Punnett squares are used by geneticists to determine the probability of different offspring genotypes. In the one shown below, what letter(s) belong in the lower right box?

Crossing is the breeding of parents to produce an offspring. Given that f is a recessive allele of smooth leaves while F is the dominant allele of fuzzy leaves. Therefore, where there is F crossing with f, the offspring will have fuzzy leaves over smooth one.
Therefore, for an offspring to have smooth leaves, ff and ff must cross to produce an offspring with smooth leaves.

From the given Punnett square, we can cross the genotypes of the two parents and established the phenotypes the offspring will have. The Punnett square is filled as:

The requested answer is aa.

We use the Punnet square to find the population of the resulting offspring from the crossing of a plant heterozygous for red flowers with a plant with white flowers.

Let R be the dominant allele for red while r is the recessive allele color for red. Therefore, the phenotype for parent 1 which is heterozygous for red flower will be Rr and the second parent with white flowers will be rr.

We cross the two phenotypes in a Punnet square as follows

Thus, there are 2 heterozygous offspring with red flowers and 2 offspring with white flowers.