Noam, a 33 year old male, comes to the clinic after trying to conceive a baby with his wife for more than a year, with no luck. Noam is very thin and quite tall. Upon physical examination, you notice that his testes are smaller than normal, and he has enlarged breast tissue. In addition, Noam doesn’t seem to have much facial and pubic hair. You decide to run a blood test, which reveals that Noam’s testosterone and inhibin B levels are decreased, while gonadotropin and estrogen are increased. In addition, you perform a karyotype analysis on his cells and find 47 chromosomes, among which there’s two X chromosomes and one Y chromosome.
Next, you see Hadas, a 17 year old girl who’s worried because she hasn’t gotten her first menstrual period yet. The first thing you notice is that Hadas is quite short for her age. Upon physical examination, she has a webbed neck, a broad chest, and poorly developed breasts with widely spaced nipples. In addition, you notice her ring fingers are very short. A blood test shows low estrogen levels and high gonadotropins, and a karyotype analysis reveals only 45 chromosomes, with one X chromosome.
Okay, based on their presentation, both Hadas and Noam seem to have some sort of disorder of sex chromosomes. Now, humans typically have 23 pairs of chromosomes, so 46 total; out of which 22 pairs are autosomal, and 1 pair consists of sex choromosomes, which can be X or Y. Generally, an individual with two X chromosomes, or 46,XX is considered to be genetically female. However, only one X chromosome gets expressed and the other is inactivated through a process called X inactivation or lyonization, becoming a Barr body. On the other hand, an individual with one X and one Y chromosome, or 46,XY is genetically male. And since there’s only one X chromosome to begin with, there’s generally no Barr body.
Now, individuals with sex chromosome disorders have aneuploidy, meaning that there’s a missing or extra sex chromosome. Most often, this results from nondisjunction, which can occur in the egg or sperm cell during meiosis 1 or 2, where a chromosome pair or sister chromatid respectively doesn’t split apart. So the child of this individual could either receive two chromosomes from that parent and one more from the other parent, resulting in trisomy; or no chromosome from that parent and one from the other parent, resulting in monosomy. Less frequently, nondisjunction may occur during mitosis, where a sister chromatid doesn’t separate.