The SLC6A4 gene is located on chromosome 17. We inherit two copies of this gene – one from each parent. Inheriting one or two copies of the short (“grouchy”) form is linked to an increased risk of depression.

• People with two copies of the short, “grouchy” form are more likely to be unhappy. They will pass this form of the SLC6A4 gene to all of their children.
• People with two copies of the long form, have a decreased risk of unhappiness and depression.
• Heterozygotes have one copy of the long form and one copy of the short form. Having one copy of the “grouchy” gene also affects the level of happiness. These people have a 50% chance of passing the grouchy gene to their children.