Explanation:

Down syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. This additional genetic material alters the course of development and causes the characteristic features of Down syndrome.

Non-disjunction of chromosome:
The most common cause of Down syndrome is non-disjunction of chromosome 21 during meiosis. Non-disjunction occurs when the chromosomes fail to separate properly during cell division, resulting in an abnormal distribution of chromosomes in the resulting cells. In the case of Down syndrome, non-disjunction of chromosome 21 occurs either during the formation of the egg or sperm, resulting in an extra copy of chromosome 21 in the fertilized egg.

Key Points:
- Non-disjunction of chromosome 21 during meiosis is the most common cause of Down syndrome.
- Non-disjunction can occur during the formation of the egg or sperm.
- Non-disjunction leads to an abnormal distribution of chromosomes in the resulting cells, resulting in an extra copy of chromosome 21.

Other Causes:
While non-disjunction is the primary cause of Down syndrome, there are other rare genetic variations that can also result in the condition. These include translocation, mosaicism, and partial trisomy.

- Translocation: In some cases, a piece of chromosome 21 breaks off and attaches to another chromosome, typically chromosome 14. This is called a translocation. If a person has a translocation involving chromosome 21, they may have a higher risk of having a child with Down syndrome.

- Mosaicism: Mosaicism occurs when there is a mixture of cells with a normal number of chromosomes and cells with an extra copy of chromosome 21. This can result in milder symptoms or features of Down syndrome.

- Partial Trisomy: In rare cases, a person may have only a portion of chromosome 21 duplicated, leading to a condition known as partial trisomy. This can result in a milder form of Down syndrome, as only a subset of genes on chromosome 21 is affected.

Conclusion:
In conclusion, the most common cause of Down syndrome is non-disjunction of chromosome 21 during meiosis. This results in an extra copy of chromosome 21 in the fertilized egg, leading to the characteristic features of Down syndrome. Other rare genetic variations, such as translocation, mosaicism, and partial trisomy, can also result in Down syndrome but are less common.