Please help I don't understand ​

In many humans, exposing the skin to sunlight over prolonged periods of time results in the production of more pigment by the skin cells (tanning). this change in skin color provides evidence that -

What is the one part of the nucleotide  that differs among the other different nucle  tides?

How would you describe an allele that be expressed and determines an organisms appearance? a.uncapitalized b.responsive c.recessive d.dominant