An organism that is heterozygous for a trait has two different alleles for that single trait.

For example, the gene for seed shape in pea plants exists in two forms. One form or allele for round seed shape (R) and the other for wrinkled seed shape (r).

A heterozygous plant would contain the following alleles for seed shape: (Rr). When the alleles are heterozygous, one is dominant and the other is recessive.

Additional information:

• Genetics Glossary
  
• Chromosomes and Gender