Chromosomal anomalies refer to any error in either the number or structure of one's chromosomes. Although some chromosomal anomalies can be inherited, most are not passed from one generation to the next. They usually occur due to an error during the process of the egg or sperm being made (meiosis). If the cell containing an error in it is involved in fertilization, then the child will develop with that chromosomal anomaly in all of his or her cells. Humans normally have 46 chromosomes, which come in 23 pairs, one set from the mother, the other from the father. If a chromosomal anomaly is numerical, then the individual will have either fewer or greater than the correct number of chromosomes. The most common types of numerical chromosomal anomalies are monosomies and trisomies. A monosomy is when a person is missing one chromosome from one pair, so they have 45 total chromosomes instead of 46. For example, in Turner syndrome, a child is born with one X chromosome instead of two X's or an X and a Y chromosome. If someone has a trisomy, then the individual has an extra copy of one chromosome, so 3 copies instead of two. This results in a person having a total of 47 chromosomes instead of 46. Down syndrome, also called trisomy 21, is a condition that results from having three copies of chromosome 21 and a total of 47 total chromosomes.
Chromosomal anomalies can also result from structural errors. This can occur in many different ways including a deletion, duplication, translocation, inversion, and rings. All cause a harmful gain, loss or rearrangement of DNA and involve multiple genes. These anomalies and the conditions that result from them are complex and many times exact expectations and prognosis is not possible due to variation. A genetic counselor can help determine risks to future children or generations and provide a better understanding of how these anomalies occur.