Explain that genes on chromosomes come in different versions called alleles, and distinguish genotype (the alleles an organism has) from phenotype (the trait that results).
Traits are characteristics passed from parents to offspring, and the instructions for them are carried by genes: sections of DNA located on chromosomes. Most genes come in more than one version, called alleles. A pea plant, for example, has a gene for flower color with two possible alleles: one for purple, one for white. Every individual gets one allele for each gene from each parent, so it ends up with two alleles per gene, one inherited from its mother, one from its father.
Some alleles are dominant, meaning their trait shows up even if only one copy is present. Others are recessive, meaning their trait only shows up if both copies present are the recessive version. Scientists call the exact pair of alleles an organism carries its genotype, and the physical trait that actually results its phenotype. Two individuals can share the same phenotype while having different genotypes underneath -- which turns out to matter a great deal when predicting what their offspring might look like.
Mastery looks like: They can convert between genotype and phenotype for a given dominant/recessive trait and explain why phenotype alone does not reveal genotype for certain.
Common stumbles: Assuming a dominant allele is always "stronger" or somehow better rather than just more likely to show up in the phenotype; forgetting a recessive allele can be silently carried without appearing.