Why do we calculate calories per gram instead of just calories?

Because an amount of the said substance emits a specific amount of heat.

In food usually calories are measured per 100 grams.

For biomolecules calories are measured per 1g (e.g. 1g glucose=4.1kcal)

dna is made up of six smaller molecules -- a five carbon sugar called deoxyribose, a phosphate molecule and four different nitrogenous bases (adenine, thymine, cytosine and guanine).

answer; a male inherits only one allele of the x-linked gene controlling hair color;