3.16 Understand that genes exist in alternative forms called alleles which give rise to differences in inherited characteristics

Genes exist in alternative forms called alleles. These control the differences in inherited characteristics. 

Extra information:
Alleles can be dominant or recessive, homozygous or heterozygous. They are written as big or small letters. Eg: Aa or AA or aa. This is known as a genotype. The physical characteristic it codes for is called a phenotype.