When egg and sperm or the sex cells first meet at point of conception, there is a coming together of the chromosomes that will help determine almost all things about a growing baby. Ideally this meeting works fine, but sometimes errors occur and people get extra, deleted, or partially missing chromosomes. This can cause a variety of chromosomal disorders, many of which are so severe that the pregnancy ends in miscarriage. Unless miscarriages have been frequent, parents may never know that a lost child had chromosomal issues because it isn’t something that is usually evaluated. In other circumstances, chromosomal disorders don’t cause death, but can impact life and health of a child to greater or lesser degree.
There are some very well known chromosomal disorders like those that involve inheriting a third copy of a chromosome. The best known of these is Down syndrome, also known as trisomy 21. It describes a state where children get three instead of two copies of the 21st chromosome, resulting in a variety of conditions that may differ in degree of expression. Some symptoms of this condition include mild to moderate retardation, changes to facial expression, and greater risk for congenital heart defects or disease. To confuse matters, some people have mosaic Down syndrome, where only some cells possess the third chromosome, and this can change Down expression a lot or a little.
In contrast to Down syndrome, there are chromosomal disorders where an additional sex chromosome is inherited, such as the X or Y, or where part of a sex chromosome is missing. Missing sex chromosomes might cause chromosomal disorders like Turner syndrome in girls, which results in infertility, slow growth, failure to reach puberty, and potential defects of other organs. When caught early, this condition can be treated with things like hormones to help with growth and to achieve natural maturation. In contrast, women can inherit a third X, which might not be apparent because it causes very few symptoms.
When boys get a second or third X, the result is not as good. Boys may develop Klinefelter syndrome, which may cause infertility or low levels of male hormones in adulthood. This might be treated with male hormones. A variety of other problems with X or Y-chromosomes may exist, each resulting in chromosomal disorders.
Additional disorders could result from partially missing, fully missing or poorly shaped or inverted chromosomes. The conditions that might result depend on chromosome number, and sometimes there is no “disorder” associated with some chromosomal errors. People are usually advised by their doctors, especially if they are over 35, to have chromosome analysis or testing while a baby is in utero. There are some risks to testing, but also risks to not knowing. It’s a matter that must be weighed carefully by each family.
Another thing that is important to note is that even with diagnosed chromosomal disorders, it is usually difficult to say how serious the expression of that disorder will be. Some conditions like trisomy 18 have a fairly predictable rate of decline, tending to cause death soon after the birth of a child. It may be much harder to predict the degree of problems or functionality a child with other types of disorders, like trisomy 21 (Down), will have.
This aspect of malfunction of the chromosomes remains mysterious in many cases, making it hard for parents to have a strong sense of what life might be like with a child that has chromosomal disorders. Additionally, there are many chromosomal errors with little to no expression. An error may simply be an error that has no impact on a child’s life.