The transmission of a hereditary trait from one generation to another can be illustrated by making a crossover diagram. The trait is denoted by a letter selected according to the recessive allele. Alleles that confer a trait are marked with either uppercase or lowercase letters.

The dominant allele is marked with an upper-case letter, whereas the recessive allele is marked with a lower-case letter. Thus, for example, the dominant gene that causes curly hair could be labeled A. In this case, the curly-haired individual may inherit the alleles AA or Aa. A straight-haired individual, on the other hand, has inherited the alleles aa.