If the same base pairs are used to code for all organisms , why such variation among organisms

Idon’t know the answer and i’ve been stuck on it for a while now skskskskks

Guinea pig coat color is determined by a single gene. the allele for black coat color is dominant to brown. in a cross between twoblack-haired guinea pigs, 20 offspring are born. if both parents were heterozygous, probability would predict that approximately howmany of the 20 offspring would have brown hair?

Why did adult crab numbers decline in he ecosystem, even though sea stars were still available?