Now you know the textbook example of a healthy human has 23 pairs of chromosomes in almost every cell of their body, but life isn't always a textbook. What happens if something causes more or less chromosomes to develop? A gain or loss of chromosomes from the standard 46 (called aneuploidy) occurs either during the formation of reproductive cells (sperm and egg), in early fetal development or in any other cell of the body after birth.
One of the more common forms of aneuploidy is "trisomy," which is the presence of an extra chromosome in the cells. One well-known result of trisomy is Down syndrome, which is a condition caused by three copies of chromosome 21 in each cell. This extra chromosome leads to a total of 47 chromosomes per cell, rather than 46.
The loss of one chromosome in a cell is called "monosomy," and describes a condition in which people have just one copy of a specific chromosome per cell as opposed to two. Turner syndrome, in which women have only one copy of the X chromosome per cell versus the regular two, is considered a form of monosomy.
There are other variations of aneuploidy as well, and in extreme cases, they may compromise a person's life. Additionally, cancer cells also have alterations in their number of chromosomes. Unlike the variations that happen in reproductive cells, these changes occur in other cells of the body, so they're not inherited.
Originally Published: Jul 3, 2019