Meaning: 'Hereditary' refers to characteristics or traits that are passed down from parents to their children, indicating that DNA carries this genetic information.

Wrong options:

• bone: The word 'bone' refers to a hard structure in the body and does not relate to the concept of genetic material, making it contextually irrelevant.
• crucial: While 'crucial' means essential, it does not specify the nature of the material as genetic, thus failing to convey the intended meaning of DNA as a carrier of hereditary information.
• circular: The term 'circular' describes shape and does not pertain to the genetic context of DNA, making it an inappropriate choice for this blank.

Correct answer: down

Explanation: The word 'down' is the correct choice as it indicates the direction of genetic material transmission from parents to offspring, fitting grammatically and contextually within the sentence structure.

Meaning: 'Down' in this context means that the hereditary material is passed from one generation to the next, implying a downward flow of genetic information.

Wrong options:

• up: The word 'up' suggests an upward direction, which contradicts the natural process of inheritance where traits are passed from parents to children, making it contextually incorrect.
• then: 'Then' implies a sequence of events rather than a direction of transmission, which does not fit the context of passing down genetic material.
• form: The term 'form' does not convey the idea of transmission and is unrelated to the context of heredity, making it an unsuitable option.

Correct answer: guiding

Explanation: The word 'guiding' fits grammatically and contextually as it describes the role of DNA in directing development, aligning with the overall theme of DNA's influence on biological processes.

Meaning: 'Guiding' means to lead or direct, indicating that DNA plays a crucial role in steering the development of an organism from a zygote to an adult.

Wrong options:

• installing: The term 'installing' implies setting something up, which does not accurately describe the role of DNA in development, making it contextually inappropriate.
• exploring: 'Exploring' suggests investigation or discovery, which does not align with the directive function of DNA in development, thus making it an incorrect choice.
• writing: While 'writing' could imply creating, it does not capture the essence of DNA's role in directing development, making it less suitable than 'guiding'.

Correct answer: thus

Explanation: The word 'thus' serves as a logical connector, indicating a conclusion or result, which fits grammatically and contextually to show the relationship between survival and reproduction.

Meaning: 'Thus' means 'as a result' or 'therefore,' linking the idea of well-constructed bodies leading to successful reproduction.

Wrong options:

• able: The word 'able' does not function as a connector and fails to convey the causal relationship intended in the sentence, making it contextually irrelevant.
• easy: 'Easy' describes simplicity and does not logically connect the concepts of survival and reproduction, rendering it an unsuitable choice.
• so: While 'so' can indicate a result, it is less formal than 'thus' and does not fit the academic tone of the passage as effectively.