Alleles probability calculation

Alleles probability calculation.  Explain: If a trait is determined by two alleles, the sum of p and q is 1. Why is this true? [Type your answer here.] D= d= D= d= 3. Calculate: Fill in the current p and q values next to the D and d alleles around the Punnett square at the right. Then calculate the probability of each offspring genotype. Write these values into the individual squares. For example, the upper left box in the square represents the probability of a DD offspring. The probability of inheriting a D allele from each parent is p, so the probability of DD is p?. What is the sum of the probabilities in the four squares