Friends in Maine tell of opening a closet in a large summer house and finding two boxes. One box is labelled ‘String’; the other ‘String Too Short To Save’. That phrase echoes because my mind is stuffed with bits of string that I save for the day I will braid it into ropes of understanding. I’ve been fiddling with string like that for the past few weeks, ever since a family party to celebrate the 80th birthday of my husband’s much-loved aunt.

For most of the years I’ve known Anne, she’s lived on her own she and her husband, Donald, were divorced in 1959 and she’s been a model for how to be alone. She’s balanced the productive solitude required for her life as a scientist, with the undistracted, happy time spent with her family and friends. She’s lived a life of material modesty that’s caused wonder to us all, the only element of lavishness the holidays with her children, their spouses and her grandchildren: skiing each winter, summers in countries requiring complicated visas. These sojourns usually included her former husband and father of her three children and his wife, a tight-knit, brainy and congenial tribe that left their cousins envious and amazed.

A couple of years ago, after Donald was widowed, he and Anne bought a house together in north London that’s a 10-minute walk from their two daughters. The top floor is Anne’s, the first floor with its kitchen and sitting room, is the communal space, and the ground floor is Donald’s, including a study for his vast library. We all find this Life Comes Full Circle arrangement touching, not because it’s ‘civilised’, but because it is an affirmation of half a century of mutual respect, love, friendship and intellectual collaboration.

Anne McLaren and Donald Michie began their married life as young scientists researching the effect of the maternal environment on mice, seeking to untangle the mystery of Nature versus Nurture. Over the years, Donald moved into the field of computers and artificial intelligence, work inspired by his wartime job as a codebreaker at Bletchley. Anne stayed with her mice. In an early experiment, she cultured mouse embryos in a test tube and placed them in the uterus of a surrogate mother. Live baby mice and the miracle of IVF were born.

The notion that ‘in dreams begin responsibilities’ was felt acutely by Anne, and she combined her research in the laboratory with her demanding work on the Warnock Committee, laying the basis for the Human Fertilisation and Embryology Act. Although she described herself as an ethical ignoramus, she was calm and lucid on the ethics of embryo research and, more recently, embryonic stem cell research. Calm and modest, a fellow of the Royal Society (she was elected Foreign Secretary, its first female officer) and a Dame. Close friends and members of her family were always the last to know her news.

Until last Saturday. Within hours, we learned that Anne and Donald had died in a car accident on the M11, driving from Cambridge to London. All week long, we’ve struggled to be philosophical. They both would have hated the loss of energy and of enchantment, hated the hidden darkness of a broken old age. We tell ourselves that, as a geneticist, Anne always looked at life appreciatively, never dictatorially or sceptically. And reading the full-page obituaries, we remember Anne’s great legacy: she had the vision and the patience to turn string too short to save into the science which creates life.