Ch. 4, Part 2: What is Down syndrome?

In the second part of the chapter on Down syndrome, I begin with the simple question: What is Down syndrome?

At its most basic, Down syndrome is a genetic condition caused by a triplicate of the 21^st chromosome. Most people only have two copies of each chromosome, for a total of 23 pairs of chromosomes, numbering 46 chromosomes in all. Down syndrome is caused when there is an extra 21^st chromosome. This is why the lab representative said Down syndrome has been at the forefront of prenatal testing because it is easy to spot: if you count past 46 chromosomes, you’ve found a trisomy, with the odds being that it is Down syndrome, the most common occurring genetic condition at birth and most commonly occurring trisomy.

Dr. Jerome Lejeune, the last of the three quoted above, discovered this genetic basis for Down syndrome. Contrary to Dr. Down’s description, the cause of Down syndrome is not due to tuberculosis in the parents. Indeed, the cause for Down syndrome remains unknown. It occurs at or near the point of conception, an embryonic moment termed “syngamy” when the separate chromosomes from the father’s sperm and the mother’s egg pair up with one another, creating the unique genetic code for that human being.

Due to this pairing up of the father’s and mother’s chromosomes, and then the subsequent cell divisions of the fertilized egg, is why there are three “versions” of Down syndrome. The overwhelmingly most common version is nondisjunctive Trisomy 21. This is the version Juliet has. It means that in every single cell in her body, there is an extra 21^st chromosome. The two other versions are called Translocation Trisomy 21 and Mosaic Down syndrome, each constituting about 2.5% of all of the individuals with Down syndrome.

In the case of Translocation, one of the parents actually only has 45 Chromosomes. This is because one of their 21^st Chromosomes is attached to another chromosome. When that parent’s chromosomes joined with their partner’s, it caused an extra amount of the 21^st Chromosome to be present in the future child. This is the one form of Down syndrome that is inheritable.

With Mosaic Down syndrome, only a percentage of the person’s cells actually have a triplicate of the 21^st Chromosome. Mosaicism occurs when, as the fertilized egg’s cells divide and divide, ever more with each division, somewhere in that division, one of the cells ends up with a triplicate of the 21^st Chromosome. These individuals then only have a certain percentage of cells that have an extra 21^st Chromosome, resulting in them being effected as differently as the percentage of cells that have the extra genetic material.

Still, though, what causes the genetic phenomena of Down syndrome is unknown. At the time of Dr. Down’s description, it was common to presume it was the “fault” of the parents: that something the parents did caused their child to have a disability. In the Judeo-Christian tradition, it is common for the parents to believe their child was born with Down syndrome as a punishment for the parents’ sins. Indeed, this was something I wondered when Juliet was born: is she being “punished” for the wrongs I’ve committed. In this sense, I understood the logic of the Disciples who asked Jesus, in referring to a blind man, “who sinned, this man or his parents, that he would be born blind.”

This view of metaphysical, cosmic causality is one that has not always been shared in all cultures. The oldest depictions of what is believed to be individuals with Down syndrome come from the ancient Olmec civilization (circa 1200 – 400 BC). In large stone sculptures, the individuals show the common facial characteristics associated with Down syndrome and are in poses many parents instantly recognize from finding their own children folded almost in half, due to the suppleness of their joints. The archaeologists who study the Olmecs consider the sculptures to be in reverence of the individuals depicted with Down syndrome.

Similarly, the first known depiction in Western Culture also shows individuals with Down syndrome in a setting of reverence. Art historians discovered a 1515 painting from Medieval Europe titled “The Adoration of the Christ Child.” In it, seated at the right hand of Mary is an angel with the distinct facial characteristics of an individual with Down syndrome. Above Mary, peering into the stable, is a shepherd with similar facial features. At the top of the painting are several cherubs that also have those facial features. In analyzing the painting, the art historians commented:

After all the speculations, we are left with a haunting late-medieval image of a person with apparent Down syndrome with all the accouterments of divinity. It is impossible to know whether any disability had been recognized or whether it simply was not relevant in that time and place.

These more ancient depictions of Down syndrome evidence the wisdom of Jesus in responding to his Disciples’ question about the cause of disability: “It was neither that this man sinned, nor his parents; but it was so that the works of God might be displayed in him.”