The Alternate may map their environment with sound.
Echolocation (also called Bisonar) is the biological ability of a DNA Alternate to determine the distance and direction of objects using sound. Echolocation works like active sonar, using sounds made by a the Alternate. Ranging is done by measuring the time delay between the Alternate's own sound emission and any echoes that return from the environment. Echolocating animals have two ears positioned slightly apart, facing forward. Somehow, Alternates who have this ability compensate.
There are two separate ways to accomplish this for Alternates. The first way is for the Alternate to emit a certain frequency that acts as their range-finder. This requires an active use the ability, meaning it's not always active. The second possible way is that the Alternate is capable of taking existing sounds and using them as a range. If this is the case then sudden loud noises could short out or distort the ability. Those with the later have a slightly enhanced sense of hearing. It is not as good as those with a dedicated ability, but certainly better than the normal human.
In either case with echolocation, the Alternate can see not only where they are going in an area void of light, but also how big another object is, what it is, and other features. However the drawbacks change depending on which form of the ability is manifested.