A digression into monosomy...
Surprise! Trisomy is not the only chromosome disorder out there.
Monosomy describes the absence of a chromosome. The normal amount of chromosomes in humans is 46. Therefore, someone with monosomy would only possess 45 chromosomes.
Only one type of true monosomy exists in humans. The disorder is called Monosomy X, but is commonly known as Turner syndrome. People with the disorder are missing one of their X chromosomes and are phenotypically female.
Turner syndrome causes many physical symptoms, including a webbed neck, broad and flat chest, and delayed physical development. Although pregnancy is possible in certain circumstances, women with Turner syndrome are usually infertile without treatment. There is no documentation of intellectual disability. The following image illustrates the some of the symptoms of Turner syndrome visible at birth.
Interestingly, whereas an extra sex chromosome usually results in tall stature, the absence of one causes stunted growth. As a result, females with Turner syndrome often look much younger than they are. In the following photo provided by the Turner Syndrome Society of the United States, you can see that the main noticeable characteristic of girls with Turner syndrome is simply their height.
Turner syndrome is now detectable as an add-on to a new noninvasive test for trisomy, which I have talked about several times in my previous blog posts. In my discussion of sex chromosome trisomy, I took a stance against terminating pregnancies with such a diagnosis. I maintain this position for prenatal diagnoses of Turner syndrome, although I do, as always, respect a woman’s right to choose.
Earlier this year, Dr. Richard Legro, a fertility and reproductive endocrinologist at the Penn State Hershey Women's Health Center, published an article summarizing the new insights into the disorder. Towards the end of the article, he notes the high rates of pregnancy termination for both mosaic and nonmosaic Turner syndrome. He writes that he was surprised by the figures he saw, considering the “overall favorable diagnosis” for women with the disorder, especially with hormone therapy.
I applaud Dr. Legro for his effort to better inform both the healthcare professionals who treat patients with Turner syndrome as well as the genetic counselors who advise expectant parents with a prenatal diagnosis.
And with that, let us return to our main topic, trisomy!
