How can you explain that not all Down syndrome cases are due to nondisjunction?

Down syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21, leading to a total of three copies instead of the normal two. This results in a set of characteristic physical and cognitive features, such as developmental delays and intellectual disabilities.

Nondisjunction is the failure of chromosomes to segregate properly during cell division, leading to the formation of gametes with an abnormal number of chromosomes. In the case of Down syndrome, nondisjunction can occur during the formation of egg or sperm cells, resulting in either parent contributing an additional chromosome 21. When an abnormal gamete fuses with a normal one during fertilization, the resulting zygote has three copies of chromosome 21, resulting in Down syndrome.

Although nondisjunction is the most common cause of Down syndrome, accounting for around 95% of cases, there are other mechanisms that can result in the presence of an extra chromosome 21. These include: 1. Robertsonian Translocation: This occurs when a piece of chromosome 21 is attached (translocated) to another chromosome, usually chromosome 14. If an individual inherits this translocated chromosome, they will have three copies of the genetic material from chromosome 21, resulting in Down syndrome. 2. Mosaicism: This occurs when some cells in the body have three copies of chromosome 21, while other cells have the normal two copies. This results from a nondisjunction event occurring after fertilization during the early stages of cell division. Mosaic Down syndrome is usually less severe than other forms, as the individual has a mixture of normal and affected cells.

In summary, although nondisjunction is the most common cause of Down syndrome, other mechanisms like Robertsonian translocation and mosaicism can also result in the presence of an extra copy of chromosome 21. Therefore, it is important to recognize that not all Down syndrome cases are due to nondisjunction.