Brown hair (b) is dominant to blond hair (b). there are 150 brown haired people in a population of 200.
a. what is the frequency of heterozygotes?
b. what is the frequency of homozygous dominant?
c. what is the frequency of the b allele?

a. What is the frequency of heterozygotes? 50%, heterozygotes are the ones that have one of each alelle (B and b). They will have brown hair as a phenotype.

b. What is the frequency of homozygous dominant? 25%, homozygotes are the ones that have the same allele twice (BB). They will also have brown hair.

c. What is the frequency of the b allele? 75%, compound of 50% brown hair and 25% blond hair.

150 represents 75% of 200. To achieve this percentage, two heterozygotes have to be cross:

                                          Bb × Bb  

                                  ↓       ↓       ↓      ↓

                                 BB     Bb    bB    bb

Because brown hair is dominant, each pair of alleles that has a B will have brown hair as a phenotype.

The pair of alleles that has 2b is the only one that will have blond hair (in this case 25%).  

the reason the two sea urchins don't compete with each other for food, resources, etc. is because the urchins have adapted to the environment but they have also adapted to each other. this is most likely because the genes of the sea urchins.