Phenotypes and GenotypesIf you think about alleles and how they are expressed, you may realize that a dominant trait can be expressed in two ways: if an individual carries two copies of a dominant allele, or if it carries one copy of the dominant allele and one copy of the recessive allele. The only way to display the recessive trait is if two copies of the recessive allele are present. We use two terms in genetics to distinguish between what alleles an individual carries and what trait is expressed. Genotype refers to the exact alleles present, and phenotype is the trait that is observed. If the two alleles present are the same (either both dominant or both recessive), the condition is called homozygous. If two different alleles are present in the genotype (the dominant and the recessive), the condition is called heterozygous.
We are now ready to examine the inheritance of traits on a molecular level and to discuss some advanced topics in genetics. Be sure you understand the basics before you move on.