According to the hardy-weinberg principle, the p stands for:

a. the most common allele

b. the recessive allele

c. the dominant allele

d. the least-favored allele

P stands for: c. the dominant allele

q therefore stands for the recessive allele. The frequency of the homozygous dominant genotype (i. e. gene T is TT), so p*p, which = p^2

The frequency of the homozygous recessive genotype tt is q*q, which = q^2. To find the heterozygous frequency Tt, we must consider that (p+1) = 1, since these are the only 2 alleles. And if we square those terms, that's how we determine the frequencies. (p+q) ^2 = 1^2

--> p^2 + 2pq + q^2 = 1, so the middle term (2pq) is our heterozygous frequency.