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.

Non-disjunction is the type of mutation that leads to Trisomy 21, where chromosomes fail to separate properly during gamete production. This results in an egg or a sperm cell carrying an extra chromosome, leading to embryos with three copies of chromosome 21 when fertilization occurs. In Trisomy 21, the abnormal process of non-disjunction can occur during meiosis, causing one gamete to contain two copies of chromosome 21. When this gamete fertilizes a normal gamete with a single chromosome 21, the resulting zygote ultimately has three copies. This extra genetic material is responsible for the characteristics associated with Down syndrome. Regarding the differences between Trisomy 21 and Mosaic Down syndrome: first, Trisomy 21 affects all somatic cells with an extra chromosome, while Mosaic Down syndrome has only a fraction of cells with the extra chromosome. Secondly, individuals with Mosaic Down syndrome may have a milder set of physical and developmental traits due to the proportions of normal versus abnormal cells, leading to varied presentations of the condition.