Take a classic monohybrid cross problem based on Mendel's work. Consider that spherical seeds are dominant to dented seeds in the genetics of pea plants. Use a capital S to represent spherical seeds and a lowercase s to represent dented seeds. You must predict what fraction of the plant's offspring should have spherical seeds if they are crossed.
Assume each parent plant is heterozygous for an allele, or DNA sequence, determining seed shape. That means each parent plant has the presence of the dominant allele, S, and the recessive allele, s. Use the symbol "Ss" to represent heterozygous plants.
Consider that the presence of the dominant allele, S, results in spherical seeds in the offspring. Only homozygous plants with both recessive alleles have dented seeds. Use the symbol "ss" to represent those plants. Note that a plant with both dominant alleles, symbolized by "SS," is also homozygous.
Set up a Punnet square to solve the problem. A Punnet square is a diagram used in biology to project the results of monohybrid crosses. A Punnet square looks like a square divided equally into four smaller squares.
Write the alleles for the first parent vertically on the left side of the Punnet square. Label them S1 and s1. Write the alleles for the second parent horizontally on top of the Punnet square. Label them S2 and s2.
Fill in the four squares of the Punnet square by pairing the symbols for the alleles you have written. The square in the upper left will read S1S2, meaning it contains the dominant allele. The squares in the upper right and lower left will read S1s2 and S2s1, meaning they both contain the dominant allele. The square in the lower right will read s1s2, meaning it contains only recessive alleles.
Calculate that only one of the four results, or 25 percent, will yield dented seeds. For your solution to the problem, predict that the results of the monohybrid cross will be 75 percent spherical seeds and 25 percent dented seeds.