A dominant allele is always expressed, even if only one copy is present. A recessive allele is only expressed if two copies are present (therefore no dominant allele present). • If the two alleles present are the same the organism is homozygous for that trait, but if the alleles are different they are heterozygous. • Most characteristics are a result of multiple genes interacting, rather than a single gene. • Students should be able to understand the concept of probability in predicting the results of a single gene cross, but recall that most phenotype features are the result of multiple genes rather than single gene inheritance. • Students should be able to complete a Punnett square diagram and extract and interpret information from genetic crosses and family trees.