Twenty years ago in Paris, Dr. Jerome Lejeune died of lung cancer.

Lejeune was the French geneticist whose breakthrough came in 1958, when he discovered the extra chromosome on the 21st pair that causes Down syndrome. Though he never did find the cure he spent the rest of his years looking for, his patients taught him something those who look at Trisomy 21 exclusively as a medical condition often miss: the great joy these trusting little souls bring into our world.

Joy is a timely message. This Sunday in Rome, two popes who brought joy to the faces of society’s weakest and most vulnerable citizens — the aged, the infirm, the prisoner, the less than physically perfect — will be canonized on St. Peter’s Square. It inspires the hope Jerome Lejeune may one day join Pope St. John XXIII and Pope St. John Paul II on the universal calendar, as the patron saint of those with Down syndrome.

It’s not as outlandish as it sounds: In Rome the case for Lejeune’s own sainthood proceeds.

Trisomy 21 families could use a patron saint. Not someone who pitied them. Someone who appreciated their most salient feature, which is not their third chromosome but their full humanity. In his own day, in addition to his research, Dr. Lejeune treated thousands — and his daughter says he called each by name.

With every canonization, the Catholic Church sends the message that saints still walk among us. And the cross Dr. Lejeune carried on earth was unique: the realization his discovery had helped make possible the prenatal testing that leads so many parents to abort when Down syndrome is confirmed.

It’s human enough. When all we know about another human being is his or her handicap, that’s how we define them. And when we define people by their handicaps, it’s hard not to look at them as balls and chains, unfairly yoking our lives to their disabilities.

Funny thing about the reality, though. Look around at the moms and dads — not to mention, the brothers and sisters — who have family members with Down syndrome. You will find an absence of sad sacks and martyrs.

Many will tell you they didn’t start out this way. Some cursed when they learned the news. Others secretly feared they wouldn’t love their babies. Even the most giving of families will tell you there are days it can seem too much.

And then they see something the rest of us take for granted — a smile breaking across the face of a child who’s learned to tie her shoes — and they find themselves smacked in the face by a joy that makes the angels sing.

The rest of us get glimmers. On YouTube there’s a video about Thomas Vander Woude, a former Navy pilot and Vietnam vet who saved his Down syndrome son, Joseph. He did it by leaping into a septic tank when his son fell in and submerging himself to push the boy up, keeping his son’s head above the sewage until he could be rescued.

In utilitarian terms, the tradeoff makes no sense: the hale and hearty giving up life for one who will always be dependent. But listen to the Vander Woude family, and instead of resentment you find awe and beauty — and gratitude. Maybe that’s because they too know the joy of Joseph, the kind of joy a strong man happily gives his life to protect.

All around us every day, people with Down syndrome elevate the ordinary into the unforgettable. A crowd going wild when a Down syndrome kid puts on a basketball uniform and scores a basket. The cheers of her fellow high school students when a classmate with Down syndrome finds herself crowned prom queen. The young man who comes home to his parents to relay the thrilling news: someone has shown enough confidence in him to give him a job.

These moments are bought with much heart-ache. But here’s the beauty. The joy isn’t limited to the Down syndrome folks. The rest of us taste it, too, when we let their dependence bring out what’s most human in us.

In the 20 years since Dr. Lejeune died, and the half-century since he made his most famous discovery, we’ve learned much more about Trisomy 21. The life expectancy for people with Down syndrome has expanded, along with the opportunities that help them live happier, healthier and fuller lives.

So what would a patron saint for those with Down syndrome offer? A reminder that there are truths outside the microscope, and among them is this: The special gift of the weak and the imperfect is to teach us how to love.