Normally humans have 23 pairs of chromosomes. These chromosomes contain the DNA that are the “instructions” for how to build our bodies.
The egg and the sperm each bring one for each pair – at least most of the time! Every now and then when the egg or sperm is forming something doesn’t go quite right and it ends up bringing a full pair. When that pair combines with another egg or sperm, the fertilized egg ends up with three of any one chromosome. When there’s an “extra” chromosome, that is called a “trisomy”
So what does that mean for the fertilized egg and its development to full term? Well, it depends on which chromosome got doubled. Some have never been seen in full term babies, so they are a possible cause of some miscarriages. Trisomy 16 and 22 are the most common ones we know play a role in miscarriage.
Most kinds of Trisomy are random, not inherited, and of no known cause.
The kinds of complete trisomy that can happen in a full-term birth:
Trisomy 13 (Patau Syndrome) Can cause some significant problems with the nervous system, muscles, skin and other parts of the body. One common feature is extra fingers.
Trisomy 18 (Edwards Syndrome) Can cause heart and intestinal issues, a small head, etc. Most babies who have Trisomy 18 live only about a week or two because of the severity of the problems it can cause.
Trisomy 21 (Down Syndrome) Most people are familiar with Down syndrome, as many who have it live long and functioning lives. The facial differences are fairly recognizable as well. As with the other trisomies, how severe the effects are varies.
The 23rd set of chromosomes are the ones that determine biological sex. Most people are either XX (which generally results in a female baby) or XY (which generally results in a male baby). It’s possible to have several different possible trisomies with this chromosome:
XXX (Triple X Syndrome) Babies with this trisomy are generally female. It has no physical features associated with it, and may have irregular periods or learning disabilities. Or no symptoms at all.
XXY (Klinefelter Syndrome) Babies with this trisomy generally appear male but have later development and lesser amounts of traditionally male characteristics like facial hair, body hair, and muscle mass. They often struggle with fertility.
XYY Syndrome Males with this trisomy appear physically normal, go through puberty normally, and have normal fertility. Some may have delayed speech or language learning disorders.
(Please note that it is also possible to have single chromosomes as well as trisomies, but there is only one monosomy that is surviveable. That is Turner’s Syndrome, when the baby has a single X chromosome. People with Turner’s syndrome are generally female, tend to be short, and need hormone treatment to go through puberty and cannot have children without fertility treatment.)