Psychology 9-1 GCSE - 

3.1.4 - the influence of genes as an explanation, including strengths and weaknesses of each explanation:

a) Depression -

People are not born with depression, but they can often be born with a raised susceptibility for it, this often runs in families. Being carried down through genes. Studies suggest that variations in many genes, each with a small effect combine and increase the risk of developing depression. The genes thought to be associated with depression have diverse functions in the brain. Some of these genes may control the production, transport and activity of chemicals called neurotransmitters, which relay chemical signals that allow nerve cells (neurones) to communicate with each other. This can result in a natural decrease of chemicals such as serotonin or endorphins being transmitted making you more susceptible to depression.

b) Addiction - 

Like depression, you can be made extremely susceptible to addiction by being born with whats called an 'addictive personality'. Symptoms of having an addictive personality don't just include having trouble with self control, it can mean you, like with depression, aren't getting the appropriate amount of chemicals which leads you to looking for something to make up for that loss e.g alcohol, marijuana. This can also often make withdrawal symptoms worse if you have an addictive personality. While being born with an addictive personality does not mean you will inevitably get addicted to something, it means you have a higher risk factor and should be careful. Sometimes the only option for people with severe addictive personalities is abstinence.