What is the smallest part of DNA molecule that can be expressed as a trait

In the practice of science, this type of reasoning is used to develop explanations.

About how many years does it take for one cycle of surface water to become deep water and then surface water again in the oceans? 101001,000

Which of the following is a reason why early living organisms on earth could not have survived on the surface? 1. the lack of an ozone layer 2. the lack of oxygen in the atmosphere 3. the fact that these organisms were single-celled