3.19 understand how genes exist in alternative forms called alleles which give rise to differences in inherited characteristics

Alleles are alternative forms of genes (sections of DNA coding for different proteins) that code for different variations of characteristics.

Genes come in a variety of forms: the gene that codes for hair, for example, can come in different colours. These different forms are different alleles, having these differences is where you vary in inherited characteristics.
*if there was only one allele for hair everyone would have the same hair!