First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of genetics. It gives a broad overview of what genetics is about without going into details. This follows the rule of Topic Sentence. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone as the first sentence. For example, Paragraph 2 starts with 'These biological traits,' which refers back to something, so it cannot be independent. Paragraph 3 uses 'Dominant and recessive genes,' which also assumes prior knowledge. Paragraph 4 starts with 'However,' which is a conjunction and cannot be the first sentence.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about genetics. It introduces a new idea about environmental factors affecting genes, which concludes the topic. This follows the Conclusive Words rule.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the topic of biological traits. This follows the Logical Order rule, as it explains how traits are passed down.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it explains how the traits are expressed. This follows the Cause and Effect Relationship rule, as it shows the effect of genes on traits.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it adds more information about genetic expression. This follows the Contrast rule, as it shows that there are other factors besides genes.