Anne Morriss and Alec at home in Cambridge, Mass. Photograph: Steve Schofield for the Observer
Two days after Anne Morriss took her newborn son home from hospital, she received a bone-chilling phone call. The stranger on the end of the line asked her whether she was sure her baby was still alive. Rushing to the next room, she was relieved to find the baby was fine, but the call was from a Massachusetts state physician who told her that a routine scan had revealed her baby had been born with a rare and often fatal genetic condition.

The condition, MCAD deficiency, is caused by mutations in a gene involved in fat metabolism. Some babies born with a severe version of the disease do not live for more than fortnight because their bodies cannot derive energy from fat by normal methods when their sugar stores run out. An infant with MCADD (Medium-chain acyl-CoA dehydrogenase) can simply sleep beyond the amount of sugar in his or her body, without an efficient way of converting fat into energy to keep the brain alive, says Morriss.

"That first year was a blur of anxious, sleepless nights. I worried that...