2.4 Down syndrome is the result of an individual having an extra copy of chromosome 21. Two genetic variations that can cause Down syndrome are: - Trisomy 21 All the somatic cells in an individual have three coples of chromosome 21 due to an abnormal process that occurs during gamete production. - Mosaic Down syndrome The individual has only some cells with an extra copy of chromosome 21 which is coused by an abnormal process during cell division after fertilisation. 2.4.1 Name the: (a) Type of mutation that leads to Trisomy 21 (b) Abnormal process during gamete production that leads to three copies of chromosome 21 (c) Type of ceil division that occurs after fertilisation 2.4.2 Describe how the process in QUESTION 2.4.1(b) leads to Trisomy 21. 2.4.3 Describe TWO differences between Trisomy 21 and Mosaic Down syndrome.

2.4.1 (a) The mutation is a chromosomal mutation known as nondisjunction.   (b) The abnormal process during gamete production is nondisjunction during meiosis.   (c) The type of cell division after fertilization is mitosis. 2.4.2 Description of how nondisjunction during meiosis leads to Trisomy 21: During meiosis (the process of gamete formation), the homologous chromosomes are supposed to separate so that each gamete receives one copy of each chromosome. In the case of nondisjunction, the two copies of chromosome 21 fail to separate properly, resulting in one gamete that contains two copies of chromosome 21 and another gamete that receives none. When the gamete with two copies is fertilized by a normal gamete (with one copy), the resulting zygote ends up with three copies of chromosome 21 in every cell. This additional copy causes the condition known as Trisomy 21 or Down syndrome. 2.4.3 TWO differences between Trisomy 21 and Mosaic Down syndrome: 1. Distribution of the extra chromosome:  • In Trisomy 21, every cell in the body contains an extra copy of chromosome 21.  • In Mosaic Down syndrome, only some cells have the extra chromosome while others have the normal chromosomal complement. 2. Timing and origin of the error:  • Trisomy 21 usually results from nondisjunction during meiosis (gamete formation) leading to a uniform presence of the extra chromosome across all cells.  • Mosaic Down syndrome results from an error (nondisjunction) during mitosis after fertilization, which means that the error occurs in only some of the cells as the embryo develops. These differences explain how the two conditions arise and why their presentations may vary.