Remember: an organism will have 2 alleles per trait; no more , no less. The paired alleles are called the "Genotype" for that trait. How that genotype is expressed or shows up in an organism is called the "Phenotype."

Those alleles could be either dominant, written with an upper-case letter or recessive, written with a lower case letter. If a dominant allele is present it will cover up or mask the recessive allele.

A genotype with two upper-case letter is homozygous dominant and the dominant trait will show. A genotype with two lower-case letters is homozygous recessive and the recessive trait will show. A genotype with one of each is heterozygous and the dominant trait will show. Recall that the prefix "homo-" means "the same" and "hetero-" means different.

Example: Tongue rolling (taco tongue) is recessive to non-tongue rolling. So we will asign a lower-case "t" to the recessive allele and an upper-case "T" to the dominant allele. A homozygous recessive genotype "tt" will have the phenotype of tongue rolling. A homozygous dominant genotype "TT" will have the phenotype of a non-roller and a heterozygous genotype "Tt" will also have the phenotype of a non-roller because that dominant allele will cover up the recessive one.  

Challenge: Go around to all of your family members and see if they are tongue rollers or not. Try to predict your parents' phenotypes.