According to scientists at the Virginia Commonwealth University, genes contribute significantly more to a woman’s risk of depression than to a man’s. I register such information anxiously; my mum has suffered from crippling depression for more than 25 years. Her mother did, too.

As if that weren’t bad enough, a study at the University of Pittsburgh found that almost 50 per cent of people who develop recurring depression in early adulthood have a family member who suffers from a mood disorder. Recurring, by the way, means a person has had at least two episodes of serious depression in their lifetime. Since she first suffered at the age of 38, my mother has been lucky to get away with less than one big episode every couple of years. Turning 40 this year, the odds seem stacked against me. I am programmed — on auto-pilot — to self-destruct; science suggests that there is little I can do about it.

Except that I will not allow myself to believe what the scientists say; I have to convince myself that mental health cannot be influenced by DNA in the same way that the colour of my eyes and the shape of my nose have been. What choice do I have? I could not bear to imagine succumbing to the same demons that regularly steal into Mum’s life; the dread would overwhelm me.

I know what those demons look like, you see, I know what it feels like to have your life manipulated by them, if not overtaken completely. My mum’s sickness has been a part of my life for longer than it hasn’t. She first fell ill when I was 13. I remember because I made a note in my diary: “January 14, 1979: my mum has Depression.” (Depression with a capital D, for a proper noun, the name of a real illness, that’s what Dad said.) A few months later I wrote: “Mum is back to her old self. I hope she never gets Depressed again.” Well she did, and does; she has just recovered from an episode that dragged on for six months.

In the beginning, before I understood the nature of the illness, the method to Mum’s particular madness, I worried that she might die. Thankfully I didn’t know the numbers back then: suicide figures are twice that of road traffic deaths in the UK. I hated it when Mum got sick; I still do. But now I can rationalise the illness in a way I couldn’t when I was younger, when it frightened me that she didn’t want to get out of bed in the morning, couldn’t summon the energy to fix lunch, seemed permanently soggy with inexplicable, fearful woe. Depression is ethereal; if you can’t see what is wrong with a person, how can you fix them, empathise even? And how, worst of all, do you explain to people that your mother really can’t pull herself together when you don’t understand why yourself? But I do understand now; partly because I have to, but mostly because Mum’s allowed me to. The first time she was admitted to hospital she gently explained the basics of the electro- convulsive therapy she was undergoing. I was horrified. But Mum used language that let me know her treatment was, in the circumstance, quite normal. And that made it almost OK.

And it set a precedent. Mum’s illness, ironically, has become a part of her normality; it would be odd not to talk about it, and Mum has never balked at trying to describe the dynamics of depression. What does it feel like, I ask her. It’s cold, she says, like living behind glass: you can see life going on around you, but you can’t engage with it. Her approach to depression is energetically proactive (an anomaly when you consider the deadening apathy that accompanies each episode).

Apart from the myriad therapies she has tried to beat her enemy (drug therapy, psychotherapy, electro-convulsive therapy, cognitive behaviour therapy), her best weapon remains her determination to acquaint herself with its nature. She never stops trying to understand why depression floors her: she owns dozens of books on the subject and is constantly alert to new research and treatment options.

The knowledge she has gleaned does not mean that she keeps depression at bay, but it does, perhaps, make each episode less terrifying. Which is why I follow suit: if I understand how depression works, maybe I can outwit the science: see it coming and duck for cover. Perhaps such a notion is simplistic but, then again, the same scientists who suggest that depression is more genetically inherent in women concede that environmental factors outweigh genetic risk.

And, so far, this strategy has worked. Of course, there are times when I have felt down, and if this persists for more than a day I worry that I’m about to succumb. But I have never suffered from depression. I once asked a clinical psychologist why Mum’s illness had not had a damaging effect on me. “Your age when your mother was first sick,” she told me. Maternal depression has a profoundly negative effect on children up to the age of 6 apparently because it inhibits a mother’s emotional availability.

My mum was quite well until I was a teenager and by then I was just about old enough to understand that it wasn’t her fault that she got sick. Nor, more importantly, was it mine.

My eldest daughter is the age that I was when Mum first suffered. She knows what depression means. Not because I have tried to explain to her that the misfiring of neurotransmitters means that Mum’s psyche is regularly short-circuited and she’s plunged into darkness. But because sometimes Granny is so overwrought by life that it makes her sad.

My mum never knew that her mother suffered from depression until she herself became sick (no reason was ever forthcoming as to why my grandmother took to her bed from time to time). I cannot know that had my mum done so she might have avoided the illness herself, but I do believe that knowledge is empowering. Forewarned is forearmed. I have to believe that it might have made a difference.

One person in six suffers from depression. I wasn’t unlucky to grow up with a parent who did. I was lucky to be raised by a mother who was honest about the fact that she does.

Can you inherit depression

• Some people are more at risk of developing depression because of the genes passed down from their parents.
  
  
• This is known because identical twins share the same genes and, if one identical twin suffers depression, there is a 60 per cent chance the other one will, too.
  
  
• No single gene causes depression and a person’s environment plays an important part. But some combinations of genes may affect brain structure or hormone balance and our physical ability to deal with stress.
  
  
• For help, contact the Depression Alliance: 0845 1232320; www.depressionalliance.org