What does trisomy indicate in terms of chromosome number?

Trisomy indicates the presence of an extra chromosome, resulting in a total of three copies of a specific chromosome instead of the usual two. This chromosomal abnormality occurs due to nondisjunction during cell division, where chromosomes fail to separate properly, leading to gametes that have an abnormal number of chromosomes. When one of these gametes fuses with a normal gamete during fertilization, the resulting zygote exhibits trisomy for that particular chromosome. A well-known example of trisomy is Down syndrome, which is caused by the presence of an extra copy of chromosome 21. Understanding trisomy is essential in genetics as it has significant implications for development and health.