What is represented by a pair of lowercase letters, such as tt, in a Punnett square?
Lower case letters are recessive alleles and upper case letters are dominant alleles.
So, "tt" would mean that both alleles are recessive. In comparison, "Tt" would mean that one allele is dominant and one is recessive.
In most cases, you need two recessive alleles for the recessive trait to be expressed whereas only one dominant allele needs to be present for the dominant trait to be expressed.
In the image below, "Y" is dominant and "y" is recessive: