mites, certain foods, etc., triggers the allergic reaction.
Therefore, both genetic and environmental risk factors contribute to the development of an allergy.
Step 3: Final Answer:
The sentence should be completed with the word genetic.