Down syndrome, also known as trisomy 21, is a genetic disorder that is caused by the presence of an extra copy of chromosome 21 in an individual's cells. This extra copy of the chromosome leads to physical and cognitive developmental delays, as well as a range of other physical characteristics and health issues.
Down syndrome is not a sex-linked disorder, meaning that it is not linked to a person's sex chromosomes (X and Y). Instead, it is an autosomal disorder, which means that it is associated with a chromosome that is not a sex chromosome. In the case of Down syndrome, it is caused by an extra copy of chromosome 21, which is an autosome. Autosomes are the chromosomes that are not involved in determining a person's sex and are present in both males and females.
Down syndrome can be inherited in several different ways. The most common form of Down syndrome is called nondisjunction, which occurs when there is an error during cell division that results in an extra copy of chromosome 21. This can happen during either meiosis (the process of cell division that leads to the formation of eggs and sperm) or mitosis (the process of cell division that occurs in all other cells of the body). Nondisjunction can be caused by a variety of factors, including age, genetics, and environmental factors.
Another form of Down syndrome is called mosaic Down syndrome, which occurs when the extra copy of chromosome 21 is present in some, but not all, of an individual's cells. This can happen when the error in cell division occurs after fertilization, leading to a mixture of cells with and without the extra chromosome. Mosaic Down syndrome tends to be less severe than other forms of the disorder because not all of the individual's cells are affected.
Down syndrome can also be caused by a chromosomal rearrangement, such as a translocation, in which a piece of chromosome 21 breaks off and reattaches to a different chromosome. This can lead to the same characteristics as Down syndrome, but it is not as common.
In conclusion, Down syndrome is an autosomal disorder that is caused by the presence of an extra copy of chromosome 21. It is not a sex-linked disorder and can be inherited in several different ways, including nondisjunction, mosaic Down syndrome, and chromosomal rearrangements.