Jacqueline and Susan Donachie were always close, sisters who looked strikingly similar; both were tall and willowy, striding out together among other, usually shorter, Glaswegian women. The fact that Jackie's eyes were her mother's blue and Susan's were her father's hazel would have been negligible had there not been a more insidious genetic distinction to set them apart.
They were pregnant together - Jackie, aged 30, with her first; Susan, aged 33, with her second - when they came across the fork in the road. Susan was induced early. The doctors said the baby had problems due to more than being premature, and that these came from the mother. They sprang from a disease that ran in families, so Jackie would have to be tested, too.
Jackie remembers standing in the corridor when she heard her name called. In that instant, she had a lurching sense that when she looked round she would see from the consultant's expression whether or not she and her unborn baby were also condemned. "That big smile," the consultant said to her, "and that firm handshake ... you don't look like someone with myotonic dystrophy."
Jackie's husband, Roddy, exploded. How could anyone make such a fateful diagnosis at a glance? In his frustration and fear, he grabbed Jackie's slender wrists and shouted, "Look at her hands, she's got really funny hands!" But the doctor was merely confirming the blood tests. Susan had it; Jackie did not.
There wasn't much space for relief. In that diagnosis everything had changed, not just for one of two sisters, but for half their extended family. There began a slow process, like ivy entwining round familial branches, of tracing a gene through relatives, to find out who had an inherited disease that affects only about one in 8,000 people in the UK, but that afflicts an average of 50% of a sufferer's children - and for which there is no cure. But the cruellest trick of myotonic dystrophy is that, generation by generation, it gets worse. Parents who are themselves succumbing have to watch as their children succumb earlier and faster.
This process of trans-generational deterioration is known by geneticists as "anticipation". So, although his symptoms had gone undetected, the gene had come through Jackie and Susan's hazel-eyed father. Working as a manager in factories that increasingly began to close down as Scottish industry declined, his health had similarly worsened until he had needed prolonged ventilator support to keep his lungs going after a heart bypass operation in his 50s. It was only later, when he was also finding it harder to walk, that previous muscle failure could retrospectively be ascribed to the disease.
Susan's symptoms were doomed to arrive earlier. In her 30s, her eyelids began to droop, her head tilted farther forward, and when her fingers gripped a hand-rail she would find that she couldn't release them. Sometimes described as the "can't be bothered" disease, myotonic dystrophy erodes muscle control throughout the body. And, in the predestinarian progress of an accelerating molecular equation, Susan's little daughter, Rhona, learned to walk only with splints.
For Jackie, gifted with the knowledge that none of her descendants would be affected, Susan became the source of an exploration into five generations of their Glaswegian relatives and Irish ancestry, navigated by a minute chromosomal clue. As an artist, she has worked with geneticists in three different continents to chart the history of how the mechanism of anticipation was finally proved in the 1990s, by observation of stretching repeats in a gene on the long arm of the 19th chromosome. And, in doing so, she has exposed something about the psychology of inheritance.
Her project, a collaboration with geneticist Darren G Monckton, is called Tomorrow Belongs To Me - a sorrowful testament to the difference in her own and her sister's fortunes. Jackie can now look at pictures of her family and see signs of the drooping eyes and slack jaws that indicate where the myotonic dystrophy gene has found its path and, in doing so, to recognise how it is not just disease that may be anticipated. The relative simplicity of an inherited gene is emblematic of more complex inheritances. "When you have children," she says, "you realise how much they're stuck with elements of your personality, or your family's."
Knowledge of fate is a weighty commodity. Susan remains glad that she did not know before she became pregnant, because she feels blessed with children she would not otherwise have had. For Jackie, now 38 and the mother of three boys, the need to have children became greater the more she knew. "I wanted them to have strength in each other when they see what's going on with their cousins," she says. "Because they're close to their cousins, and things are going to get much darker for my family. There will be funerals."
